1. 10.2 Chemical Equilibrium
In most chemical reactions, the reactants are not completely converted to products because a reverse reaction takes place in which products collide to form the reactants.
Learning Goal Use the concept of reversible reactions to explain chemical equilibrium.

2. Reversible Reactions
When a reaction proceeds in both a forward and a reverse direction, it is said to be a reversible reaction. As the reactants, H2 and I2, collide, the forward reaction begins. HI molecules begin to form and collide with each other to form reactants in the reverse reaction. This reversible reaction is written with a double arrow.
H2(g) + I2(g) HI(g)
forward
H2(g) + I2(g) HI(g)
reverse

3. Reversible Reactions A reversible reaction
occurs in both the forward and reverse direction at the same time.
forward
H2(g) + I2(g) HI (g)
reverse
has two rates, a rate for the forward reaction and a rate for the reverse reaction.

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

5. Solution
Write the forward and reverse reactions for the following: CH4(g) + 2H2S(g) CS2(g) + 4H2(g) The forward reaction is The reverse reaction is CS2(g) + 4H2(g) CH4(g) + 2H2S(g)

6. Rate of Reversible Reactions
As the reaction progresses, the rate of the forward reaction decreases and that of the reverse reaction increases. At equilibrium, the rates of the forward and reverse reactions are equal.

7. Equilibrium and Reversible Reactions
Equilibrium is reached when there are no further changes in the concentrations of reactants and products.

8. Rates of Forward and Reverse Reactions

9. Equilibrium In the reaction H2(g) + I2(g) 2HI(g)
the forward reaction is H2(g) + I2(g) HI(g).
the reverse reaction is 2HI(g) H2(g) + I2(g).
As HI product builds up, the rate of the reverse reaction increases, while the rate of the forward reaction decreases.

10. Equilibrium
A reaction reaches equilibrium when no further changes take place in the concentration of the reactants and products.
At equilibrium,
the rate of the forward reaction is equal to the rate of the reverse reaction.
the forward and reverse reactions continue at the same rate.

11. Study Check
Complete each of the following with is/are equal or is/are not equal, change(s) or do(es) not change.
Before equilibrium is reached, the concentrations of the reactants and products ______.
At equilibrium, the rate of the forward reaction ______ to the rate of the reverse reaction.

12. Forward and Reverse Reactions
If we start with reactants SO2 and O2, the reaction to form SO3 takes place until equilibrium is reached.
If we start with only the product SO3, the reaction to form SO2 and O2 takes place until equilibrium is reached.

13. Study Check
Complete each sentence 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. Products form reactants in the _______ reaction.

14. Solution
Complete each sentence with 1) equal 2) not equal 3) forward 4) reverse 5) changes 6) does not change A. Reactants form products in the forward reaction. B. At equilibrium, the reactant concentration does not change. C. Products form reactants in the reverse reaction.