CHEMISTRY CHAPTER 8. CHEMICAL EQUATIONS AND REACTIONS SECTION 1. DESCRIBING CHEMICAL REACTIONS 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.

Reactants are usually written on the left side of an equation, and products on the right side. 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 is a representation of what is happening in a chemical reaction and the relative amounts of reactants and products.

(However, some of these could also be produced by physical changes) Some Indications of a Chemical Reaction (However, some of these could also be produced by physical changes) Energy produced as heat and light Production of gas Formation of a precipitate Color change

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Additional Symbols Used in Chemical Equations

Coefficients are numbers written in front of reactants and products to indicate the relative numbers of atoms, molecules, or moles of substances. “1” is not written as a coefficient.

Example: H2 (g) + Cl2 (g) → 2HCl (g) 1 molecule of hydrogen gas reacts with 1 molecule of chlorine gas to form two molecules of hydrogen chloride gas.

Interpreting a Chemical Reaction

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From the coefficients and molar masses, the relative mass of reactants and products can be calculated (Chapter 9).

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Balancing Chemical Equations Law of conservation of mass: in a chemical reaction, mass is neither created nor destroyed. Balancing a chemical reaction is the adjusting of coefficients so that the number of atoms of an element in the reactants equals the number of atoms of that element in the products. This must be done for each element.

In balancing equations, do not change subscripts because that would mean changing the formulas (giving different compounds). Balancing may be a trial and error process. It is easiest to start with an element that appears in only one item on each side.

Example 1: C3H8 + O2 → CO2 + H2O Start with carbon (3 on left, 1 on right)

Example 1: C3H8 + O2 → CO2 + H2O Start with carbon (3 on left, 1 on right) C3H8 + O2 → 3CO2 + H2O

Example 1: C3H8 + O2 → CO2 + H2O Start with carbon (3 on left, 1 on right) C3H8 + O2 → 3CO2 + H2O Next fix hydrogen (8 on left, 2 on right)

Example 1: C3H8 + O2 → CO2 + H2O Start with carbon (3 on left, 1 on right) C3H8 + O2 → 3CO2 + H2O Next fix hydrogen (8 on left, 2 on right) C3H8 + O2 → 3CO2 + 4H2O

Example 1: C3H8 + O2 → CO2 + H2O Start with carbon (3 on left, 1 on right) C3H8 + O2 → 3CO2 + H2O Next fix hydrogen (8 on left, 2 on right) C3H8 + O2 → 3CO2 + 4H2O Finally fix oxygen (2 on left, 10 on right)

Example 1: C3H8 + O2 → CO2 + H2O Start with carbon (3 on left, 1 on right) C3H8 + O2 → 3CO2 + H2O Next fix hydrogen (8 on left, 2 on right) C3H8 + O2 → 3CO2 + 4H2O Finally fix oxygen (2 on left, 10 on right) C3H8 + 5O2 → 3CO2 + 4H2O

Check: reactants products C 3 3 H 8 8 O 10 10 C3H8 + 5O2 → 3CO2 + 4H2O Check: reactants products C 3 3 H 8 8 O 10 10

Example 2: H2O + C → H2 + CO

Example 2: H2O + C → H2 + CO already balanced

Example 3: Ac(OH)3 → Ac2O3 + H2O option 1: balance Ac

Ac(OH)3 → Ac2O3 + H2O option 1: balance Ac 2Ac(OH)3 → Ac2O3 + H2O Example 3: Ac(OH)3 → Ac2O3 + H2O option 1: balance Ac 2Ac(OH)3 → Ac2O3 + H2O

Ac(OH)3 → Ac2O3 + H2O option 1: balance Ac 2Ac(OH)3 → Ac2O3 + H2O Example 3: Ac(OH)3 → Ac2O3 + H2O option 1: balance Ac 2Ac(OH)3 → Ac2O3 + H2O Then balance H:

Ac(OH)3 → Ac2O3 + H2O option 1: balance Ac 2Ac(OH)3 → Ac2O3 + H2O Example 3: Ac(OH)3 → Ac2O3 + H2O option 1: balance Ac 2Ac(OH)3 → Ac2O3 + H2O Then balance H: 2Ac(OH)3 → Ac2O3 + 3H2O

Example 3: Ac(OH)3 → Ac2O3 + H2O option 2: balance H

Ac(OH)3 → Ac2O3 + H2O option 2: balance H 2Ac(OH)3 → Ac2O3 + 3H2O Example 3: Ac(OH)3 → Ac2O3 + H2O option 2: balance H 2Ac(OH)3 → Ac2O3 + 3H2O

In many equations involving polyatomic ions, the ions appear on both sides of the equation. In that case the entire group can be treated as if it were a single atom.

Example 4 (Math Tutor, p. 294) (NH4)2SO4 + AgNO3 → NH4NO3 + Ag2SO4 Balance the ammonium ion: (NH4)2SO4 + AgNO3 → 2NH4NO3 + Ag2SO4 Balance the nitrate: (NH4)2SO4 + 2AgNO3 → 2NH4NO3 + Ag2SO4 (balanced)

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SECTION 2. TYPES OF CHEMICAL REACTIONS There are several ways to classify chemical reactions. Scheme used in this section: synthesis decomposition single-displacement double-displacement combustion

Abbreviations used in this section: M = metal Nm = nonmetal ′, ′′ = different examples of the same type

1. Synthesis Reactions: two or more substances combine to form a new compound. General reaction: A + X AX A and X can be elements or compounds. AX is a compound

Synthesis Reactions: examples M + Nm → MNm (subscripts will depend on charges) 2Mg + O2 → 2MgO 2Na + F2 → 2NaF 8Ba + S8 → 8BaS

Nm + Nm′ → NmNm′ C + O2 → CO2 2H2 + O2 → 2H2O c. MO + H2O → MOH CaO + H2O → Ca(OH)2 (occurs in setting of cement) compound + compound′ → compound′′ NH3 + HCl → NH4Cl

2. Decomposition: a single compound forms two or more simpler substances. In general, they are the opposite of synthesis reactions. General reaction: AX → A + X

Sometimes this is brought about by electric current (electrolysis) Decomposition Reactions: examples Binary compounds: MNm → M + Nm Sometimes this is brought about by electric current (electrolysis) 2H2O 2H2 + O2 or by heat: 2HgO 2Hg + O2 electricity 

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metal hydroxides (except group 1): MOH → MO + H2O examples – ternary compounds metal carbonates: MCO3 → MO + CO2 ex: CaCO3 → CaO + CO2 metal hydroxides (except group 1): MOH → MO + H2O ex.: Ca(OH)2 → CaO + H2O 

metal chlorates: MClO3 → MCl + O2 ex: 2KClO3 → 2KCl + 3O2 acids: ex.: H2CO3 → CO2 + H2O H2SO4 → SO3 + H2O

NH4Nm → NH3 + HNm MSO4 → MO + SO2 + O2 MSO3 → MO + SO2 other examples NH4Nm → NH3 + HNm MSO4 → MO + SO2 + O2 MSO3 → MO + SO2 MPO4 → MO + P2O5 MPO3 → MO + P2O3 MSiO3 → MO + SiO2 MAsO4 → MO + As2O5 MNO3 → MO + NO2 + O2

3. Single-Displacement (or “Replacement”) Reactions: one element replaces a similar element in a compound. General equations: A + BX → AX + B Y + BX → BY + X (A, B, X, Y are elements)

Single-Displacement Reactions - Examples Displacement of a metal by another metal: M′ + MNm → M + M′Nm ex.: Ca + MgO → Mg + CaO Displacement of a hydrogen in water by a metal: ex.: 2Na + 2H2O → 2NaOH + H2

Displacement of hydrogen in an acid by a metal ex.: Mg + 2HCl → MgCl2 + H2 Displacement of halogens ex.: Cl2 + 2KBr → 2KCl + Br2

4. Double-Displacement Reactions: positive and negative ions of two compounds are exchanged. Usually the ions are in aqueous solution, and one of the products is a precipitate, a gas, or a molecular compound (such as water). General equation: AX + BY → AY + BX

Formation of a precipitate: ex: 2KI(aq) + Pb(NO3)2(aq) → PbI2(s) + 2KNO3 (aq) (How else could you indicate a precipitate?) The precipitate forms as a result of the very strong attractive forces between the Pb2+ cations and the I− anions.

Formation of a precipitate: ex: 2KI(aq) + Pb(NO3)2(aq) → PbI2(s) + 2KNO3 (aq) (How else could you indicate a precipitate?) ↓ The precipitate forms as a result of the very strong attractive forces between the Pb2+ cations and the I− anions.

Formation of a gas. Ex.: FeS(s) + 2HCl(aq) → H2S(g) + FeCl2(aq) Formation of water. Ex: HCl(aq) + NaOH(aq)→ NaCl(aq) + H2O(l)

5. Combustion Reaction: a substance combines with oxygen, releasing a large amount of energy in the form of light and heat. Ex.: combustion of hydrogen 2H2(g) + O2(g) → 2H2O(g) (how else could we classify this?)

5. Combustion Reaction: a substance combines with oxygen, releasing a large amount of energy in the form of light and heat. Ex.: combustion of hydrogen 2H2(g) + O2(g) → 2H2O(g) (how else could we classify this?) synthesis

Ex.: combustion of propane C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(g) Ex.: combustion of ammonia 4NH3 + 7O2 → 4NO2 + 6H2O

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Identifying Reactions and Predicting Products

(usually)

SECTION 3. ACTIVITY SERIES OF THE ELEMENTS 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.

More active metals lose electrons more easily to form positive ions More active metals lose electrons more easily to form positive ions. So they have a greater tendency to form compounds instead of remaining as metallic elements. More active nonmetals gain electrons more easily.

Activity Series of the Elements

Activity series are used to help predict whether certain chemical reactions will occur. An element can displace an element below it in a single-displacement reaction, but not an element above it.

Examples: Al(s) + ZnCl2(aq) Aluminum is above zinc in the table. It is more reactive. The reaction will occur. Write a balanced equation for the reaction.

What compound will be formed if aluminum replaces zinc in ZnCl2? So the equation (unbalanced) is: Al(s) + ZnCl2(aq) →

What compound will be formed if aluminum replaces zinc in ZnCl2? AlCl3 So the equation (unbalanced) is: Al(s) + ZnCl2(aq) →

What compound will be formed if aluminum replaces zinc in ZnCl2? AlCl3 So the equation (unbalanced) is: Al(s) + ZnCl2(aq) → Zn + AlCl3

What compound will be formed if aluminum replaces zinc in ZnCl2? AlCl3 So the equation (unbalanced) is: Al(s) + ZnCl2(aq) → Zn + AlCl3 Balance the equation:

What compound will be formed if aluminum replaces zinc in ZnCl2? AlCl3 So the equation (unbalanced) is: Al(s) + ZnCl2(aq) → Zn + AlCl3 Balance the equation: 2Al + 3ZnCl2 → 3Zn + 2AlCl3

2. Co(s) + NaCl(aq) Cobalt is below sodium in the table. It is less reactive. There will not be a reaction. 3. Br2(l) + KI(aq) Bromine is above iodine in the table, so the reaction will occur. What is it?

2. Co(s) + NaCl(aq) Cobalt is below sodium in the table. It is less reactive. There will not be a reaction. 3. Br2(l) + KI(aq) Bromine is above iodine in the table, so the reaction will occur. What is it? Br2(l) + 2KI(aq) → 2KBr + I2

Note that any metal above H in the table (such as Mg) will react with acid, replacing H, but metals below H (such as Cu) will not.

The table also tells which metals react with cold water, with steam, or with O2. Ex. – Al reacts with steam but not cold water, and reacts with O2.

Note that Ag, Pt, and Au are at the bottom of the table Note that Ag, Pt, and Au are at the bottom of the table. These (and some other, more rare elements) are called the noble metals because they are resistant to corrosion.

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