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1.12Physical Chemistry

Equilibrium Constant Kp for Homogeneous Systems — Oxford AQA International A Level Chemistry Revision Notes & Topical Questions

5 min read 8 parts Lesson 12 of 13

📚 Oxford AQA International A Level Chemistry Hub / Topical Revision
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Lesson summary

Complete PDF with figures and original page layoutPage 1 Oxford AQA International Your notes A Level (IAL) Chemistry Equilibrium Constant Kp for Homogeneous Systems Contents Equilibrium Constant Kp for Homogeneous Systems Kp Calculations …

Part 1 of 6K<sub>p</sub> expressions

1Gaseous equilibria
  • Gaseous equilibria for aA(g) + bB(g) ⇌ cC(g) + dD(g), Kp = p(C)c p(D)d ÷ (p(A)a p(B)b)
    What the symbols mean: Kp = equilibrium constant in terms of partial pressures · p = partial pressure of each gas at equilibrium (Pa or kPa) · powers come from the balanced equation
2Terms
  • Terms a, b, c and d are the coefficients, and p are equilibrium partial pressures in kPa
3Phases
  • Phases solids and liquids are ignored
4Temperature
  • Kp is constant and changes only if the temperature changes
5Notation
  • Notation do not use square brackets, as they imply concentration and therefore Kc
EquilibriumKp expressionUnits
N2 + 3H2 ⇌ 2NH3p(NH3)2 ÷ (p(H2)3 p(N2))kPa-2
N2O4 ⇌ 2NO2p(NO2)2 ÷ p(N2O4)kPa
2SO2 + O2 ⇌ 2SO3p(SO3)2 ÷ (p(SO2)2 p(O2))kPa-1

Part 2 of 6Effect of conditions on K<sub>p</sub>

Le Chatelier
  • Le Chatelier it applies to gaseous equilibria as to aqueous ones, predicting the position and the value of Kp
Temperature
  • Exothermic forward reactionheating shifts the equilibrium left, the ratio of products to reactants falls, so Kp decreases.
  • Endothermic forward reactionheating shifts right, the ratio rises, so Kp increases.
Pressure
  • Kp is unaffected by pressure.
  • A pressure change shifts the position to a new position that restores the value of Kp.
Catalyst
  • Kp is unaffected because the forward and reverse reactions are sped up equally.
  • A catalyst only makes equilibrium be reached faster.

Part 3 of 6Partial pressure and mole fraction

Partial pressure and mole fraction
Why Kp
  • Why Kp for gas mixtures it is easier to measure pressure than concentration
Partial pressure = the pressure a gas would have if it were alone in the container
Total pressure
  • Ptotal = PA + PB + PC + …
Mole fraction = moles of one gas ÷ total moles of all gases
Equation
  • Equation partial pressure = mole fraction × total pressure
Checks
  • Checks mole fractions add up to 1, and partial pressures add up to the total pressure

Part 4 of 6Mole fractions and partial pressures with an ICE table

Mole fractions and partial pressures with an ICE table
Example
  • 0.25 mol N2 and 0.75 mol H2 react, leaving 0.16 mol N2 at 75 kPa
Moles
  • N2 changes by −0.09, H2 by −0.27 and NH3 by +0.18, so equilibrium moles are 0.16, 0.48 and 0.18, totalling 0.82
Mole fractions
  • 0.195, 0.585 and 0.220
Partial pressures
  • 14.625, 43.875 and 16.5 kPa, which add to 75 kPa

Part 5 of 6Calculating K<sub>p</sub> from partial pressures

Calculating K<sub>p</sub> from partial pressures
Write the Kp expression.
Substitute the equilibrium partial pressures.
Work out the units from the expression.
  • Example
    • 2SO2 + O2 ⇌ 2SO3 with partial pressures 1.0 × 106, 7.0 × 106 and 8.0 × 106 Pa gives Kp = 9.1 × 10-6 Pa-1

Part 6 of 6Calculating K<sub>p</sub> from moles and total pressure

1
Find the equilibrium moles of every gas.
2
Calculate the mole fractions.
3
Calculate the partial pressures.
4
Write the Kp expression.
5
Calculate Kp and its units.
Example
  • H2 + I2 ⇌ 2HI with 1.71 × 10-3, 2.91 × 10-3 and 1.65 × 10-2 mol at 100 kPa has mole fractions 0.0810, 0.1378 and 0.7813, partial pressures 8.10, 13.78 and 78.13 kPa,
    • and Kp = 54.7 with no units
Full question
  • Full question it gives initial moles, equilibrium moles of at least one gas and total pressure,
    • so the equilibrium moles of all gases must be deduced first
Mole fraction approach
  • Mole fraction approach if H2 and Br2 are in a 1 : 1 ratio, their mole fractions are equal, and the mole fraction of HBr is the remainder up to 1

Key terms

TermMeaning
KpThe equilibrium constant in terms of partial pressures of gases.
Partial pressureThe pressure a gas would exert if it were alone in the container.
Mole fractionThe moles of one gas divided by the total moles of gas.
Homogeneous equilibriumAn equilibrium in which all species are in the same phase.

Avoid these mistakes

1
Using square brackets in Kp — use p for partial pressure.
2
Including solids or liquids in Kp — leave them out.
3
Thinking pressure changes Kp — only temperature does.
4
Using total pressure instead of partial pressures — convert with mole fractions.

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