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Chemical Equilibrium and Le Chatelier's Principle
Chemical Equilibrium and Le Chatelier's Principle
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Chemical equilibrium occurs when a reversible reaction continues in both directions but the amounts of reactants and products stay constant. It matters because many real chemical systems, from blood chemistry to industrial ammonia production, depend on controlling equilibrium. At equilibrium, the forward and reverse reaction rates are equal, not zero. This makes equilibrium dynamic rather than static.
Le Chatelier’s Principle explains how an equilibrium system responds when stress is applied. A change in concentration, pressure, volume, or temperature causes the system to shift in the direction that reduces that change. For a general reaction A + B ⇌ C + D, adding reactants tends to form more products, while removing products also pulls the reaction forward. Understanding these shifts helps chemists improve reaction yield and predict how systems behave.
Key Facts
- At equilibrium, rateforward = ratereverse.
- For A + B ⇌ C + D, Kc = [C][D] / ([A][B]).
- If Q < K, the reaction shifts forward toward products.
- If Q > K, the reaction shifts reverse toward reactants.
- Increasing pressure shifts a gas equilibrium toward the side with fewer moles of gas.
- A catalyst speeds up both forward and reverse reactions but does not change K or the equilibrium position.
Vocabulary
- Chemical equilibrium
- A state in which the forward and reverse reactions occur at equal rates and concentrations remain constant.
- Dynamic equilibrium
- An equilibrium in which reactions are still happening even though no overall concentration change is observed.
- Equilibrium constant
- A number, written as K, that compares product concentrations to reactant concentrations at equilibrium.
- Reaction quotient
- A value, written as Q, calculated like K but using current concentrations that may not be at equilibrium.
- Le Chatelier’s Principle
- The rule that an equilibrium system shifts to oppose an applied stress such as concentration, pressure, or temperature change.
Common Mistakes to Avoid
- Saying equilibrium means the reaction has stopped is wrong because both forward and reverse reactions continue at equal rates.
- Putting solids and pure liquids into K expressions is wrong because their activities are treated as constant and are not included.
- Assuming a catalyst increases product yield is wrong because a catalyst only helps the system reach equilibrium faster and does not change the equilibrium composition.
- Forgetting that temperature changes K is wrong because adding or removing heat changes the energy balance and can shift the equilibrium constant.
Practice Questions
- 1 For A + B ⇌ C + D, the equilibrium concentrations are [A] = 0.20 M, [B] = 0.50 M, [C] = 0.80 M, and [D] = 0.25 M. Calculate Kc.
- 2 For N2(g) + 3H2(g) ⇌ 2NH3(g), predict the direction of shift when the pressure is increased. Then state which side has fewer moles of gas.
- 3 A sealed container at equilibrium for A + B ⇌ C + D has extra C added. Explain how the system shifts and why this response follows Le Chatelier’s Principle.