Chapter 7. Chemical Reactions: An Introduction

7.1 Evidence for a Chemical Reaction

Color change

Formation of a solid

Formation of a gas

Change in temperature

7.2 Chemical Equations

Chemical Reaction- a chemical change occurs

Chemical Equation

Reactants Products

Law of Conservation of Matter (therefore, must balance)

Example: hydrogen and oxygen make water

Indicate physical states of reactants & products

Solid (s)

Liquid (l)

Gas (g)

Aqueous (aq) dissolved in water

Figure 7.4: The reaction

between methane and oxygen.

CH4 + O2 CO2 + H2O

7.3 Balancing Chemical Equations

1. Formulas must be written correctly FIRST!

(Don’t forget diatomic molecules!)

2. You may not change subscripts on formulas in order to balance equations

3. Use coefficients; it multiplies everything in the formula.

3O2 = 6 O atoms total

4NaOH = 4 Na atoms, 4 O atoms, 4 H atoms

___N2 + ___H2 ___NH3

___C2H6 + ___O2 ___CO2 + ___H2O

___H2 + ___O2 ___H2O

___O2 ___O3

___Cl2 + ___NaBr ___NaCl + ___Br2