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7.4Equilibrium

Calculating the Equilibrium Constant — AP CHEMISTRY Revision Notes & Topical Questions

5 min read 8 parts Lesson 11 of 19 1 quiz

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Lesson summary

7.4 Calculating the Equilibrium Constant — AP CHEMISTRY notes: Calculate Kc and Kp from equilibrium data.Kc — products over reactants using molar concentrations at equilibrium, each raised to its coefficientKp — the same using partial pressures in atmWrite the balanced equation a…

  • Equilibrium values
  • Mole fraction method
  • Equilibrium only
  • Write the balanced equation and the K expression
  • Substitute into the expression
  • Calculate K, a unitless number

Section 1 of 8Expressions

Kc
  • Kc products over reactants using molar concentrations at equilibrium, each raised to its coefficient
Kp
  • Kp the same using partial pressures in atm

Section 2 of 8Calculation from a graph or table

1
Write the balanced equation and the K expression.
2
Read equilibrium values where the lines become constant.
3
Substitute into the expression.
4
Calculate K, a unitless number.

Section 3 of 8Pattern: H₂ + CO₂ ⇌ H₂O + CO at 700 °C

Equilibrium values
  • [H2O] = 0.8 M, [CO] = 0.2 M, [H2] = 0.6 M, [CO2] = 0.5 M
Result
  • Kc = (0.8 × 0.2)/(0.6 × 0.5) = 0.5

Section 4 of 8Partial pressures from moles

Partial pressures from moles
Mole fraction method
  • Pspecies = (nspecies/ntotal) × Ptotal
Pattern: 2NO2 ⇌ 2NO + O2
  • n(NO2) = 0.200, n(NO) = 1.700, n(O2) = 1.100 mol at 6.00 atm total.
  • P = 0.400, 3.40 and 2.20 atm, so Kp = (3.40)2(2.20)/(0.400)2 ≈ 159.

Section 5 of 8Checks

Units
  • Units use mol L-1 in Kc and atm in Kp
Equilibrium only
  • Equilibrium only do not substitute values taken before equilibrium

Section 6 of 8More from the specification

1Law of mass action equilibrium ratio
  • Law of mass action equilibrium ratio computing the ratio of equilibrium product concentrations to reactant concentrations characterizes the closed reaction system.
2Forward reaction
  • Reaction converting reactants into products.
3Reaction quotient
  • The ratio of product to reactant concentrations at any moment.
4Temperature
  • A measure of the average kinetic energy of the particles.

Section 7 of 8Key terms

TermMeaning
KcEquilibrium constant from concentrations.
KpEquilibrium constant from partial pressures.
Mole fractionMoles of a gas divided by total moles.
Partial pressurePressure of one gas in a mixture.

Section 8 of 8Exam technique

Avoid these mistakes
Using non-equilibrium values.
Forgetting to raise to the coefficient power.
Using moles instead of concentration in Kc.

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