Short on time? This page is the recap. For explanations and worked examples, use the full study guide.
Recap
- A reversible reaction runs forward and in reverse at the same time. What you observe is the net change.
- Forward rate > reverse rate: net conversion of reactants into products.
- Reverse rate > forward rate: net conversion of products into reactants.
- Forward rate = reverse rate: equilibrium, no net change.
- As the net reaction proceeds, the rate of the faster direction falls and the rate of the slower direction rises, until they meet.
Key relationships
| Relationship | Use it to |
|---|---|
| net rate = forward rate − reverse rate | find the direction from the sign (+ right, − left, 0 equilibrium) |
| A ⇌ B (elementary): forward = k_f[A], reverse = k_r[B] | calculate both rates from concentrations |
| 2A ⇌ A₂ (elementary): forward = k_f[A]², reverse = k_r[A₂] | remember the power from the coefficient |
| At equilibrium for A ⇌ B: k_f[A] = k_r[B] | find the equilibrium mixture from a known total |
Assumptions behind the numbers
- Rate laws can be written from the equation only for elementary steps. For other reactions, compare measured rates.
- The temperature is constant, so k_f and k_r do not change.
- The system is closed, so the total amount of each element is fixed.
Mistakes to avoid
- Direction is not fixed by the equation. Too much product means the net change goes right to left.
- Use concentrations as well as rate constants. The same k values can give either direction.
- Square where the coefficient says so for an elementary step such as 2A ⇌ A₂.
- Neither rate falls to zero on the way to equilibrium (except a rate that starts at zero because a species is absent).
- One snapshot shows amounts, not direction. Compare snapshots over time.
- Equal counts in a snapshot do not mean equilibrium. Constant counts over time do.
Quick self-check
- Forward rate 2.0 × 10⁻³ M s⁻¹, reverse rate 5.0 × 10⁻³ M s⁻¹. Which way is the net change? (To the left, products → reactants, at 3.0 × 10⁻³ M s⁻¹.)
- A ⇌ B with k_f = 0.060 s⁻¹ and k_r = 0.020 s⁻¹: is [A] = 0.20 M, [B] = 0.60 M at equilibrium? (Yes: both rates are 0.012 M s⁻¹.)
- A reaction starts with only products. Which rate is zero at the start? (The forward rate.)
Next: practice questions.