IB — chemistryHard2 min read

Equilibrium

Master equilibrium constants, Le Chatelier's principle, Kc/Kp calculations, and Gibbs free energy for IB Chemistry.

Tutor AI Team
Tutor AI Team

# Equilibrium (IB)

Dynamic equilibrium occurs in closed systems when forward and reverse reaction rates are equal. The IB requires understanding of KcK_c, KpK_p, Le Chatelier's principle, and at HL, the relationship between ΔG\Delta G and KK.

1. Dynamic Equilibrium

Conditions:

  • Closed system
  • Forward rate = reverse rate
  • Concentrations remain constant (but not equal)
  • Both reactions still occurring

2. Equilibrium Constant

For aA+bB⇌cC+dDa\text{A} + b\text{B} \rightleftharpoons c\text{C} + d\text{D}:

Kc=[C]c[D]d[A]a[B]bK_c = \frac{[\text{C}]^c[\text{D}]^d}{[\text{A}]^a[\text{B}]^b}

  • Large KcK_c → products favoured
  • Small KcK_c → reactants favoured
  • Only temperature changes KcK_c
  • Solids and pure liquids excluded

KpK_p for gases uses partial pressures: pA=xA×Ptotalp_A = x_A \times P_{total}

3. Le Chatelier's Principle

If a system at equilibrium is disturbed, it shifts to oppose the change.

Change Effect
Increase [reactant] Shifts forward
Increase temperature (exo forward) Shifts backward; KK decreases
Increase temperature (endo forward) Shifts forward; KK increases
Increase pressure Shifts to side with fewer gas moles
Catalyst No shift; equilibrium reached faster

4. $\Delta G$ and $K$ (HL)

ΔG⊖=−RTln⁡K\Delta G^\ominus = -RT\ln K

  • ΔG<0⇒K>1\Delta G < 0 \Rightarrow K > 1 (products favoured)
  • ΔG>0⇒K<1\Delta G > 0 \Rightarrow K < 1 (reactants favoured)
  • ΔG=0⇒K=1\Delta G = 0 \Rightarrow K = 1 (at equilibrium)

5. ICE Tables

Initial → Change → Equilibrium concentrations

Example: 1.0 mol H₂ and 1.0 mol I₂ in 1.0 dm³. At equilibrium, 0.80 mol HI.

H2+I2⇌2HI\text{H}_2 + \text{I}_2 \rightleftharpoons 2\text{HI}

H₂ I₂ HI
I 1.0 1.0 0
C −0.40 −0.40 +0.80
E 0.60 0.60 0.80

Kc=(0.80)2(0.60)(0.60)=1.78K_c = \frac{(0.80)^2}{(0.60)(0.60)} = 1.78

6. Practice Questions

    1. Write KcK_c for: N2(g)+3H2(g)⇌2NH3(g)\text{N}_2(g) + 3\text{H}_2(g) \rightleftharpoons 2\text{NH}_3(g)
    1. Predict the effect on equilibrium of increasing pressure for the Haber process.
    1. 0.50 mol A and 0.50 mol B in 2.0 dm³ reach equilibrium with 0.30 mol C. Calculate KcK_c for A+B⇌C+D\text{A} + \text{B} \rightleftharpoons \text{C} + \text{D}.
    1. Explain why a catalyst doesn't change KK.
    1. Calculate KK from ΔG⊖=−5.7\Delta G^\ominus = -5.7 kJ/mol at 298 K.

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Summary

  • Equilibrium: forward rate = reverse rate in closed system
  • KcK_c expression: products/reactants with stoichiometric powers
  • Le Chatelier's: system opposes changes
  • Only temperature changes KK
  • HL: ΔG⊖=−RTln⁡K\Delta G^\ominus = -RT\ln K
Tags:
#ib#chemistry#equilibrium

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