1. Chapter 9 Chemical Equilibrium 9.2Chemical Equilibrium

2. Reversible Reactions In a reversible reaction, there are both forward and reverse reactions. • Suppose SO2 and O2 are present initially. As they collide, the forward reaction begins. 2SO2(g) + O2(g) 2SO3(g) • As SO3 molecules form, they also collide in the reverse reaction that forms reactants. This reversible reaction is written with a double arrow. forward 2SO2(g) + O2(g) 2SO3(g) reverse

3. Chemical Equilibrium At equilibrium, • the rate of the forward reaction becomes equal to the rate of the reverse reaction • the forward and reverse reactions continue at equal rates in both directions

4. Chemical Equilibrium (continued) When equilibrium is reached, • there is no further change in the amounts of reactant and product

5. Equilibrium At equilibrium, • the forward reaction of N2 and O2 forms NO • the reverse reaction of 2NO forms N2 and O2 • the amounts of N2, O2, and NO remain constant N2(g) + O2(g) 2NO(g)

6. Learning Check Write the forward and reverse reactions for the following: CH4(g) + 2H2S(g) CS2(g) + 4H2(g)

7. Solution Write the forward and reverse reactions for the following: Forward: CH4(g) + 2H2S(g) CS2(g) + 4H2(g) Reverse: CS2(g) + 4H2(g) CH4(g) + 2H2S(g) or CH4(g) + 2H2S(g) CS2(g) + 4H2(g)

8. Learning Check Complete with 1) equal 2) not equal 3) forward 4) reverse 5) changes 6) does not change A. Reactants form products in the ________ reaction. B. At equilibrium, the reactant concentration _______. C. When products form reactants, it is the _______ reaction. D. At equilibrium, the rate of the forward reaction is ______ to the rate of the reverse reaction. E. If the forward reaction is faster than the reverse, the amount of products ________.

9. Solution A. Reactants proceed in the (3) forward reaction. B. At equilibrium, the reactant concentration (6) does not change. C. When products form reactants, it is the (4) reverse reaction. D. At equilibrium, the rate of the forward reaction is (1) equal to the rate of the reverse reaction. E. If the forward reaction is faster than the reverse, the amount of products (5) changes.