CHAPTER 7: Chemical Reactions

Note: Review all information in text.

Introduction:

1.Chemical Equations represent Chemical Reactions or Chemical Change.

2. They are formed by combining symbols and formulas.

E.g. H2 + O2 ---------> H2O 3. According to Law of Conservation of Mass, equation must be balanced. Number of atoms of each element must be same on each side of equation.

4. 5 types of reactions.

7.1 Chemical Reactions

1. Reactants ---------> Products

2. John Dalton's Atomic Theory: In a chemical reaction, the ways in which atoms are joined together are changed. Bonds that hold atoms together are broken and new bonds are formed. According to Law of Conservation of Mass atoms in products are same as atoms in reactants, just rearranged.

3. In a chemical equation, an arrow separates the formulas of the reactants from the formulas of the products. When writing chemical reactions use symbols, see Table 7.1:

4. A skeleton equation is a chemical equation that does not indicate the relative amounts of the reactants and products. It is a necessary first step in equation writing.

E.g. Fe(s) +O2(g) --------->Fe2O3(s) 5. To write a skeleton equation, you must write the correct formulas of the reactants and products.

6. A catalyst is a substance that speeds up a reaction without being used up. E.g., MnO2 , page 178

7. Do problems. 3. a, b, c.

7.3 Balancing Everyday Equations.

1. F+W+H+P----->FW2HP2 (Bicycle)

7.4 Balancing Chemical Equations

1. Each side of the equation has the same number of atoms of each element.

Determine the correct formulas for all reactants and products.

Write the formulas for the reactants on left and products on right with yields sign in between.

Count number of atoms on each side. Polyatomic ions should be considered as a unit.

Use coefficients to balance elements one at a time. Do not change subscripts of formulas.

Check each atom or polyatomic ion.

All coefficients should be in lowest common denominator.

 2. Example 3 (burning of hydrogen).

Examples 4, 5, problems 8&9.

7.5 Types of Reactions

There are 5 types of chemical equations.

7.6 Combination Reactions

1. two or more substances react to form a single substance. Also called a synthesis reaction.

2. Reactants = 2 elements or 2 compounds. A+B=AB.

3. 2 nonmetals

4. Transition metal and nonmetal (more than one product).

e.g., Cu + Cl2 -------->CuCl or CuCl2

4. (See page 188 for more)

5. Group A metal & nonmetal is simple combination of ions

e.g. K + Cl2 ----->2KCl 6. Nonmetal oxides & water form acid

e.g. SO2 +H2O-------->H2SO3

(Hint add formulas of two molecules) 7. Metallic oxides & water give a base (OH)

e.g., CaO + H2O ----------> Ca(OH)2

8. Do example 6 and problems 13.- 15.

7.7 Decomposition Reactions

1. A single compound is broken down into two or more products.

2. Difficult to predict results; simple binary compounds break down into elements. Most decomposition reactions need energy: heat, light, or electricity.

e.g., CaCO3 ------> CaO + CO2 or decomposition of water.

3. Example 7, problems 16, 17

7.8 Single replacement reactions

1. Atoms of an element replace the atoms of a second element in a compound. (Also called single-displacement)(occurs in aqueous solutions).

2. One metal replaces another based on relative reactivity. See Table 7.2, page 191. "Activity Series of Metals".

3. Activity Series of metals is a list of metals in order of decreasing reactivity.

4. Reactive metal replaces listed below it.

5. Nonmetal can replace nonmetal. Usually limited to halogens - F2, Cl2, Br2, I2. Activity decreases going down group 7A.

6. Example 8, problems 18, 19

7.9 Double replacement reactions

1. Exchange of positive ions between compounds, usually between 2 ionic compounds in aqueous solution.

2. One product usually comes out in some way.

3. Three possible situations:

One product is precipitate

One product is a Gas

One product is a molecular compound, such as water.

 4. Example 9; problem 20.

7.10 Combustion reactions

1. An element or a compound reacts with oxygen, producing energy in the form of heat and light.

2. Complete combustion of hydrocarbon produces CO2 and H2O.

e.g., Methane-CH4, Propane-C3H8, etc.

3. Incomplete combustion( not enough oxygen) results in production of toxic CO.

4. An element and oxygen may also combine in combustion.

e.g., Mg, or S

2Mg + O2------>2MgO

 5. Example 10; problems 21-23.

7.11 Predicting products of a chemical reaction

1. The only way to be certain of results of reaction: carry out reaction.

2. Problems 26, 27.

7.12 Net Ionic Equations

1. May write single and double replacement reactions as net ionic equations.

2. Ions are dissolved in water.

3. AgNO3 + NaCl ---->AgCl(s) + NaNO3

4. Net Ionic equations shows dissolved ionic compouns as free ions.

5. Ag+ + NO3 + Na+ + Cl---->AgCl(s) + Na+ + NO3-

6. Cancel ions on both sides of equation (spectator ions, not involved in reaction).

7. Ag+ + Cl---->AgCl(s)

8. Balance for charge and elements.

9. See examples, page 199, and example 11, p. 200, probs 28, 29.

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