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Topic 17. Equilibrium HL

17.1 The equilibrium law

The equilibrium law quantifies the position of equilibrium.

There are 3 ways to calculate Kc: 1) mathematical manipulation 2) ICE method 3) the ∆G equation

To solve equilibrium HL problems, use the ICE table

  • I: initial concentration

  • C: change concentration

  • E: equilibrium concentration

Leave large answers in SCIENTIFIC NOTATION 


Pearson textbook pg. 332

Calculating the equilibrium concentrations when Kc is very small

When the given Kc value is less than 10⁻³, we make the assumption that:  [reactant]initial ≈ [reactant]equilibrium


Pearson textbook pg. 334

Free energy and equilibrium

∆G⁰ is the free energy change. It can be calculated for a reaction by the equation:  ∆G⁰ = ∆G⁰products - ∆G⁰reactants 

  • (The symbol is supposed to be θ not 0, but I can't make θ a superscript)

∆G⁰ = negative → reaction proceeds in the forward direction

∆G⁰ = positive  → reaction proceeds in the backward direction

∆G⁰ = 0 → reaction is at equilibrium

  • at equilibrium, ∆G⁰products = ∆G⁰reactants 

Kc can be calculated from thermodynamic data with the equation ∆G⁰ = - R T lnK

∆G⁰ : standard free energy change of the reaction

R : gas constant 8.31 J/K mol

T : temperature in KELVIN

lnK : natural log of Kc

Pearson textbook pg. 337


Pearson textbook pg. 337


Linking kinetics and equilibrium

rate of forward reaction = k [A] [B]

rate of backward reaction = k' [C] [D]

At equilibrium, the rate of the forward reaction = rate of the background reaction

k [A] [B] = k' [C] [D]


If k > k', then Kc is large -> reaction progresses towards completion

If k < k', then Kc is small -> reaction barely takes place

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