IGCSE Chemistry revision guide 2012
- 1. This revision guide is designed to help you study for the chemistry part of the IGCSE Coordinated Science course. The guide contains everything that the syllabus says you need you need to know, and nothing extra. The material that is in the supplementary part of the course (which can be ignored by core candidates) is highlighted in dashed boxes: Some very useful websites to help you further your understanding include: •http://www.docbrown.info/ - whilst not the prettiest site this contains a lot of very useful and nicely explained information. •http://www.bbc.co.uk/schools/gcsebitesize/scienc e/ - well presented with many clear diagrams, animations and quizzes. Can occasionally lack depth. •http://www.chemguide.co.uk/ - whilst mostly targeted at A-Levels this site contains very detailed CHEMISTRY REVISION GUIDE for CIE IGCSE Coordinated Science (2012 Syllabus) CHEMISTRY REVISION GUIDE for CIE IGCSE Coordinated Science (2012 Syllabus) Whilst this guide is intended to help with your revision, it should not be your only revision. It is intended as a starting point but only a starting point. You should make sure that you also read your text books and use the internet to supplement your study in conjunction with your syllabus document. Whilst this guide does contain the entire syllabus, it just has the bare minimum and is not in itself sufficient for those candidates aiming for the highest grades. If that is you, you should make sure you read around a range of sources to get a deeper knowledge and understanding. targeted at A-Levels this site contains very detailed information suitable for those looking to deepen their knowledge and hit the highest grades. Finally, remember revision is not just reading but should be an active process and could involve: •Making notes •Condensing class notes •Drawing Mind-maps •Practicing past exam questions •Making flashcards The golden rule is that what makes you think makes you learn. Happy studying, Mr Field.
- 2. C1: THE PARTICULATE NATURE OF MATTER Solids, Liquids and GasesAtom: The smallest particle of matter An atom: Some atoms: Molecule: A small particle made from more than one atom bonded together Molecules of an element: Molecules of a compound: Element: A substance made of only one type of atom A solid element: A gaseous element: SOLIDS LIQUIDS AND GASES The particles in solids, liquids and gases are held near to each other by forces of attraction. The strength of these forces determines a substance’s melting and atom Compound: A substance made from two or more different elements bonded together A solid compound A gaseous compound: Mixture: A substance made from two or more elements or compounds mixed but not joined A mixture of compounds and elements: attraction. The strength of these forces determines a substance’s melting and boiling points. In a solid, the forces of attraction are strongest, holding the particles tightly in position. As the solid is heated, and the particles vibrate faster, these forces are partially overcome allowing the particles to move freely as a liquid – this is called melting. As the liquid is heated more, the particles gain so much energy that the forces of attraction break completely allowing particles to ‘fly around’ as a gas – this is called boiling. The reverse of the these processes are condensing and freezing. Under specific conditions, some solids can turn straight to gases – a process called subliming (the reverse is called desubliming). PROPERTIES Solids •Have a fixed shape •Can’t be compressed •Particles close together in a regular pattern •Particles vibrate around a fixed point Liquids •Take the shape of their container •Can’t be compressed •Particles close together but disordered •Particles move freely Gases •Take the shape of their container •Can be compressed •Particles widely spaced in random order •Particles moving very fast.
- 3. C2: EXPERIMENTAL TECHNIQUES FILTRATION Used to separate solids from liquids. The mixture is poured through a filter paper in a funnel. The liquid can pass through the small holes in the filter paper (to become the filtrate) and the solid gets left behind (called the residue). PAPER CHROMATOGRAPHY Paper chromatography is a technique that can be used to separate mixtures of dyes or pigments and is used to test the purity of a mixture or to see what it contains. Firstly a very strong solution of the mixture is prepared which is used to build up a small intense spot on a piece of absorbent paper. This is then placed in a jar of solvent (with a lid). As the solvent soaks up the paper, it dissolves the mixture-spot, causing it to move up the paper with the solvent. However since compounds have different levels of solubility, they move up the paper at different speeds causing the individual components to separate out. The solvent or combination of solvents can be changed to get the best possible separation of spots. CRYSTALLISATION Crystallisation is used to separate mixtures of solid dissolved in liquid and relies on the fact that solids are more soluble at higher temperatures. A solution containing a solid is cooled FRACTIONAL DISTILLATION When the liquids being distilled have similar boiling points, normal distillation can’t separate them completely but simply gives a purer mixture. In this case a fractionating column is PURITY It is important for chemists to be able to purify the compounds they make, this is because the impurities could be dangerous or just un-useful. This is especially true for chemists making compounds that are consumed by people such as WHICH TECHNIQUE? You need to be able to select appropriate methods to separate a given mixture. The key to this is look for differences in the properties of the components of the mixture such as their state, solubility, melting/boiling point and so on. Then pick the method that best takes advantage of this difference. higher temperatures. A solution containing a solid is cooled down until crystals form in the solution, these can then be collected by filtration. The related technique of recrystallisation can be used to separate a mixture of two soluble solids by taking advantage of the difference in their solubility. The mixture is dissolved in the smallest possible amount of hot solvent. As the solution cools, the less soluble compound forms crystals that can be collected by filtration whilst the more soluble compound stays dissolved. DISTILLATION In distillation a mixture of liquids is separated using the differences in their boiling points. The mixture is heated until the liquid with the lowest boiling point boils, the vapours then condense on the cold surface of the condenser and the pure(er) liquid is collected. used. This provides a large surface area for condensation meaning much purer ‘fractions’ are produced. The most important use of this is separating crude oil into it’s useful components. compounds that are consumed by people such as drugs or food additives since the impurities may be toxic which would be very bad news! MELTING/BOILING POINTS No two substances have the exact same melting and boiling points. We can take advantage of this to test the purity of a compound we have made. If we know what the melting or boiling point of the pure compound should be, we can then measure the melting or boiling point of a sample we have produced and the closer it is to the pure value, the more pure it is likely to be.
- 4. C3: ATOMS, ELEMENTS ANDC3: ATOMS, ELEMENTS AND COMPOUNDS – Structures and Bonding STRUCTURE OF THE PERIODIC TABLE (PT on last page!) Elements arranged in order of increasing proton number. Periods: The rows in the periodic table. •For example Li, C and O are all in period 2. Non-metals Transition Metals GroupVIII:NobleGases GroupI:AlkaliMetals GroupII:Alkali-Earth Lanthanides and Actinides (metals) Other Metals H GroupVII:Halogens ELECTRON ARRANGEMENT/CONFIGURATION Electrons are arranged around atoms in specific shells. The most important shell is the outer one as this controls an atom’s chemistry. We call the electrons in the outer shell ‘valence electrons’ because they are used for bonding. The number of electrons in the outer shell is the same an element’s group number. The number of electrons around an atom is given by the atom’s proton number. They are arranged in shells as follows: •1st Shell – Holds two electrons •2nd/3rd/4th Shells – Hold 8 electrons •Example 1: Carbon. Proton number is 6 which means there are 6 electrons: 2 in the 1st shell and 4 in the second •Example 2: Chlorine. Proton number is 17 which means there are 17 electrons: 2 in the 1st shell, 8 in the second A NOBLE MATTER The Noble Gases (He, Ne, Ar etc) have full outer shells containing either 2 or 8 electrons. This is very stable which is why the Noble gases are so unreactive. Other elements tend to react in such a way as to achieve a full outer shell by gaining or losing electrons until they achieve this Noble Gas configuration. CHEMICAL VS PHYSICAL CHANGES Physical changes are reversible whereas chemical changes are not. ISOTOPES Isotopes are atoms with the same proton number but different nucleon number. •For example Li, C and O are all in period 2. Groups: The columns in the PT. •Use roman numbers: I, II, III, IV, V, VI, VII, VIII •Eg. F, Cl, Br, I are all in Group VII •Elements in the same group have similar properties and react in similar ways: the halogens all react in the same way with sodium to form sodium fluoride (NaF), sodium chloride (NaCl), sodium bromide (NaBr) and sodium iodide (NaI) Checking Your Answer: To check you are right, the period gives the number of shells and the group gives the number of electrons in the outer shell. For example chlorine is in Period 3 and Group VII so it has 3 shells and 7 electrons in the outer shell. Ions: The configuration of ions is the same as for atoms but you have to take electrons away from positive ions and add extra for negative ions. For example O/O2- Li/Li+ 1st shell and 4 in the second the 1st shell, 8 in the second and 7 in the 3rd. C Cl Li Li+O2-O ATOMIC STRUCTURE Atoms are made of: Protons: mass = 1, charge = +1 Neutrons: mass = 1, charge = 0 Electrons: mass = 0, charge = -1 The numbers of each vary from element to element but it is the number of protons which decides what the element is. In a square on the periodic table the smaller number, the proton number, gives the number of protons or electrons and the bigger number, the nucleon number the number of protons and neutrons together. Eg 1: Boron has 5 protons, 6 neutrons (ie 11-5) and 5 electrons Eg 2: Phosphorus has 15 protons, 16 neutrons (ie 31-16) and 15 electrons For example if you melted some solid sugar to a liquid and then left it to cool, it would freeze back to solid sugar – this is a physical change. If you took the same sugar and burned it to produce carbon dioxide and water, there would be no easy way to turn those back to sugar – this is a chemical change – new substances are made. For example carbon has two main isotopes – C-12 and C-13. Carbon has a proton number of 6 so they both contain 6 protons and 6 electrons but C-12 has 6 neutrons and C- 13 has 7.
- 5. C3: ATOMS, ELEMENTS ANDC3: ATOMS, ELEMENTS AND COMPOUNDS – Bonding and Structure Non-metals GroupVIII:NobleGases GroupVII:Halogens COVALENT BONDING A covalent bond forms between two atoms and is the attraction of two atoms to a shared pair of electrons. Small groups of IONIC BONDING An ionic bond is the attraction between two oppositely charged ions. Cations (positive) are formed when atoms (usually metals) lose electrons. Anions (negative) are formed when atoms (usually non- metals) gain electrons. Atoms will lose or gain electrons until they have a complete outer shell: elements in Groups I, II and III will lose 1, 2 and 3 electrons respectively to form 1+, 2+ and 3+ ions. Atoms in Groups V, VI and VII gain 3, 2 and 1 electrons to form 3-, 2- and 1- ions. In an ionic compound the number of positive and negative and charges must cancel out to neutral. Example: NaF, sodium in Group I forms a 1+ ion and fluorine in group VII forms a 1- ion so one Na+ is needed to balance out one F- Example: Li2O, lithium in Group I forms a 1+ ion but oxygen in Group VI forms a 2- ion so two Li+ are needed to balance out one O2- Li+ O2-Li+Na+ F- MOLECULES A molecule is a small particle made from (usually) a few non-metal atoms bonded together. The atoms in a molecule are joined by strong covalent bonds. In a solid each molecule is held close to its neighbour by weak intermolecular forces. When a substance melts, it is these weak intermolecular forces that break NOT the strong covalent bonds. Molecular compounds have low melting points and are volatile (evaporate easily) due to the weak intermolecular forces, and insulate electricity as all electrons are stuck in bonds and so unable to move. GIANT IONIC LATTICES The positive and negative ions in an ionic compound don’t formof two atoms to a shared pair of electrons. Small groups of covalent bonded atoms can join together to form molecules. The atoms share enough electrons to complete their outer shells. *Nb: In these diagrams only draw the outer shell and use different shapes/colours to show where electrons have come from. You should be able to draw at least: H2O, CH4, Cl2, HCl, H2, N2, O2, CO2, C2H4 Example: H2O*, hydrogen is has one valence electron and needs one more to complete the 1st shell, oxygen has six valence electrons electrons so needs two more. Thus one oxygen will react with two hydrogens: Example: CO2*, carbon is has four valence electrons so needs four more to complete its outer shell, oxygen needs two more. Thus each carbon will react with two oxygens, sharing two electrons with each one. A bond involving two shared pairs is a double bond. H HO O OC an ionic compound don’t form molecules but form crystals made of a repeating pattern of positive and negative ions called a giant ionic lattice. Eg sodium chloride: Properties of Ionic Compounds When you melt or dissolve an ionic compound it conducts electricity because the ions are free to move towards the positive and negative electrodes. When solid the ions are stuck in position and there are no free electrons so they don’t conduct. GIANT COVALENT LATTICES A crystal made of a repeating pattern of atoms joined with covalent bonds that repeats millions of times in all directions. Diamond is made of carbon atoms arranged so that each C is bonded in a pyramid arrangement to 4 others. This makes it very hard, ideal for use in industrial drills: Graphite: made of carbon atoms arranged in hexagonal sheets with long weak bonds between the sheets. This means the sheets can easily separate making graphite a good lubricant: Silicon (IV) oxide (SiO2) has a structure with each Si joined to 4 O and each O joined to 2 Si. It is the main ingredient in glass.
- 6. C4: STOICHIOMETRY – Formulas and Equations CHEMICAL FORMULAS Formulas tell you the atoms that make up a compound Eg 1. H2O – two H, one O Eg 2. C2H6O – two C, six H, one O Eg 3. Mg(OH)2 – one Mg, two O, two H* Eg 4. CH2(CH3)2 – three C, 8 H* *In this case everything in brackets is doubled You may be asked to write a formula given a diagram of a molecule for example glucose. By counting you can see there are 6 carbons, 12 hydrogens and 6 oxygens so the formula is C6H12O6 SYMBOL EQUATIONS •Show the reactants you start with and the products you make using symbols not words •Must contain an arrow ( ) NOT an equals sign (=) •Must be balanced – same number of atoms on each side. •Balancing is done by placing numbers called coefficients in front of the formulas for the compounds/elements. For example, ‘O2‘ means there is one oxygen molecule involved in a reaction but ‘2O2’ would mean there are two. Example:. CH4(g) + O2(g) CO2)g) + H2O(g) * This is unbalanced as there are 4 ‘H’ on the left but only 2 ‘H’ on the right. This must be corrected by placing a ‘2’ in front of the ‘H2O’ so there are now 2 waters: CH4 (g) + O2(g) CO2(g) +2H2O(g) Now the ‘H’ balances but there 4 ‘O’ on the right and only 2 on the left. This must be balanced by placing a ‘2’ in front of WORD EQUATIONS •These tell you the names of the chemicals involved in reaction •The left hand side shows you what you start with and is called the reactants •The right hand side shows you what you make and is called the products •The left and right are connected by an arrow ( not ‘=‘) which means ‘makes’ or ‘becomes’ •When you react a metal with oxygen to make a metal oxide, the equation might be: Iron + oxygen iron oxide IONIC FORMULAS You can deduce the formula of an ionic compound if you know the charges on the ions involved. The total positive charge must balance out the total negative charge so you must look for the lowest common multiple (LCM) of the charges. Eg1. Calcium nitrate is made of Ca2+ ions and NO3 - ions. The LCM of 2 and 1 is 2 which means you need 1 Ca2+ ion and 2 NO3 - ions so the formula is Ca(NO3)2on the left. This must be balanced by placing a ‘2’ in front of the ‘O2’ so that there are 2 oxygen molecules: CH4(g) + 2O2(g) CO2(g) + 2H2O(g) Now there is 1 ‘C’, 4 ‘H’ and 4 ‘O’ on each side so it balances. In ionic equations, we tend to look only at the ions that actually change. For example, when iron reacts with copper sulphate to form iron sulphate and copper the equation is: Fe(s) + Cu2+( aq) + SO4 2- (aq) Fe2+ (aq) + SO4 2- (aq) + Cu(s) In this case, the sulphate ion (SO4 2-) remains unchanged (we call it a spectator ion) so it can be left out of the equation to give: Fe(s) + Cu2+( aq) Fe2+ (aq) + Cu(s) This allows us to see more clearly the actual chemical changes taking place. Note: You can’t change the little numbers (ie the 2 in H2O ) as this changes the compound to something completely different. *The state symbols (s), (l), (g) and (aq) are used to indicate solid, liquid, gas and ‘aqueous solution’ (dissolved in water). Iron + oxygen iron oxide •Many fuels burn in oxygen to produce carbon dioxide and water for example: Methane + oxygen carbon dioxide + water so the formula is Ca(NO3)2 Eg2. Aluminium oxide is made of Al3+ ions and O2- ions. The LCM of 2 and 3 is 6 which means you need 2 Al3+ ions and 3 O2- ions so the formula is Al2O3. CHEMICAL MASSES The relative atomic mass (Ar) of an element is the mass of one atom relative to 1/12th the mass of C- 12. It is just a number that allows us to compare the mass of atoms of different elements. Ar can be found on the periodic table as the ‘large’ number in each square. For example Ar for carbon is 12.01 and for iron is 55.85. Ar has no units since it is only a relative number, allowing us to compare things. The relative formula mass (Mr) is the combined Ar of all the elements in the formula for a substance. Mr also has no units for the same reason as above. Example 1: Water, H2O The Ar for H and O are 1.01 and 16.00 so: Mr(H2O) = 2 x 1.01 + 1 x 16.00 = 18.02 Example 2: Magnesium Hydroxide, Mg(OH)2 The Ar for Mg, O and H are 24.31, 16.00 and 1.01: Mr(Mg(OH)2) = 1 x 24.31 + 2 x 16.00 + 2 x 1.01 = 58.33 Example 3: Decane, CH3(CH2)8CH3 The Ar for C and H are 12.01 and 1.01 Mr(decane) = 10 x 12.01 + 22 x 1.01 = 142.34
- 7. C4: STOICHIOMETRY – The Mole Concept THE MOLE A mole is 6.02x1023 of something. It is chosen so that a mole of something has the same mass in grams (molar mass, Mm) as its formula mass. For example the Mr of water is 18.02 so the Mm of water is 18.02g; the Mr of decane is 142.34 so the Mm of decane is 142.34g. Importantly this means that 18.02 g of water and 142.34g decane contains the same number of molecules. EQUATIONS AND MOLE RATIOS Equations can be used to help us calculate the numbers of moles of substances involved in a reaction. We can see this by studying the following reaction: 2C2H6 + 7O2 4CO2 + 6H2O Q1: How many moles of CO2 are produced by burning 1.0 mol of C2H6? We say that C2H6 is our ‘known’ and CO2 is our ‘unknown’ so: Moles CO2 = moles known/knowns in eqn x unknowns in eqn = 1.0 /2 x 4 = 1.0 x 2 = 2.0 mol Q2: If 0.01 mol of CO2 is produced, how much H2O must also be produced? This time CO2 is our known and H2O is our unknown so: Moles H2O = moles known/knowns in eqn x unknowns in eqn = 0.01/4 x 6 = 0.0025 x 6 = 0.015 mol *You must make sure your equation is balanced or your mole ratio will be wrong. THE MOLES AND MASSES If you know the mass in grams of substance, you can calculate the number of moles as follows: Moles = Mass */ Molar mass Eg 1. How many moles is 27.03 g of H2O? Moles (H2O) = Mass / Molar mass = 27.03 / (2 x 1.01 + 16.00) = 1.50 mol Eg 2. What is the mass of 0.05 mol of H2O. This time the equation must be rearranged to give: Mass (H2O) = Moles x molar mass = 0.05 x (2 x 1.01 + 16.00) = 0.901g *Mass must be given in grams – you may need to convert from kg: x1000 THE MOLES AND GASES One mole of any gas has a volume of 24.0 dm3 (remember dm3 is the symbol for decimetres cubed, aka litres) at room temperature and pressure. So for a gas: CALCULATING REACTING QUANTITIES Using what we know about calculating moles, we can now answer questions like: If Moles = Volume / 24.0 Eg 1. How many moles of CO2 are present in 60 dm3? Moles (CO2) = Volume / 24.0 = 60/24.0 = 2.50 mol Eg 2. What is the volume of 0.20 mol of H2 gas?.This time the equation must be rearranged to give: Volume (H2) = Moles x 24.0 = 0.20 x 24.0 = 4.80 dm3 *The volume must be in dm3 – to convert from cm3 divide by 1000 THE MOLE AND SOLUTIONS The concentration (strength) of a solution is measured in mol dm-3 (moles per decimetre cubed). A 1.0 mol dm-3 solution contains 1 mol of substance dissolved in each litre. Moles = Concentration x Volume* Eg 1. How many moles of NaOH are present in 2.5 dm3 of a 1.5 mol dm-3 solution? Moles (NaOH) = concentration x volume = 1.5 x 2.5 = 3.75 mol Eg 2. 0.15 mol NaCl is dissolved in 250 cm3 water. What concentration is this? This time you must rearrange the equation to: Concentration = moles/volume = 0.15/(250/1000)* = 0.60 mol dm-3 *The volume must be in dm3 – to convert from cm3 divide by 1000 Using what we know about calculating moles, we can now answer questions like: If I have 100g X, how much Y is made? The key is to convert the known to moles 1st. Example: What volume of H2 gas would be produced by reacting 12.15g magnesium with excess hydrochloric acid? First we need a balanced equation: Mg + 2HCl MgCl2 + H2 Then calculate moles of Mg (our known) we start with: Moles Mg = mass/molar mass = 12.15/24.30 = 0.50 mol Next we work out how many moles of H2 ( our unknown) we expect to produce: Moles H2 = moles known/knowns in eqn x unknowns in eqn = 0.50/1 x 1 = 0.50 mol Finally we calculate the volume using our equations for a gas: Volume H2 = moles x 24.0 = 0.50 x 24.0 = 12.0 dm3 LIMITING REACTANTS This is the reactant that will run out first. It is important as this is the one you should then use for your calculations. You calculate it by dividing the number of moles of reactant by the number of times they appear in the equation. For example 2H2 + O2 2H2O. How many moles of H2O could you make from 3 mol of H2 and 3 mol of O2. H2: 3/2 = 1.5, O2: 3/1 = 3. This means there is enough O2 to do the reaction 3 times but only enough H2 for 1.5 times so H2 is the limiting reactant. Thus, moles H2O = 1.5 x (2/2) = 1.5 mol.
- 8. C5: ELECTRICITY AND CHEMISTRY ELECTROLYSIS Electrolysis is a process in which electricity is used to break compounds down into their elements. The mixture being electrolysed is called an electrolyte and must be liquid (either melted or dissolved) to allow the ions to move. Cations (positive ions – remember they are ’puss-itive’) ions move to the cathode (the negative electrode) where they gain electrons, usually forming a metal (or H). Anions (negative ions) move to the anode (the positive electrode) where they lose electrons, usually forming a non-metal (other than H). In the electrolysis of copper chloride (CuCl2) (right) positive copper ions move to the cathode and form copper metal. Negative chloride ions more to the anode and form chlorine gas. Cu2+ Cu2+ Cu2+ Cu Cu CuCl- Cl- Cl- Cl- Cl- Cl- Cl Cl Cl Cl Cathode (negative electrode) Anode (positive electrode) Anions move to anode Cations move to cathode Layer of metal formed Bubbles of gas formed EXTRACTING ALUMINIUM Aluminium can’t be extracted by reduction of aluminium oxide (Al2O3) using carbon as carbon is less reactive than aluminium. Instead Aluminium oxide ( the electrolyte) is dissolved in molten ‘cryolite’ and placed in a large carbon lined vessel which acts as the cathode. A large anode made of carbon is lowered into the ELECTROLYSIS OF COPPER SULPHATE When copper sulphate is electrolysed using carbon electrodes, you produce O2 gas at the anode and a layer of Cu metal at the cathode. This can be used to electroplate items by setting them as the cathode. However, when two copper electrodes are used, what ends up happening is a transfer of copper from the anode to the cathode, this is used to purify copper. Molten Salt Salt Solution Cathode Metal Metal, except with reactive metals (K, Na, Li Ca, Mg) in which case H2 gas is produced plus a solution of metal hydroxide Anode Non-metal Non Metal, except sulphates in which case O2 is less reactive than aluminium. Instead aluminium is produced by electrolysis. anode made of carbon is lowered into the electrolyte. The processes that take place are: At the cathode: Aluminium ions gain electrons to make liquid aluminium Al3+ + 3e- Al(l) At the anode: Oxide ions lose electrons to make oxygen gas O2- ½ O2(g) + 2e- The oxygen reacts with the carbon anode so it has to be replaced regularly ELECTROLYSIS OF BRINE When sodium chloride solution (brine) is electrolysed , chlorine gas is produced at the anode and hydrogen gas at the cathode (because sodium is too reactive). A solution of sodium hydroxide is left behind. the anode to the cathode, this is used to purify copper. When copper is made it contains lots of impurities. The copper is purified by electrolysis. A large lump of impure copper is used as the anode, the electrolyte is copper sulphate solution and the cathode is made of pure copper. At the anode, instead of anions losing electrons, neutral copper atoms lose electrons to become copper ions . Cu(s) Cu2+ (aq) + 2e- These then move through the electrolyte to the cathode where they become copper atoms again. Cu2+ (aq) + 2e- Cu(s) The anode loses mass as copper atoms leave it and the cathode gains mass as copper atoms join it. The impurities sink to the bottom as a pile of sludge.
- 9. C6: ENERGY CHANGES IN CHEMICAL REACTIONS EXOTHERMIC REACTIONS Exothermic reactions get hotter – the temperature increases. The energy given out can be used to keep the reaction going so that once started, they don’t stop until they have run out of reactants. Important examples of exothermic reactions include: •Combustion of fuels •Acid-base neutralisations •Displacement reactions •Respiration in cells ENDOTHERMIC REACTIONS QUANTIFYING ENERGY Using the ideas you learn in physics about specific heat capacity, you may have to calculate the amount of energy released by one mole of a substance. Example: When 0.250 mol of Metal X reacts fully with 500 cm3 of 2.0 mol dm-3 HCl solution, the temperature increases by 15.4OC. How much energy is released when 1.0 mol X reacts with HCl? First calculate the heat evolved: Heat evolved = m.c.ΔT = 500 x 4.2 x 15.4 = 32340 J* Then calculate heat released per mole: Heat per mole = heat evolved / moles = 32340/0.250 = 129360J = 129.4 kJ *ΔT is the temperature rise, m is the mass of the solution in grams which is assumed to equal its volume in cm3, c is the specific heat capacity of water which is 4.2 J K-1 g-1 ENERGY CHANGES In exothermic reactions, chemical energy stored in the reactants gets converted to heat energy. The products have less chemical energy than the reactants and the difference is the amount of heat released. In endothermic reactions, heat energy gets converted to chemical energy. The products have more chemical energy than the reactants and the difference between the two is the energy that had to be supplied to make the reaction go. Endothermic reactions reactions get colder – the temperature decreases. Generally endothermic reactions need a constant energy supply to keep them going Important examples of exothermic reactions include: •Dissolving of many (but not all) salts •Thermal decompositions •Photosynthesis •Cooking!!! Yes this unit really is this small – in fact you don’t even really need the stuff about quantifying energy, I just put it in there as it often proves useful!!
- 10. C7: CHEMICAL REACTIONS MEASURING REACTION RATES On a graph showing the change in concentration of reactants or products, the gradient of the line tells you the reaction rate: steeper = faster, flat = stopped RATES OF REACTION The ‘speed’ of a reaction is called its rate and is simply the amount of new product formed every second. For a chemical reaction to happen, the reacting particles need to collide with enough energy. Anything that increases the number of collisions or their energy will increase the rate. Temperature Increasing temperature increases the rate of a reaction. This is because particles are moving faster which means more collisions and higher energy collisions. Concentration Increasing the concentration of a solution increases the rate of a reaction. MEASURING REACTION RATES If a reaction produces gas, you can easily measure the reaction rate by collecting the gas (either in an upturned measuring cylinder full of water or a gas syringe) and recording how much has been collected each second. INVESTIGATING REACTION RATES To investigate a factor influencing reaction rate, you must change it whilst keeping the others constant. For example, investigating the effect of concentration, you could a reaction. This is because it means there are more particles available to react which leads to more collisions. Surface Area/Particle size Increasing the total surface area of particles (by using finer powder) increases the rate of a reaction because it means more particles at the surface are exposed to collisions. Catalysts Catalysts are substances that speed up a reaction without getting used up. Whenever a catalyst is present, the rate of reaction increases. carry out the reaction at 5 different concentrations whilst making sure the temperature, particle size and presence/absence of a catalyst remains the same. REDOX REACTIONS Reduction means a substances loses oxygen. Oxidation means a substance gains oxygen. For example: 2Fe2O3 + 3C 4Fe + 3CO2 Fe2O3 is reduced because it loses oxygen to become Fe. C is oxidised because it gains oxygen to become CO2. C is called a reducing agent because it causes Fe2O3 to get reduced. Reactions like this are called redox reactions because an oxidation AND a reduction take place together. Another way to look at this is to think of oxidation as the loss of electrons and reduction as the gain of electrons (OILRIG). Eg: in the electrolysis of molten sodium bromide. At the anode: 2Br- Br2 + 2e- This is an oxidation because the bromide ions lose electrons. At the cathode: Na+ + e- Na This is a reduction because the sodium ions gain electrons. DANGEROUS RATES Factories that produce flammable powders (for example bread flour) have to be careful about sparks since the very fine powder particles burn with a VERY high reaction rate causing explosions. Similar is true underground in coal mines where gas can build up. Gas can be thought of as the finest possible powder so they too react explosively fast.
- 11. C8: ACIDS, BASES AND SALTS – Reactions of Acids REACTIONS OF ACIDS You need to memorise these reactions, each one shows the general word equation then a specific example with symbols. Acids and Metals Acid + Metal Salt + Hydrogen •Hydrochloric acid + lithium lithium chloride + hydrogen • 2HCl(aq) + 2Li(s) 2LiCl(aq) + H2(g) Acids and Base (like alkali but not always soluble) Acid + Base Salt + Water •Sulphuric acid + sodium hydroxide sodium sulphate + water • H2SO4(aq) + 2NaOH(aq) Na2SO4(aq) + 2H2O(l) Acids and Carbonates Acid + Carbonate Salt + Water + Carbon Dioxide •Nitric acid + calcium carbonate calcium nitrate + water + THE pH SCALE Neutral substances have a pH=7 Acids have a pH of less than 7 Alkalis have a pH greater than 7 pH can be measured with colour changing indicators or digital pH meters Litmus indicator turns red in acids and blue in alkalis. Universal indicator has many colours (see chart). WHAT IS THE SALT? To work out which salt is formed during neutralisation reactions you need to know the ions formed by the acid or alkali when it dissolves. Working out the name is easy, you just combine the name of the cation from the alkali with the anion from the acid. For example potassium sulphate and sulphuric acid makes potassium sulphate. Magnesium hydroxide and Substance Cation(s) Formed Anion(s) Formed Hydrochloric acid, HCl 1 H+ Cl- , chloride Nitric acid, HNO3 1 H+ NO3 - , nitrate Sulphuric acid, H2SO4 2H+ SO4 2- , sulphate Phosphoric acid, H3PO4 3 H+ PO4 3- , phosphate Sodium hydroxide, NaOH Na+ , sodium 1 OH- Potassium hydroxide, KOH K+ , potassium 1 OH- Magnesium hydroxide, Mg(OH) Mg2+ , magnesium 2 OH- •Nitric acid + calcium carbonate calcium nitrate + water + . carbon dioxide • 2HNO3(aq) + CaCO3(s) Ca(NO3)2(aq) + H2O(l) + CO2(g) Magnesium hydroxide and phosphoric acid makes magnesium phosphate Working out the formula of the salt is a little more complicated, the key is to make sure the positive and negative charges on the cancel each other out to zero. Finally, to write a balanced equation, you need to get the right number of waters, the simplest way is to remember that each ‘H+’ from an acid makes one water. Magnesium hydroxide, Mg(OH)2 Mg2+ , magnesium 2 OH- Ammonium hydroxide, NH4OH NH4 + , ammonium 1 OH- Eg 1. Potassium nitrate K+ has one plus charge SO4 2- has two minus charges You need two K+ to balance out one SO4 2- so the formula is K2SO4 Eg 2. Magnesium phosphate Mg2+ has two plus charges PO4 3- has three minus charges So you need three Mg2+ to balance out two PO4 3- so the formula is Mg3(PO4)2 Eg 1. Potassium hydroxide and sulphuric acid As we have seen it makes K2SO4 which requires one H2SO4 and two KOH. Two H2O are made since the one H2SO4 produces two H+ ions H2SO4 + 2KOH K2SO4 + 2H2O Eg 2. Magnesium phosphate As we have seen it makes Mg3(PO4)2 which requires two H3PO4 and three Mg(OH)2. Six H2O are made since each of the two H3PO4 produces three H+ ions. 2H3PO4 + 3Mg(OH)2 Mg3(PO4)2 + 6H20 PREPARING SALTS To prepare any given salt, you first need to work out which acid and alkali to react together (see right). Then react them in appropriate quantities so they exactly neutralise each other. You can either calculate the right amounts (see Unit C4) or find it experimentally from a titration. Once you have done this you can use the appropriate techniques to separate the salt from the rest of the solution (See Unit C2). ACID ALKALI + INDICATOR
- 12. C8: ACIDS, BASES AND SALTS – Chemical Testing TESTING GASES Hydrogen: •A test tube of hydrogen produces a ‘squeaky pop’ with a lighted splint Oxygen: •A test tube of oxygen can re-light a glowing splint. Chlorine: •Bleaches the colour from damp litmus paper. Ammonia: •Turns damp red litmus paper blue. Carbon dioxide: •Turns limewater cloudy. TESTING FOR IONS: Most of these involve forming insoluble precipitates – they go cloudy. Test for... By.... Positive result The reaction Chloride ions, Cl- Add acidified silver nitrate White precipitate Forms insoluble silver chloride: Cl- (aq) + AgNO3(aq) AgCl(s) + NO3 - (aq) Sulphate ions, SO4 2- Add acidified barium nitrate White precipitate Insoluble barium sulphate formed: SO4 2- (aq) + Ba(NO3)2(aq) BaSO4(s) + 2NO3 - (aq) Carbonate ions, CO3 2- Add acid and bubble the gas formed in limewater Rapid gas formation which turns limewater cloudy The acid reacts with carbonate to make carbon dioxide gas: CO3 2- (s) + 2H+ (aq) CO2(g) + H2O(l) The CO2 reacts with limewater to make insoluble calcium carbonate. Nitrate ions, NO3 - Boil with sodium hydroxide and aluminium foil. Test the gas with Red litmus paper turns blue The nitrate gets reduced by aluminium which is a strong reducing agent and forms ammonia. Yet More Tests You need to remember these chemical tests: •Oxygen (see Unit C7) •Hydrogen – lighting a test-tube of H2 with a splint gives a squeaky pop •Carbon dioxide – when bubbled through limewater it turns it cloudy. OXIDES The oxides of most metals are basic (the opposite of acidic). For example sodium oxide (Na2O) forms the alkali sodium hydroxide when it reacts with water. Most oxides of non-metals are acidic. For example, sulphur trioxide (SO3) forms sulphuric acid when it dissolves in water. Some oxides can behave like acids or bases and are called amphoteric. For example aluminium oxide (Al2O3) can react with the alkali NaOH to from sodium aluminium hydroxide (NaAl(OH)4) or with hydrochloric acid to form aluminium chloride (AlCl3) Test the gas with damp red litmus paper. Ammonia is an alkali so can turn the red litmus paper blue. Copper (II), Cu2+ Add sodium hydroxide followed by ammonia solution Blue precipitate that dissolves when ammonia added Insoluble copper (II) hydroxide formed: Cu2+ (aq) + 2NaOH(aq) Cu(OH)2(s) + 2Na+ (aq) When ammonia is added a soluble complex forms so the precipitate dissolves. Iron (II), Fe2+ Add sodium hydroxide followed by ammonia solution. Green precipitate insoluble in ammonia Insoluble iron (II) hydroxide formed: Fe2+ (aq) + 2NaOH(aq) Fe(OH)2(s) + 2Na+ (aq) Ammonia does not react with the iron (II) hydroxide so it does not dissolve. Iron (III), Fe3+ Add sodium hydroxide followed by ammonia solution. Brown precipitate insoluble in ammonia Insoluble iron (III) hydroxide formed: Fe3+ (aq) + 3NaOH(aq) Fe(OH)3(s) + 3Na+ (aq) Ammonia does not react with the iron (III) hydroxide so it does not dissolve. Zinc, Zn2+ Add sodium hydroxide followed by ammonia solution or more sodium hydroxide. White precipitate soluble in both ammonia or more sodium hydroxide Insoluble zinc hydroxide formed: Zn2+ (aq) + 2NaOH(aq) Zn(OH)2(s) + 2Na+ (aq) Both ammonia and sodium hydroxide react with the zinc hydroxide to form a soluble complex. ACID ENVIRONMENTS Acid soils grow poor crops so the acidity is reduced by neutralising it with lime (CaO, calcium oxide) Acidic gases from factory chimneys (like sulphur dioxide) can dissolve in the water in clouds to form harmful acid rain.
- 13. C9: THE PERIODIC TABLE THE PERIODIC TABLE The periodic table is arranged in order of increasing proton number – starting at Hydrogen with a proton number of one and working along the rows. Periods: The rows in the periodic table are called periods. Going along a period, the elements change from metals to non-metals. Usually, one or two elements in the period are called metalloids – these have some properties of a metal and some properties of a non-metal. Groups: These are the columns in the periodic table. Elements in the same group share similar properties. Groups I and II are always metals. Groups VII and 0/VIII are GROUP I (Li, Na, K....) The metals of Group I (aka the alkali metals) are soft, silvery grey, reactive metals. Down the group they get: •Softer •Lower melting point •More reactive They all react with water as follows: Metal + water metal hydroxide + hydrogen •Lithium + water lithium hydroxide + hydrogen • Li + H2O LiOH + H2 •Lithium reacts the slowest, Na reacts faster, K reacts faster still and so on. GROUP VII (F, Cl, Br, I...) The elements of Group VII are better known as the halogens. As we go down the group they get: •Less reactive •Higher melting point (Cl2 is gas, Br2 is liquid, I2 is solid) •Darker colour (Cl2 is pale green, Br2 is reddy-brown, I2 is dark brown) They will react with ions of other halogens (halide ions) that are below them in the group. For example: Cl2 + 2Br- 2Cl- + Br2 Because Cl is more reactive than Br. However, Br2 + Cl- no reaction As Br is less reactive than Cl. GROUP 0/VIII (He, Ne, Ar, Kr....) The gases of Group 0 are called the Noble Gases because they are very unreactive. This is because they have full outer shells of electrons which is very stable. TRANSITION ELEMENTS These are the metals in the long middle block of the periodic table. Their important properties include: •High melting/boiling pointsGroups I and II are always metals. Groups VII and 0/VIII are always non-metals and elements in groups III, IV, V and VI can be metals, metalloids or non-metals depending on the period. The Periodic Table and Atomic Structure: The periodic table can be used to work out the arrangement of electrons: •Period number = number of shells •Group number = electrons in outer shell For example: Chlorine is in Period 3 and Group VII so it has 3 electron shells and 7 electrons in the outer shell. They exist as single atoms rather than molecules. They are used whenever an inert (unreactive) atmosphere is needed. For example: •Light Bulbs – Argon surrounds the coiled filament as even when white hot, it won’t react. Helium has a very low density (1/7th that of air) so is used to make airships and blimps float. •High melting/boiling points •High densities •Form strongly coloured compounds •(Often) Act as catalysts – both as elements and when combined in compounds METALS (really belongs in C10 but didn’t quite fit) Most of the known elements are metals. All metals: Conduct electricity, conduct heat, are shiny Most metals are also: •Malleable – can be beaten into shape •Strong •High melting/boiling point •Sonorous – ‘ring’ when hit •Ductile – can be pulled into wires Many metals react with: •Acids – to form salt and hydrogen •Oxygen – to form (basic) oxides •Sulphur – to form sulphides When metals bond to non metals they form ionic bonds. Cl
- 14. C10: METALS REACTIVITY SERIES MOST REACTIVE Potassium, K Sodium, Na Calcium, Ca Magnesium, Mg Aluminium, Al (Carbon, C) Zinc, Zn Iron, Fe Tin, Sn Lead, Pb (Hydrogen, H) Copper, Cu Silver, Ag Gold, Au Platinum, Pt LEAST REACTIVE REACTIVITY REACTIVITY OF METALS The reactivity of metals can be seen by the way they react with steam or with acid (see Unit C6 for the reactivity series). Reaction with water (see Unit C2 for details of this reaction): The most reactive metals (K-Ca) react with cold water, fairly reactive metals (Mg-Fe) will only react with steam whereas the least reactive metals (Sn- Pt) don’t react at all. Reaction with dilute acids (see Unit C9 for details) The reaction of metals with acids shows a similar patter with the most reactive metals (K-Ca) reacting violently, the fairly reactive metals (Mg-Pb) reacting gradually more slowly and the least reactive metals (Cu-Pt) not reacting at all. Displacement Reactions The reactivity of metals relates to how easily they form ions, more reactive metals like K form K+ ions much more easily than less reactive metals like Cu can form Cu+ ions. A more reactive metal will reduce a less reactive metal: Eg 1. Reaction with aqueous ions Zinc + Copper sulphate Zinc sulphate + copper Zn + Cu2+ + SO 2- Zn2+ + SO 2- + Cu2+ EXTRACTING METALS FROM THEIR ORES Rocks that contain a significant amount of a metal are called ores. The metals are present as compounds – often oxides or sulphides of the metal. For example lead can be extracted from an ore called galena (PbS, lead sulphide). Metals that are less reactive than carbon can be extracted by using carbon as a reducing agent (to steal the oxygen/ sulphur). More reactive metals are extracted by electrolysis. Iron is less reactive than carbon so can be reduced by it. This is done in a blast furnace. Study the diagram then read the following: •Step 1: Carbon (coke) reacts with oxygen (from the hot air blast) C (s)+ O2(g) CO2(g) •Step 2: Carbon dioxide reacts with more carbon to make USES OF METALSZn(s) + Cu2+ (aq) + SO4 2- (aq) Zn2+ (aq) + SO4 2- (aq) + Cu2+ (aq) This happens because Zn is more reactive than Cu so is able to reduce it. Eg 2. Reaction with metal oxides Iron oxide + aluminium aluminium oxide + iron This happens since Al is more reactive than Fe so is able to reduce it. These are called displacement reactions because the more reactive metal takes the place of the less reactive metal. •Step 2: Carbon dioxide reacts with more carbon to make carbon monoxide CO2(g) + C(s) 2CO(g) •Step 3: Carbon monoxide reduces the iron oxide (iron ore) to make molten liquid iron. Fe2O3(s) + CO(g) Fe(l) + CO2(g) The limestone (CaCO3) reacts with impurities such as silicon to form an easy-to-collect waste called slag (calcium silicate, CaSiO3): CaCO3 +SiO2 CaSiO 3+ CO2 Step 1 happens here Step 2 happens here Step 3 happens here ALLOYS Alloys are ‘mixtures of metals’ (although sometimes they can contain a non-metal) that are made by mixing molten metals. Alloys often have very different properties to the metals they are made from and by varying their metals can be tailored to have specific desirable properties – this is called metallurgy. Alloys are often harder than the metals they are made from. In pure metals atoms are neatly lined up meaning they can slip past each easily when hit. In alloys there are atoms of different sizes which don’t line up neatly so can’t slip past each other so easily making them harder. USES OF METALS Metals have many uses including: •Aluminium – and its alloys used for aircraft as they have low density and great strength •Aluminium – used for food containers as the waterproof oxide layer on its surface prevents corrosion which could taint the food. •Zinc - used to protect steel either by coating it (galvanising) or as sacrificial protection – i.e. on a ship’s hull – a lump of zinc prevents rust as it is more reactive so corrodes instead of the steel hull. AlloyElement
- 15. C11: AIR AND WATER WATER, H2O Water is the most useful compound known to man. In the home it is used for cooking, cleaning and transporting waste. In industry it is used for cooling hot machinery, cleaning and as a solvent. Water is useful for cleaning as it is able to dissolve many types of ‘dirt’. A simple test for water is that it is able to turn cobalt chloride paper from blue to pink. Drinking Water Water drawn from rivers can contain pollutants such as fertilizers, dissolved organic matter, harmful bacteria and industrial waste that make it unfit to drink. At treatment plants, two main processes are used to make water safe: Filtration – the water is passed through a series of increasingly fine filters that trap suspended particles. Activated carbon is used to filter out dissolved pollutants. CARBON DIOXIDE, CO2 There are many ways to produce CO2 including: Burning carbon-containing fuels: CH4 + 2O2 CO2 + 2H2O Thermal decomposition of carbonates e.g.: CaCO3 CaO + CO2 As a by product of respiration in living cells: C6H12O6 + O2 CO2 + H2O AIR POLLUTION Many of man’s activities pollute the air. Pollutants include: Carbon monoxide, CO •Formed when fuels burn without enough O2. •CO prevents the blood from carrying oxygen leading to death by suffocation Sulphur dioxide, SO2 •Formed by burning fossil fuels containing sulphur impurities. •Dissolves in water in clouds to form sulphurous acid which falls as acid rain •Acid rain corrodes buildings and damages ecosystems •Irritates the respiratory system when inhaled. Nitrogen Oxides, NOx •Formed by burning fuels in engines and power stations. •Dissolves in cloud water to form nitric acid thus acid rain. RUSTING Rust (hydrated iron (III) oxide) affects most structures made of iron (or steel) and causes huge damage: Iron + oxygen + water hydrated iron (III) oxide Rust can be prevented by taking steps making sure either oxygen or water can’t reach the iron. The main ways to do this involve covering the metal with: paint (bridges and other structures); oil/grease (moving machine parts) or another metal such as zinc (galvanising). FERTILISERSpollutants. Chlorination – chlorine is added to the water which destroys bacteria. AIR Air is a mixture of gases comprising: The ‘other’ is mostly argon with CO2, water vapour and many trace gases. Although the proportion of carbon dioxide is very small (~0.04%) it is increasing due to man’s activities such as burning fossil fuels and deforestation. This is a concern as CO2 is able to absorb the infrared radiation (heat) radiated by the ground when the sun heats it up (the greenhouse effect). More CO2 means more trapped heat leading to global warming. Global warming is a major problem because temperatures are rising faster than nature’s ability to adapt – this makes the future of both farming and of our ecosystems very uncertain. •Dissolves in cloud water to form nitric acid thus acid rain. •Irritates the respiratory system when inhaled. FERTILISERS Fertilisers are chemicals applied to plants to improve their growth and increase the amounts of products such as fruits, nuts, leaves, roots and flowers that they produce for us. They work by supplying plants with the vital elements they need including Nitrogen - in the form or nitrate (NO3 - containing) salts; phosphorous – in the form of phosphate (PO4 3- containing) salts and potassium (K+ containing) salts. CATALYTIC CONVERTERS Fit to a car’s exhaust and use a platinum or palladium catalyst to convert harmful gases to safer gases: for example nitrogen oxides are reduced back to nitrogen gas and oxygen gas. NITROGEN AND AMMONIA Ammonia (NH3) is a smelly gas. One way to produce it is to react ammonium (NH4 +) salts with an alkali (OH-) eg: NH4Cl + NaOH NH3 + H2O . + NaCl Ammonia is vital to produce the nitrates used in fertilisers and explosives. It is produced by the Haber process: N2(g) + 3H2(g) 2NH3 The reaction is reversible which means much of the product turns back to reactants as soon as it is made, this means it takes a long time to make an economical amount of ammonia. To speed it up, the reaction is done at high temperature (~450OC) with an iron oxide catalyst. High pressure (~200 times atmospheric pressure) is used to increase the proportion of NH3 formed. The nitrogen comes from the air and hydrogen comes from reacting methane (CH4) gas with steam. Nitrogen and oxygen can be separated from air by cooling it to a liquid and using fractional distillation.
- 16. C12: SULPHUR THE CONTACT PROCESS Sulphuric acid is produced by the Contact Process. This involves are three chemical reactions. First sulphur is burnt in air to produce sulphur dioxide (SO2): S + O2 SO2 Secondly SO2 is reacted with further oxygen to make sulphur trioxide (SO3): 2SO2 + O2 SO3 This reaction is reversible, so to maximise the amount of SO3 made, they use a high temperature (425OC), medium-high pressure (1-2 times atmospheric pressure) and a catalyst (vanadium (V) oxide, V2O5). Finally, the sulphur trioxide is produced by first dissolving it in sulphuric acid to make oleum (H2S2O7) which then makes more sulphuric acid on the addition of water: SO3 + H2SO4 H2S2O7 H2S2O7 + H2O 2H2SO4 SULPHURIC ACID, H2SO4 Sulphuric acid is a very important compound used in many industrial processes including: •Fertiliser production •Oil refining •Paper making •Steel making It is also the acid found in car batteries. Sulphuric acid is a strong acid which when diluted in water produces two protons and a sulphate ion: H2SO4(l) 2H+ (aq) + SO4 2- (aq) It exhibits all the reactions typical of an acid as seen by its reactions with metals, alkalis, metal oxides and carbonates. Note: trying to dissolve SO3 directly in water produces a very fine mist of sulphuric with limited uses. reactions with metals, alkalis, metal oxides and carbonates. (see Unit C8 for details). This is another tiny unit with very little to learn.
- 17. C13: CARBONATES CALCIUM CARBONATE, CaCO3 Calcium carbonate is a very common mineral and makes up the bulk of many common rocks including: •Chalk •Limestone •Marble Whilst solid limestone is often used in construction, powdered limestone has many industrial uses. USES OF CALCIUM CARBONATE Powdered calcium carbonate can be added directly to soils to raise their pH (reduce their acidity). We can also make calcium oxide (CaO, aka ‘quicklime’) by heating powdered calcium carbonate to about 1000OC, producing carbon dioxide as a by-product: CaCO3(s) CaO(s) + CO2(g) This is called a thermal decomposition as heat is used to break down or decompose the calcium carbonate. Calcium oxide is one of the key ingredients in cement. Another useful product, calcium hydroxide (Ca(OH)2, ‘slaked lime’) is made by adding water to calcium oxide: CaO + H2O Ca(OH)2 Slaked lime has many uses including: •Raising soil pH quickly (when powdered calcium carbonate might take too long) •Neutralising acidic industrial waste •Sewage treatment – it helps small particles of waste to clump together into easily removed lumps. Another mini-unit with very little in it!
- 18. C14: ORGANIC CHEMISTRY - Oil CHEMICAL FAMILIES Organic chemistry is the chemistry of compounds containing carbon. You need to know the structure of four organic compounds: methane, ethane, ethene and ethanol (check the diagram below). Methane and ethane are both members of the ‘alkane’ family – you can tell this because their names end ‘–ane’. Ethene is an alkene, as shown by the ‘–ene’ ending and ethanol is an alcohol which has the ending ‘–ol’. OIL Oil is a mixture of hundreds of hydrocarbons (compounds containing only H and C). This mixture must be separated into its useful components by fractional distillation. Very hot crude oil is pumped into the fractionating column where the hydrocarbons separate out by their boiling points, rising through the column until they get cold enough to condense. The compounds that condense at a particular temperature are called a FRACTION. refinery gas, 1-4 carbons gasoline, 5-9 carbons naptha, 6-11 carbons kerosene, 11-18 carbons diesel, 15-21 carbons COOLER Bubble Caps: the gaseous fractions bubble up through these until they get cool enough when they then condense. Homologous Series: These are families of compounds that differ only in the length of their carbon chain. For example, looking at the diagram above you can see all alcohols contain an ‘-OH’ group bonded to a carbon, all alkenes contain a ‘C=C’ double bond and all alkanes contain only single C-C and C-H bonds. The beginning of a name tells you the number of carbons in the chain: ‘meth’ means 1 C, ‘eth’ means 2, ‘prop’ is 3 and ‘but’ is 4 carbons. fuel oil, 20-27 carbons Greases and wax, 25-30 carbons bitumen, 35+ carbonsHOTTER How does it work? Larger molecules with longer carbon chains have higher boiling points because the intermolecular forces holding each molecule near its neighbour are stronger so take more energy to break. Three important fractions: Refinery gas: this is bottled and used for cooking and heating Gasoline: the petrol used to fuel our cars Diesel oil: used in diesel engines – particularly for large vehicles Fossil Fuels: Coal, oil and natural gas are all fossil fuels formed by the action of heat and pressure over millions of years on the remains of living organisms. All of them release carbon dioxide when burnt which contributes to global warming. Because coal is contains the most carbon, it also produces the most carbon dioxide so is not an environmentally sustainable fuels. Natural gas (made mostly of methane, CH4) contains much less carbon and so is an environmentally better fuel.
- 19. C14: ORGANIC CHEMISTRY – Classes of Compounds HYDROCARBONS Hydrocarbons are compounds made of only hydrogen and carbon atoms. Hydrocarbons – for example methane (CH4) – burn very well producing only carbon dioxide (CO2) and water (H2O): CH4 + 2O2 CO2 +2H2O Alkanes (see structure on previous page) These are the simplest hydrocarbons. They are ‘saturated’ which means they only contain single bonds. They are pretty unreactive but burn well making them good fuels. Alkenes (see structure on previous page) These are hydrocarbons containing a C=C double bond. The double bond makes them quite reactive and they are used as a starting material to make many other organic compounds. ALCOHOLS Alcohols such as ethanol are very important compound with many uses including as solvents and fuels. They can be made from alkenes (see left) by reacting them with steam. Alcohols burn very cleanly producing very little soot and smoke: C2H5OH + 3O2 2CO2 + 3H2O CRACKING Because there is a greater need for hydrocarbons with shorter carbon chains we sometimes need to cut longer chains into shorter ones using the process of cracking. A long alkane is heated, vaporised and passed over a ceramic catalyst produce a shorter alkane and an alkene. Eg. 1: C8H18 C4H10 + C4H8 Eg. 2: C10H22 C7H16 + C3H6 Note: •The with the alkenes for each carbon there are 2 H (CnH2n); with the alkanes, for each C there are 2 H plus 2 extra (CnH2n+2). •Any combination of alkene and alkane can be made, including straight and branched chains, so long as the numbers of atoms balance. MACROMOLECULES These are large molecules made from lots of smaller molecules – called monomers - joined together. Different monomers lead to different macromolecules. Natural Macromolecules Proteins and starch are both examples of natural macromolecules. In a protein the monomer is various different amino acids: They are condensation polymers where the ‘acid’ end of one amino acid joins to the ‘amino’ end of Addition reaction of alkenes with bromine: When an orange solution of bromine is added to alkenes in the presence of UV light, the bromine reacts with the double bond on the alkene to make a bromoalkane. The bromine water loses its colour so this makes it a good test for alkenes: C2H4 + Br2 C2H4Br2 Addition reaction of alkenes with steam: Ethene reacts with steam in the presence of a phosphoric acid catalyst to make ethanol which can be used as a solvent or to make other useful compounds. C2H4(g) + H2O(g) C2H5OH(g) Addition reaction of alkenes with hydrogen: Alkenes reacts with hydrogen in the presence of a nickel catalyst to make alkanes. C2H4(g) + H2O(g) C2H6 (g) Whilst not very useful in itself, this reaction applies to C=C double bonds in much more complex molecules to, and for example is one of the key steps in producing margarine. Polythene Poly(ethene) is a synthetic polymer (plastic) made from many ethene molecules joined together. It is formed by addition polymerisation whereby many individual monomers (in this case ethene) join together in one long chain. The diagram shows a section of poly(ethene) made from three ethene monomers joined. end of one amino acid joins to the ‘amino’ end of the next, forming an amide linkage and one molecule of water each time. Proteins can be broken back down to amino acids by strong acids or strong alkalis. This process is called hydrolysis. Nb: The ‘R’ on an amino acid means any small group of atoms as is different in each amino acid. Condensation Polymers In condensation polymerisation, each time two monomers join, one molecule of water is produced. In the case of nylon (pictured) there are two monomers – one with two acid ends (-COOH, black) and one with two amine ends (-NH2, white). They join with an ‘amide’ linkage, producing water.