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Credit Sprint's Guide to Chemistry Unit 7: Equilibrium

AP® weight 7–9%CLEP® weight 6–8%

At equilibrium the forward and reverse rates are equal and concentrations stop changing. The equilibrium constant K sizes up how far a reaction goes, Q compares the current mixture with K, and Le Chatelier's principle predicts how a stress shifts the system.

Approach to equilibriumTimeConcentration equilibrium reached [reactants][products]

What you need to know

  • K = [products]^coefficients ÷ [reactants]^coefficients. Solids and pure liquids are left out.
  • K > 1 favors products; K < 1 favors reactants. K depends only on temperature.
  • Compare Q with K: Q < K shifts forward; Q > K shifts reverse; Q = K is equilibrium.
  • Reverse a reaction: K → 1/K. Multiply coefficients by n: K → Kⁿ. Add reactions: multiply their K values.
  • Kp = Kc(RT)^Δn, where Δn is the change in moles of gas.
  • Le Chatelier: adding a reactant or removing a product shifts forward; higher pressure favors the side with fewer gas moles; heating favors the endothermic direction.
  • A catalyst or inert gas (at constant volume) doesn't shift equilibrium.
  • ICE tables (Initial, Change, Equilibrium) solve for unknown concentrations.
  • Ksp for slightly soluble salts: AgCl s², PbI₂ 4s³. A common ion lowers solubility; acid raises the solubility of salts of weak-acid anions.

Equations and rules

K = [C]ᶜ[D]ᵈ ÷ [A]ᵃ[B]ᵇ
Kp = Kc(RT)^Δn
Ksp = [cation]ᵃ[anion]ᵇ

Exam traps

Watch forChanging concentration or pressure changes Q, not K.
Watch forPure solids and liquids don't appear in K.
Watch forA large K says nothing about speed.
Watch forTemperature is the only thing that changes K.

Key terms

  • Equilibrium: A state in which forward and reverse rates are equal and concentrations are constant.
  • Equilibrium constant (K): Ratio of product to reactant concentrations, each raised to its coefficient.
  • Reaction quotient (Q): Same expression as K evaluated at any moment; compare with K to predict direction.
  • Le Chatelier's principle: A system at equilibrium shifts to counteract an applied stress.
  • Common ion effect: Adding an ion already in the equilibrium lowers solubility of a salt.
  • Solubility product (Ksp): The equilibrium constant for dissolving a slightly soluble salt.
  • ICE table: Initial, Change, Equilibrium table used to solve equilibrium problems.
  • What changes K?: Only temperature changes K.

Practice questions

Five of the 52 Unit 7 questions in Credit Sprint. Every choice is explained in the app.

1. Which is the equilibrium constant expression for N₂(g) + 3 H₂(g) ⇌ 2 NH₃(g)?
  1. Kc = [N₂][H₂]³ ÷ [NH₃]²
  2. Kc = [NH₃] ÷ ([N₂][H₂])
  3. Kc = 2[NH₃] ÷ (3[H₂] + [N₂])
  4. Kc = [NH₃]² ÷ ([N₂][H₂]³)
Show answer

Kc = [NH₃]² ÷ ([N₂][H₂]³). Products over reactants, each raised to its coefficient.

2. For PCl₅ ⇌ PCl₃ + Cl₂, K = 0.50. A mixture has [PCl₅] = [PCl₃] = [Cl₂] = 1.0 M. Which statement is correct?
  1. Q = 1.0 < K, so the reaction shifts right
  2. Q = K, so the system is at equilibrium
  3. Q = 1.0 > K, so the reaction shifts left
  4. Q = 0.50, so the reaction shifts left
Show answer

Q = 1.0 > K, so the reaction shifts left. Q > K means too many products.

3. A student adds more reactant to a system at equilibrium. According to Le Chatelier's principle, the system
  1. shifts toward reactants
  2. shifts toward products to consume some of the added reactant
  3. does not change
  4. changes K
Show answer

shifts toward products to consume some of the added reactant. The system opposes the change.

4. Adding an inert gas to a rigid container at equilibrium will
  1. shift toward the side with more gas
  2. shift toward products
  3. not shift the equilibrium
  4. increase K
Show answer

not shift the equilibrium. Partial pressures of the reacting gases are unchanged.

5. For which reaction does Kp equal Kc?
  1. H₂(g) + I₂(g) ⇌ 2 HI(g)
  2. N₂O₄(g) ⇌ 2 NO₂(g)
  3. N₂(g) + 3 H₂(g) ⇌ 2 NH₃(g)
  4. PCl₅(g) ⇌ PCl₃(g) + Cl₂(g)
Show answer

H₂(g) + I₂(g) ⇌ 2 HI(g). Δn(gas) = 0.