Reverse reaction

1. General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
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. General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
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) 2HI(g)
H2(g) + I2(g) 2HI(g)
forward
reverse

3. General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Reversible Reactions
A reversible reaction
• occurs in both the forward and reverse
direction at the same time.
forward
H2(g) + I2(g) 2HI (g)
reverse
• has two rates, a rate for the forward reaction
and a rate for the reverse reaction.

4. General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Study Check
Write the forward and reverse reactions for the following:
CH4(g) + 2H2S(g) CS2(g) + 4H2(g)

5. General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Solution
Write the forward and reverse reactions for the following:
CH4(g) + 2H2S(g) CS2(g) + 4H2(g)
The forward reaction is
CH4(g) + 2H2S(g) CS2(g) + 4H2(g)
The reverse reaction is
CS2(g) + 4H2(g) CH4(g) + 2H2S(g)

6. General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
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. General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Equilibrium and Reversible Reactions
Equilibrium is reached when there are no further changes in
the concentrations of reactants and products.

8. General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Rates of Forward and Reverse Reactions

9. General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Equilibrium
In the reaction
H2(g) + I2(g) 2HI(g)
• the forward reaction is H2(g) + I2(g) 2HI(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. General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
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. General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Study Check
Complete each of the following with is/are equal or
is/are not equal, change(s) or do(es) not change.
A. Before equilibrium is reached, the concentrations
of the reactants and products ______.
B. At equilibrium, the rate of the forward reaction
______ to the rate of the reverse reaction.

12. General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
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. General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
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. General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
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.