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Unit 7 · 7–9% of the exam

Equilibrium

Unit 7 covers reversible reactions that reach a dynamic balance in which the forward and reverse rates are equal. You will write and manipulate K expressions, compare Q with K to predict how a reaction shifts, calculate equilibrium concentrations and apply Le Châtelier's principle. It ends with solubility equilibria (Ksp), which sets up Unit 8.

Le Chatelier's principle

Disturb an equilibrium mixture and watch Q chase K as concentrations shift back.

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Topics in this unit

  1. 7.1

    Introduction to Equilibrium

    At equilibrium the forward and reverse rates are equal, so concentrations stay constant even though both reactions continue.

  2. 7.2

    Direction of Reversible Reactions

    A reversible reaction proceeds in whichever direction is faster until the two rates become equal.

  3. 7.3

    Reaction Quotient and Equilibrium Constant

    Q has the same form as K but uses current concentrations or pressures; at equilibrium Q = K. Pure solids and liquids are left out.

  4. 7.4

    Calculating the Equilibrium Constant

    K is calculated from equilibrium concentrations (Kc) or partial pressures (Kp), often with an ICE table.

  5. 7.5

    Magnitude of the Equilibrium Constant

    K ≫ 1 means products are favored at equilibrium; K ≪ 1 means reactants are favored.

  6. 7.6

    Properties of the Equilibrium Constant

    Reversing a reaction gives 1/K, multiplying by n gives Kⁿ, and adding reactions multiplies their Ks.

  7. 7.7

    Calculating Equilibrium Concentrations

    Use an ICE table with K to find equilibrium concentrations; when K is small, x can often be neglected relative to the initial concentration.

  8. 7.8

    Representations of Equilibrium

    Particle diagrams and graphs of concentration vs time show equilibrium as unchanging amounts after a certain time, not equal amounts.

  9. 7.9

    Introduction to Le Châtelier's Principle

    A system at equilibrium responds to a stress (change in concentration, pressure/volume or temperature) by shifting to partially offset it.

  10. 7.10

    Reaction Quotient and Le Châtelier's Principle

    Changing a concentration or volume changes Q (not K), and the system shifts to bring Q back to K; only temperature changes K.

  11. 7.11

    Introduction to Solubility Equilibria

    Ksp is the equilibrium constant for a slightly soluble salt dissolving into its ions, and molar solubility can be calculated from it.

  12. 7.12

    Common-Ion Effect

    Adding an ion already in the equilibrium lowers the solubility of a slightly soluble salt.

  13. 7.13

    pH and Solubility

    Salts whose anion is a weak base (e.g., OH−, CO32−, F−) become more soluble as pH decreases, because H+ removes the anion.

  14. 7.14

    Free Energy of Dissolution

    Whether a salt dissolves depends on both enthalpy (breaking the lattice, hydrating the ions) and entropy changes, summarized by ΔG of dissolution.

Must know

  • For aA + bB ⇌ cC + dD: K = [C]ᶜ[D]ᵈ / [A]ᵃ[B]ᵇ (omit pure solids and liquids)
  • Q < K → net forward; Q > K → net reverse; Q = K → at equilibrium
  • Reverse reaction: K' = 1/K; multiply by n: K' = Kⁿ; add reactions: K' = K1 × K2
  • Kp = Kc(RT)^Δn, with Δn = moles of gaseous products − moles of gaseous reactants
  • Only temperature changes the value of K
  • Exothermic reaction: raising T lowers K; endothermic: raising T raises K
  • For MX: Ksp = s2; for MX2: Ksp = 4s3
  • Decreasing volume shifts toward the side with fewer moles of gas

Common mix-ups

  • At equilibrium, the concentrations of reactants and products are equal.

    At equilibrium the rates are equal; the concentrations are constant but usually quite different from each other.

  • Adding more reactant increases K.

    Adding reactant changes Q, and the system shifts to restore Q = K; K is unchanged unless the temperature changes.

  • Once equilibrium is reached, the reactions stop.

    Equilibrium is dynamic: the forward and reverse reactions continue at equal rates (isotope labeling experiments show this).

Practice questions

AP-style multiple choice. Every answer comes with the reasoning, including why the tempting wrong answer is wrong.

1 / 6

What is the equilibrium constant expression for N2(g) + 3H2(g) ⇌ 2NH3(g)?

Free-response practice

For H2(g) + I2(g) ⇌ 2HI(g), Kc = 49 at a certain temperature. A flask initially contains 0.200 M H2 and 0.200 M I2 and no HI. (a) Calculate the equilibrium concentrations of all three species. (b) The volume of the flask is then halved at constant temperature. Predict the direction of any shift and justify it.

Flashcards

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See it in real life

Everyday reactions that use Unit 7 ideas, animated atom by atom.