Reversible Reactions (College Board AP® Chemistry)
Study Guide
Written by: Fallon
Reviewed by: Stewart Hird
Reversible Reactions
When the species involved in a reversible reaction are combined, both the forward and reverse reactions occur at the same time
The overall direction of the reaction can be determined by considering the rate of the forward and reverse reactions
Rate of forward reaction > rate of reverse reaction
Overall conversion of reactants into products
Rate of forward reaction < rate of reverse reaction
Overall conversion of products into reactants
The reversible reaction will proceed in an overall direction until equilibrium is established and the rate of the forward reaction is equal to the rate of the reverse reaction
Worked Example
2HI (g) ⇌ H2 (g) + I2 (g)
A mixture of HI(g), H2(g), and I2(g) was placed in a rigid, sealed container at a constant temperature and allowed to react, according to the equation above. The partial pressures of HI(g) and I2(g) in the container over time are shown in the table below
Time elapsed (s) | Partial Pressure HI (g) (atm) | Partial Pressure I2 (g) (atm) |
|---|---|---|
0 | 2.042 | 0.020 |
20 | 2.018 | 0.032 |
40 | 1.970 | 0.056 |
60 | 1.874 | 0.104 |
80 | 1.680 | 0.201 |
100 | 1.560 | 0.261 |
120 | 1.560 | 0.261 |
Based on the data, what claim can be made about the system between 60 and 80 seconds?
(A) The forward and reverse reactions are occurring simultaneously, but the rate of the reverse reaction is greater than that of the forward reaction
(B) The forward and reverse reactions are occurring simultaneously at the same rate
(C) The forward and reverse reactions are occurring simultaneously, but the rate of the forward reaction is greater than that of the reverse reaction
(D) Only the forward reaction is occurring, and the rate of the reverse reaction is zero
Answer:
Equilibrium is established when:
The rate of the forward and reverse reactions remain constant and equal
The partial pressures of the reactants and products remain constant
The partial pressures are still changing between 60 and 80 seconds. Therefore, the system is not in equilibrium and the rate of the forward and reverse reactions are not equal
For all reversible reactions, the forward and reverse reactions occur simultaneously. So, the answer cannot be D
Between 60 and 80 seconds, the partial pressure of HI decreases and the partial pressure of I2 increases indicating an overall net increase in the amount of products
So during this time period, the forward and reverse reactions are occurring simultaneously, but the rate of the forward reaction must be greater than that of the reverse reaction
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