1. Chapter 9 Reaction Rates and Chemical Equilibrium
General, Organic, and Biological Chemistry
Fourth Edition
Karen Timberlake
Chapter 9
Reaction Rates and Chemical Equilibrium
9.2
Chemical Equilibrium
Lectures
© 2013 Pearson Education, Inc.

2. Reversible Reactions
A reversible reaction proceeds in both the forward and
reverse directions. As a result there are two reaction
rates: the rate of the forward reaction and the rate of the
reverse reaction.
When molecules begin to react, the rate of the forward reaction is faster than the rate of the reverse reaction.
As reactants are consumed and products accumulate, the rate of the forward reaction decreases, whereas the rate of the reverse reaction increases.

3. Reversible Reactions

4. Reversible Reactions
Suppose SO2 and O2 are present initially. As they collide,
the forward reaction begins.
2SO2(g) + O2(g) SO3(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) SO3(g)
reverse

5. 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.
no further changes occur in the concentration of reactants and products.
Insert bottom page 7 right column, reaction diagram. 5

6. Equilibrium Given the reaction, H2(g) + I2(g) 2HI(g) at
equilibrium, the forward and reverse reactions occur at
the same time and are shown together using a double
arrow.
Forward reaction: H2(g) + I2(g) HI(g)
Reverse reaction: 2HI(g) H2(g) + I2(g)
H2(g) + I2(g) HI(g)

7. Equilibrium H2(g) + I2(g) 2HI(g)
Initially, the reaction flask contains only the reactants H2 and I2.
The forward reaction begins to produce HI.
As the reaction proceeds, there is less H2 and I2 and more HI, which increases the rate of the reverse reaction.
At equilibrium, the concentrations of reactants and product are constant.
The reaction continues, with the rate of the forward reaction equal to the rate of the reverse reaction.

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

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

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

11. Solution Complete with equal, not equal, forward, reverse,
changes, or does not change.
1. Reactants form products in the forward reaction.
2. At equilibrium, the reactant concentration does not change.
3. When products form reactants, it is the reverse reaction.
4. At equilibrium, the rate of the forward reaction is equal to the rate of the reverse reaction.
5. If the forward reaction is faster than the reverse reaction, the amount of products changes.