Chapter 8 Notes

Getting Started The photograph in the textbook provides evidence that an exothermic chemical reaction is occurring. How would you convey to other scientists what is occurring in the photograph? A chemical equation is a shorthand way of communicating the reaction that is occurring. A chemical equation packs a great deal of information into relatively few symbols.

A chemical reaction is the process by which one or more substances are changed into one or more different substances. In any chemical reaction, the original substances are known as the reactants and the resulting substances are known as the products . According to the law of conservation of mass, the total mass of reactants must equal the total mass of products for any given chemical reaction.

A chemical equation represents, with symbols and formulas, the identities and relative molecular or molar amounts of the reactants and products in a chemical reaction. example: The following chemical equation shows that the reactant ammonium dichromate yields the products nitrogen, chromium(III) oxide, and water. (NH 4 ) 2 Cr 2 O 7 ( s ) N 2 ( g ) + Cr 2 O 3 ( s ) + 4H 2 O( g )

Indications of a Chemical Reaction Certain easily observed changes usually indicate that a chemical reaction has occurred. 1. Evolution of energy as heat and light 2. Production of a gas 3. Formation of a precipitate . A solid that is produced as a result of a chemical reaction in solution and that separates from the solution is known as a precipitate. 4. Color change

Characteristics of Chemical Equations The following requirements will aid you in writing and reading chemical equations correctly. 1. The equation must represent known facts. 2. The equation must contain the correct formulas for the reactants and products. 3. The law of conservation of mass must be satisfied. A coefficient is a small whole number that appears in front of a formula in a chemical equation.

Elements That Normally Exist as Diatomic Molecules

Characteristics of Chemical Word and Formula Equations The first step in writing a chemical equation is to identify the facts to be represented. A word equation is an equation in which the reactants and products in a chemical reaction are represented by words. A word equation is qualitative example: methane + oxygen carbon dioxide + water

Characteristics of Chemical Equations The next step in writing a correct chemical equation is to replace the names of the reactants and products with appropriate symbols and formulas. A formula equation represents the reactants and products of a chemical reaction by their symbols or formulas. example: The formula equation for the reaction of methane and oxygen is CH 4 ( g ) + O 2 ( g ) CO 2 ( g ) + H 2 O( g ) (not balanced)

To complete the process of writing a correct equation, the law of conservation of mass must be taken into account. The relative amounts of reactants and products represented in the equation must be adjusted so that the numbers and types of atoms are the same on both sides of the equation. This process is called balancing an equation and is carried out by inserting coefficients.

Begin by counting carbon atoms. To balance the equation, begin by counting atoms of elements that are combined with atoms of other elements and that appear only once on each side of the equation. CH 4 ( g ) + O 2 ( g ) CO 2 ( g ) + 2 H 2 O( g ) (not balanced) Carbon is already balanced in the equation. Two additional hydrogen atoms are needed on the right side of the equation.

CH 4 ( g ) + O 2 ( g ) CO 2 ( g ) + 2H 2 O( g ) (partially balanced) Now consider the number of oxygen atoms. Increase the number of oxygen atoms on the left side to four by placing the coefficient 2 in front of the molecular formula for oxygen. The correct chemical equation, or balanced formula equation, for the burning of methane in oxygen is CH 4 ( g ) + 2O 2 ( g ) CO 2 ( g ) + 2H 2 O( g )

Additional Symbols Used in Chemical Equations

Symbols Used in Chemical Equations

Sample Problem A Write word and formula equations for the chemical reaction that occurs when solid sodium oxide is added to water at room temperature and forms sodium hydroxide (dissolved in the water). Include symbols for physical states in the formula equation. Then balance the formula equation to give a balanced chemical equation.

Sample Problem A Solution The word equation must show the reactants, sodium oxide and water, to the left of the arrow. The product, sodium hydroxide, must appear to the right of the arrow. sodium oxide + water sodium hydroxide Sodium has an oxidation state of +1, that oxygen usually has an oxidation state of  2, and that a hydroxide ion has a charge of 1  . The unbalanced formula equation is Na 2 O + H 2 O NaOH (not balanced)

Adding symbols for the physical states of the reactants and products and the coefficient 2 in front of NaOH produces a balanced chemical equation . Na 2 O( s ) + H 2 O( l ) 2NaOH( aq )

Sample Problem B Translate the following chemical equation into a sentence: BaCl 2 ( aq ) + Na 2 CrO 4 ( aq ) BaCrO 4 ( s ) + 2NaCl( aq )

Sample Problem B Solution Aqueous solutions of barium chloride and sodium chromate react to produce a precipitate of barium chromate plus sodium chloride in aqueous solution.

Significance of a Chemical Equation Some of the quantitative information revealed by a chemical equation includes 1 molecule H 2 : 1 molecule Cl 2 : 2 molecules HCl 1. The coefficients of a chemical reaction indicate relative, not absolute, amounts of reactants and products. H 2 ( g ) + Cl 2 ( g ) 2HCl( g ) This ratio shows the smallest possible relative amounts of the reaction’s reactants and products.

The relative masses of the reactants and products of a chemical reaction can be determined from the reaction’s coefficients. An amount of an element or compound in moles can be converted to a mass in grams by multiplying by the appropriate molar mass. example:

The reverse reaction for a chemical equation has the same relative amounts of substances as the forward reaction. An equation gives no indication of whether a reaction will actually occur. Chemical equations give no information about the speed at which reactions occur. Equations do not give any information about how the bonding between atoms or ions changes during the reaction.

Balancing Chemical Equations The following procedure demonstrates how to master balancing equations by inspection using a step-by-step approach. Identify the names of the reactants and the products, and write a word equation. water hydrogen + oxygen

Write a formula equation by substituting correct formulas for the names of the reactants and the products. H 2 O( l ) H 2 ( g ) + O 2 ( g ) (not balanced)

Balance the formula equation according to the law of conservation of mass. Balance the different types of atoms one at a time. First balance the atoms of elements that are combined and that appear only once on each side of the equation. Balance polyatomic ions that appear on both sides of the equation as single units. Balance H atoms and O atoms after atoms of all other elements have been balanced.

Balance the formula equation according to the law of conservation of mass. Balance oxygen atoms by increasing the number of H 2 O molecules. 2H 2 O( l ) H 2 ( g ) + O 2 ( g ) (partially balanced) Balance the hydrogen atoms by placing the coefficient 2 in front of hydrogen, H 2 . 2H 2 O( l ) 2H 2 ( g ) + O 2 ( g ) (balanced)

Count atoms to be sure that the equation is balanced. 2H 2 O( l ) 2H 2 ( g ) + O 2 ( g ) (4H + 2O) = (4H) + (2O) If the coefficients do not represent the smallest possible whole-number ratio of reactants and products, divide the coefficients by their greatest common factor in order to obtain the smallest possible whole-number coefficients.

Sample Problem C The reaction of zinc with aqueous hydrochloric acid produces a solution of zinc chloride and hydrogen gas. Write a balanced chemical equation for the reaction.

Sample Problem C Solution Write the word equation. zinc + hydrochloric acid zinc chloride + hydrogen Write the formula equation. Zn( s ) + HCl( aq ) ZnCl 2 ( aq ) + H 2 ( g ) (not balanced)

Sample Problem C Solution Adjust the coefficients. Balance chlorine first because it is combined on both sides of the equation. Zn( s ) + 2HCl( aq ) ZnCl 2 ( aq ) + H 2 ( g ) Count atoms to check balance. (1Zn) + (2H + 2Cl) = (1Zn + 2Cl) + (2H) Zn( s ) + 2HCl( aq ) ZnCl 2 ( aq ) + H 2 ( g )

Sample Problem D Solid aluminum carbide, Al 4 C 3 , reacts with water to produce methane gas and solid aluminum hydroxide. Write a balanced chemical equation for this reaction.

Sample Problem D Solution The reactants are aluminum carbide and water. The products are methane and aluminum hydroxide. The formula equation is Al 4 C 3 ( s ) + H 2 O( l ) CH 4 ( g ) + Al(OH) 3 ( s ) (not balanced) Balance Al atoms Al 4 C 3 ( s ) + H 2 O( l ) CH 4 ( g ) + 4Al(OH) 3 ( s ) (partially balanced)

Balance the carbon atoms. Al 4 C 3 ( s ) + H 2 O( l ) 3CH 4 ( g ) + 4Al(OH) 3 ( s ) (partially balanced) Balance oxygen atoms. Oxygen, unlike hydrogen, appears only once on each side of the equation. Al 4 C 3 ( s ) + 12H 2 O( l ) 3CH 4 ( g ) + 4Al(OH) 3 ( s ) The hydrogen atoms are balanced.

Count atoms to check balance. Al 4 C 3 ( s ) + 12H 2 O( l ) 3CH 4 ( g ) + 4Al(OH) 3 ( s ) (4Al + 3C) + (24H + 12O) = (3C + 12H) + (4Al + 12H + 12O) The equation is balanced.

There are several ways to classify chemical reactions. The classification scheme described in this section provides an introduction to five basic types of reactions: synthesis decomposition single-displacement double-displacement combustion reactions Types of Chemical Reactions

Synthesis Reactions In a synthesis reaction, also known as a composition reaction, two or more substances combine to form a new compound. This type of reaction is represented by the following general equation. A + X AX A and X can be elements or compounds. AX is a compound

Group 2 elements react in a similar manner, forming oxides with the formula MO, where M represents the metal. Reactions of Elements with Oxygen and Sulfur One simple type of synthesis reaction is the combination of an element with oxygen to produce an oxide of the element. Almost all metals react with oxygen to form oxides. example: 2Mg( s ) + O 2 ( g ) 2MgO( s )

The Group 1 metals form oxides with the formula M 2 O. example: Li 2 O The Group 1 and Group 2 elements react similarly with sulfur, forming sulfides with the formulas M 2 S and MS, respectively. 8Ba( s ) + S 8 ( s ) 8BaS( s ) 16Rb( s ) + S 8 ( s ) 8Rb 2 S( s )

Nonmetals also undergo synthesis reactions with oxygen to form oxides. example: Sulfur reacts to form sulfur dioxide. S 8 ( s ) + 8O 2 ( g ) 8SO 2 ( g ) example: Hydrogen reacts with oxygen to form dihydrogen monoxide (water). 2H 2 ( g ) + O 2 ( g ) 2H 2 O( g )

Reactions of Metals with Halogens Most metals react with the Group 17 elements, the halogens, to form either ionic or covalent compounds. Group 1 metals react with halogens to form ionic compounds with the formula MX, where M is the metal and X is the halogen. example: 2Na( s ) + Cl 2 ( g ) 2NaCl( s )

Group 2 metals react with the halogens to form ionic compounds with the formula MX 2 . example: Mg( s ) + F 2 ( g ) MgF 2 ( s ) Fluorine is so reactive that it combines with almost all metals.

Synthesis Reactions with Oxides Active metals are highly reactive metals. Oxides of active metals react with water to produce metal hydroxides. example: Calcium oxide reacts with water to form calcium hydroxide. CaO( s ) + H 2 O( l ) Ca(OH) 2 ( s )

Many oxides of nonmetals in the upper right portion of the periodic table react with water to produce oxyacids. example: SO 2 ( g ) + H 2 O( l ) H 2 SO 3 ( aq ) Certain metal oxides and nonmetal oxides react with each other in synthesis reactions to form salts. example: CaO( s ) + SO 2 ( g ) CaSO 3 ( s )

Decomposition Reactions In a decomposition reaction, a single compound undergoes a reaction that produces two or more simpler substances. Decomposition reactions are the opposite of synthesis reactions. They are represented by the following general equation. AX A + X AX is a compound. A and X can be elements or compounds.

Decomposition of Binary Compounds The decomposition of a substance by an electric current is called electrolysis. example: Oxides of the less-active metals, which are located in the lower center of the periodic table, decompose into their elements when heated. example:

Decomposition Reactions Decomposition of Metal Carbonates Decomposition of Metal Chlorates Decomposition of Metal Hydroxides

Decomposition of Acids Certain acids decompose into nonmetal oxides and water. example: Carbonic acid is unstable and decomposes readily at room temperature to produce carbon dioxide and water.

Single-Displacement Reactions In a single-displacement reaction, also known as a replacement reaction, one element replaces a similar element in a compound. Many single-displacement reactions take place in aqueous solution. Single-displacement reactions can be represented by the following general equations. A + BX AX + B or Y + BX BY + X A, B, X, and Y are elements. AX, BX, and BY are compounds.

Displacement of a Metal in a Compound by Another Metal Aluminum is more active than lead. 2Al( s ) + 3Pb(NO 3 ) 2 ( aq ) 3Pb( s ) + 2Al(NO 3 ) 3 ( aq )

Displacement of Hydrogen in Water by a Metal The most-active metals, such as those in Group 1, react vigorously with water to produce metal hydroxides and hydrogen. 2Na( s ) + 2H 2 O( l ) 2NaOH( aq ) + H 2 ( g ) Less-active metals, such as iron, react with steam to form a metal oxide and hydrogen gas. 3Fe( s ) + 4H 2 O( g ) Fe 3 O 4 ( s ) + 4H 2 ( g )

Displacement of Hydrogen in an Acid by a Metal The more-active metals react with certain acidic solutions, such as hydrochloric acid and dilute sulfuric acid, replacing the hydrogen in the acid. The reaction products are a metal compound (a salt) and hydrogen gas. Mg( s ) + 2HCl( aq ) H 2 ( g ) + MgCl 2 ( aq )

Displacement of Halogens Fluorine is the most-active halogen. It can replace any of the other halogens in their compounds. In Group 17 each element can replace any element below it, but not any element above it. F 2 ( g ) + 2NaCl( aq ) 2NaF( aq ) + Cl 2 ( g ) Cl 2 ( g ) + 2KBr( aq ) 2KCl( aq ) + Br 2 ( l ) Br 2 ( l ) + KCl( aq ) no reaction

Double-Displacement Reactions In double-displacement reactions, the ions of two compounds exchange places in an aqueous solution to form two new compounds. One of the compounds formed is usually a precipitate, an insoluble gas that bubbles out of the solution, or a molecular compound, usually water. The other compound is often soluble and remains dissolved in solution.

Double-Displacement Reactions A double-displacement reaction is represented by the following general equation. AX + BY AY + BX A, X, B, and Y in the reactants represent ions. AY and BX represent ionic or molecular compounds.

Formation of a Precipitate The formation of a precipitate occurs when the cations of one reactant combine with the anions of another reactant to form an insoluble or slightly soluble compound. example: 2KI( aq ) + Pb(NO 3 ) 2 ( aq ) PbI 2 ( s ) + 2KNO 3 ( aq ) The precipitate forms as a result of the very strong attractive forces between the Pb 2+ cations and the I − anions.

Double-Displacement Reactions Formation of a Gas FeS( s ) + 2HCl( aq ) H 2 S( g ) + FeCl 2 ( aq ) Formation of Water HCl( aq ) + NaOH( aq ) NaCl( aq ) + H 2 O( l )

Combustion Reactions In a combustion reaction, a substance combines with oxygen, releasing a large amount of energy in the form of light and heat. example: combustion of hydrogen 2H 2 ( g ) + O 2 ( g ) 2H 2 O( g ) example: combustion of propane C 3 H 8 ( g ) + 5O 2 ( g ) 3CO 2 ( g ) + 4H 2 O( g )

Identifying Reactions and Predicting Products

The ability of an element to react is referred to as the element’s activity . The more readily an element reacts with other substances, the greater its activity is. An activity series is a list of elements organized according to the ease with which the elements undergo certain chemical reactions. For metals, greater activity means a greater ease of loss of electrons, to form positive ions. For nonmetals, greater activity means a greater ease of gain of electrons, to form negative ions.

The order in which the elements are listed is usually determined by single-displacement reactions. The most-active element is placed at the top in the series. It can replace each of the elements below it from a compound in a single-displacement reaction. Activity series are used to help predict whether certain chemical reactions will occur. Activity series are based on experiment.

Activity Series of the Elements

Chapter 8 Notes

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Chapter 8 Notes