Additional Science-Chemistry.
- 1. Ionic Bonding: •Ionic bonding is when you transfer electrons. •In ionic bonding the atoms involve in losing or gaining electrons to form charged ions. •When non metals react with metals, the non metals gain electrons to have a stable Nobel gas structure. •When metal atoms need to lose electrons it reacts with a non metal which needs to gain electrons. •To represent the atoms and ions involved in forming ionic bonds you use the dot and cross diagram. •Overall charge of a ionic compound is zero. Compound: Two or more elements which are chemically combined. Covalent bond: •Covalent bonding is when non metals react together their atoms share pairs of electrons to form molecules. •The atoms of a non metal need to gain electrons to get a stable outer energy level/shell. This is why the two non metals do not give away electrons when reacting instead they share so they both can have a stable outer energy level/ shell. •They have giant structures where huge numbers of atoms are held together by a network of covalent bonds. These are sometimes referred to as macromolecules.. Metallic Bonding: •Metals are an example of giant structures. •The outer electrons from each metal atom can easily move throughout the giant structure. The outer electrons form a sea of free electrons (delocalized electrons) surrounding positively charged metal ions. Strong electrostatic attraction between the negatively charged electrons and positively charged ions bond the metal ions to each other. Structure and bonding.
- 2. Structure and properties. Giant ionic structures: •The attractive electrostatic forces between the oppositely charged ions act in all directions and are very strong. This force holds the ions in the lattice together. •To separate the ions it has to overcome the electrostatic forces of attraction. This means that the ionic compounds have high melting and boiling points. •The ionic solid melts and then becomes a liquid. •The ions are then free to move anywhere in this liquid which means that they can carry electrical charge through the molten liquid. •A solid ionic compound CANNOT conduct electricity as its ions are in a fixed position in the lattice. •Ionic compounds will dissolve in water as the lattice is split up by the water molecules. The ions are then free to move around in the solution formed. Simple Molecules: •When atoms of non metals react to form compounds they share electrons in their outer shells. In this way each atom gains the electron. •Covalently bonded molecules have a low melting and boiling point. This means that the substances made up of simple molecules are liquids or gases at room temperature. •The attraction between the individual molecules in a covalent substance is small so they are called weak intermolecular forces between the molecules. •Ionic compounds will conduct electricity when it is a liquid. •There is no overall charge ion the simple molecule in a compound like ethanol so their molecules doesn’t carry an electrical charge. This makes it impossible for the substances made up of simple molecules to conduct electricity. Giant Covalent structures: •Covalently bonded substances are made up of individual molecules. •They do not have a relatively small number of atoms in simple molecules. •They form huge networks of atoms held together by covalent bonds. (Giant Covalent Structures). Graphite: •Carbon is not always found as diamond it is also found as graphite. In Graphite the carbon atoms are bonded to three other carbon atoms only. They form hexagons which are arranged in giant layers but there are no covalent bonds between the layers so this means that they can slide over each other easily. •Carbon atoms have 4 electrons in their outer shell available for bonding. This means that the electrons are free to move along the layers of the carbon atoms. These are called delocalized electrons. •The free electrons allow graphite to conduct electricity. Fullerenes: •This is a large molecule formed from the hexagonal ring of carbon atoms. •The rings are joined together to form a cage like shape which has different numbers of carbon atoms.
- 3. Giant metallic structures: •Metals can be hammered and bent into different shapes because the layers of atoms in a pure metal are able to slide easily over each other. •Alloys are a mixture of metals which have different sizes of atoms. This makes it difficult for the layers in the metal’s giant structure to slide over each other. Properties of a metal: •The positive ions in a metal’s giant structure are held together by a sea of delocalized electrons. Their negative charge between the positively charged ions hold the ions in position. •The electrons are able to move throughout the whole giant lattice because they can move around and hold the metal ions together. •The delocalized electrons enable the lattice to distort. •Metals are a good conductor of heat and electricity because the delocolised electrons can flow through the metallic lattice. Shape memory alloys: Mixture of metals which respond to changes in temperature. Properties of polymers: •Polymers can be made from chemicals in crude oil. •Small molecules called monomers join together to make bigger molecules called polymers. •Properties of a polymer depends on the monomer used to make it and the conditions chosen to carry out the reaction. Reaction conditions: •Two types of poly(ethene) one is called high density and the other is low density. They are both made from ethene monomers but they are formed under different conditions. •Using a catalyst at 50 degrees and a bit of pressure and ethene makes high density poly(ethene). This is made up of a straighter poly(ethene) molecules. •Using very high pressures and a trace of oxygen and ethene forms low density of poly(ethene). Thermosoftening polymers: Polymers that forms plastics which can be softened by heat, then remolded into different shapes as they cool down and set. Thermosetting polymers: Polymers that can form extensive cross-linking between chains, resulting in rigid materials which are heat resistant. Bonding Polymers: •In Thermosoftening the forces between the polymer chains are weak. When polymer is heated the weak intermolecular forces are broken which causes the polymer to become soft. But when the polymer cools down the intermolecular forces brings the molecules back together and the polymer becomes hard. •The monomers makes the covalent bond between the polymer chains when they are first heated to make the shape. The covalent bond is strong and cans top the polymer from softening. The covalent cross links between the chains and does not allow them to separate.
- 4. How Much?. Subatomic particle: Relative Mass: Protons. 1 Neutrons. 1 Electrons. Very small. Mass of proton and neutron is the same this means that the relative mass of a neuron compared with a proton is 1. •Mass number and proton is the top number. •Neuron= Top number-bottom number. •Atomic number is the bottom number. Isotopes: •Isotopes is the name for the atoms of the same element with different number of neutrons. •Isotopes always have the same atomic number but different mass numbers. •The extra neutrons makes the nucleus unstable so it is radio active. •Isotopes have different density and they may or may not be radioactive. •They always have the same chemical properties. •Their atoms will have the same number of electrons. Relative atomic masses: Working with the real masses of atoms and just focusing on the relative masses of different elements. Relative atomic mass: •We use an atom of carbon as a standard atom and compare the masses of all the other atoms with this. •The relative atomic mass of an element is (Ar) is an average value that depends on the isotopes the element contains. Mole: Relative atomic mass in grams. Percentage and formulae: •To work out the percentage of an element in a compound you write down the formula of the compound and then use the Ar value to work out the Mr of the compound. Then write the mass of the element you are investigating as a fraction of Mr. Then multiple the fraction by a hundred to get the percentage. •Example: Mass of magnesium 24 ----------------------- = -------- times 100% = 60% Total mass of a 40 Compound. •To work out the percentage of the masses you have to change the percentages given to the masses of each element in 100g of compound. Then change the masses to moles of atoms by dividing the masses by the Ar values. This will tell you how many moles each different element are present and this will tell you the ratio of the atoms of the different elements in a compound. The simplest ratio will give you the empirical formula for the compound.
- 5. Percentage Yield= Amount of product produced ---------------------------------------- times 100% Maximum amount of products possible Few chemical reactions have a yield of 100% because the reaction can be reversible, unexpected products given, products may be left behind in the apparatus, not completely pure, chemical reactions produce more than one product and when it produces more than one it can be hard to separate the product from the mixture. •A reversible reaction can go in both directions so we use two arrows in the equations. •A+B C+D Reactants Products. •Example: •Ammonium+hydrogen chloride ammonium chloride •Ammonium ammonia + Hydrogen Chloride Chloride. Analysing substances: •A substance that is added to food to extend its shelf life or to improve its taste or appearance a food addictive. •One way is by using chromatography. This works because some compounds in a mixture dissolve better than others in particular solvents. Their solubility determines how far they travel across the paper. •Once the compound in a food has been separated using chromatography it can be identified.Advantages: Disadvantages: Highly accurate and sensitive. Expensive. Quick. Special training for it to be used. Enable very small samples to be analysed. Gives results that can often be interpreted only by comparison with data from known substances. Analysing Mixtures: •Samples to be analysed are often mixtures of different compounds so the first thing you do is to separate the compounds. •First use the gas chromatography to separate the compounds that are easily vaporized. Then the separated compounds pass into the mass spectrometer which can identify them. Gas chromatography: •Sample mixture is vaporised. •Carrier gas moves the vapour through the coiled column. •Compounds in the sample have different attractions to the material. • Compounds with weak attractions to the material in the column will leave first. Mass Spectrometry: •To ensure that we can identify the unknown substances in the gas chromatography apparatus can be attached directly to a mass spectrometer which identifies substances quickly.
- 6. Rates and energy. Rate of reactions: •The rate of a chemical reaction tells us how fast reactants turn into products. •If it takes too long to produce it will be hard to make profit when it is sold but if the rate of the reaction is fast and can be made quickly and safely it can make more money/profit. •To find out the rate of a chemical reaction you can work out how quickly the reactants are used up as the product is being made or you can find out how quickly the products of the reactions are made. •Rate of reactions= Amount of react used -------------------------- Time. 4 main factors that affect the rate of chemical reactions are: Surface area. Temperature. Concentration of solution/pressure of gases. Presence of a catalyst. Collision Theory: An explanation of chemical reactions in terms of reacting particles colliding with sufficient energy for a reaction to take place. Activation Energy: The minimum energy needed to start off a reaction. Reactions are most likely to happen between reactant particles if we increase the chance of reacting particles colliding with each other or increasing the energy that they have when they collide. Surface are and reaction rate: When a solid reacts in a solution the size of the pieces of a solid affects the rate of the reaction because the particles inside the solid are not in contact with the solution so they cannot react. The particles inside the solid have to wait for the particles on the surface to react first. Collision theory tells us why raising temperature increases the rate of a reaction. One of the reason is that particles collide more often and the second one is that particles collide with more energy. Particles colliding: •When substances are heated up energy is transferred to its particles. In solution and gases the particles move around faster and when particles move around faster they collide more often. •When particles react more there are more chances for it react. This causes to increase the rate of reaction. •Particles that move around quickly have more energy. This means that their collision is energetic. •When we increase the temperature of reacting mixture there is a higher proportion that the collision will result in making a reaction take place this is because a higher proportion of particles have more energy than the activation energy.
- 7. Effects of concentration of pressure: •Limestone and marble are mainly calcium carbonate. •Calcium carbonate reacts with acids leaving the stone soft and crumbly. •The reaction rate increases because the concentration of acids in the rain water has been increasing steadily. •When increasing the concentration of reactants in a solution increases the rate of reaction this is because there are more particles of the reactants moving around in the same volume of solution. •When you increase the pressure of reacting gases it has the same effect. •It squashes the particles together and then the particles of the gas are given space. So increasing the pressure speeds up the rate of reaction. Catalysts: •A catalyst is a substance which increases the rate of a reaction. •Catalysts DO NOT change the products it only changes the rate of reaction! •Catalysts save money and help the environment as it uses a low temperature and pressure conserves the non renewable resources. It also stops more C02 entering the atmosphere. Exothermic reactions: •Exothermic reaction is a reaction that transfers energy from the reacting chemicals to their surroundings. • Example: Respiration involves in sugar reacting with oxygen's inside the cells of living things. This reaction produces water and C02 as waste. Neutralisation reactions between acids and alkalis. Endothermic reaction: •Endothermic reaction is a reaction that transfer energy from the surrounding to the reacting chemicals. •Example: Thermal decomposition. Energy/reversible reactions: •Energy cannot be created or destroyed in a chemical reaction. •When energy is released it goes in one direction in a reversible reaction it must be exactly the same energy absorbed when it goes in the opposite direction. •Crystals contain water as a part of the lattice formed when the copper sulfate is crystallised. •The copper sulfate is hydrated. •When copper sulfate drives off the water from the crystal it produces white anhydrous.(Without water) (Endothermic reaction). •When water is added to anhydrous copper sulfate hydrated copper sulfate is produced. Colour changed in the reaction test for water. (Exothermic reaction.) Energy transferring from reactions: •Iron turns into hydrated iron(iii) oxide in an exothermic reaction. •Sodium chloride is used as a catalyst. •These are based on the formation of crystals from solution of a salt. The salt is often sodium ethanoate. •Supersaturated solution is prepared by dissolving as much of salt in water. The solution is then cooled.