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Chemistry
Equilibrium
How Reactions Balance Forward and Reverse Rates
Chemical equilibrium occurs when a reversible reaction continues in both directions but the amounts of reactants and products stop changing. It matters because many important systems, from blood chemistry to industrial ammonia production, depend on controlling equilibrium. At equilibrium, the reaction is not finished or frozen. Instead, the forward and reverse reactions happen at equal rates, so there is no net change in concentration.
Key Facts
- At equilibrium, forward rate = reverse rate.
- For aA + bB ⇌ cC + dD, Kc = [C]^c[D]^d / ([A]^a[B]^b).
- If K > 1, products are favored at equilibrium; if K < 1, reactants are favored.
- The reaction quotient is Qc = [C]^c[D]^d / ([A]^a[B]^b) using current concentrations.
- If Q < K, the reaction shifts forward; if Q > K, the reaction shifts reverse.
- Changing temperature can change K, but changing concentration or pressure shifts the position without changing K at constant temperature.
Vocabulary
- Dynamic equilibrium
- A state in which forward and reverse reactions continue at equal rates so concentrations remain constant.
- Equilibrium constant
- A number, written as K, that describes the ratio of products to reactants at equilibrium for a specific temperature.
- Reaction quotient
- A value, written as Q, that compares product and reactant concentrations at any moment before or at equilibrium.
- Le Châtelier's principle
- A rule stating that a system at equilibrium shifts to reduce the effect of a stress such as added reactant, pressure change, or temperature change.
- Reversible reaction
- A reaction in which products can turn back into reactants under the same conditions.
Common Mistakes to Avoid
- Saying equilibrium means the reaction has stopped is wrong because molecules still react in both directions at equal rates.
- Assuming equal rates mean equal concentrations is wrong because equilibrium concentrations depend on K and the balanced equation.
- Leaving out coefficients as exponents in the K expression is wrong because stoichiometric coefficients determine the powers in the equilibrium expression.
- Thinking a catalyst changes the equilibrium amount is wrong because a catalyst speeds up both directions and helps the system reach equilibrium faster without changing K.
Practice Questions
- 1 For N2O4(g) ⇌ 2NO2(g), write the expression for Kc and calculate Kc if [NO2] = 0.040 M and [N2O4] = 0.020 M at equilibrium.
- 2 For H2(g) + I2(g) ⇌ 2HI(g), Kc = 49. If [H2] = 0.10 M, [I2] = 0.10 M, and [HI] = 0.50 M, calculate Qc and decide whether the reaction shifts forward, reverse, or is at equilibrium.
- 3 A sealed container at equilibrium contains both reactants and products. Extra reactant is added while temperature stays constant. Explain how the forward and reverse rates change immediately and how the system responds as it returns to equilibrium.