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7.2Equilibrium

Equilibrium Constants — AP CHEMISTRY Revision Notes & Topical Questions

5 min read 9 parts Lesson 8 of 19 1 quiz

📚 AP CHEMISTRY Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Lesson summary

7.2 Equilibrium Constants — AP CHEMISTRY notes: Write, calculate, interpret and manipulate equilibrium constants.Kc — [C]c[D]d / [A]a[B]b for aA + bB ⇌ cC + dD, using equilibrium concentrationsKp — the same with partial pressures and no bracketsOmissions — pure solids and liquids…

  • Temperature
  • Very large K
  • Very small K
  • Reaction quotient
  • Extent of reaction
  • Omissions — pure solids and liquids are left out because their concentrations are constant

Part 1 of 7Expressions

Kc
  • [C]c[D]d / [A]a[B]b for aA + bB ⇌ cC + dD, using equilibrium concentrations
Kp
  • Kp the same with partial pressures and no brackets
Omissions
  • Omissions pure solids and liquids are left out because their concentrations are constant
Units
  • K is unitless

Part 2 of 7Q and K

Q
Q uses values at any time; K — uses equilibrium values only
Temperature
  • K depends only on temperature

Part 3 of 7Direction from Q

Q compared with KShift
Q < KForward
Q > KReverse
Q = KEquilibrium

Part 4 of 7Calculating K

Calculating K
Concentrations
  • Concentrations substitute equilibrium molarities
Partial pressures
  • P = (n/ntotal) × Ptotal
  • For H2 + CO2 ⇌ H2O + CO with 0.8, 0.2, 0.6, 0.5 M, Kc = 0.5.

Part 5 of 7Magnitude of K

K > 1
  • K > 1 products favoured; K < 1 — reactants favoured
Very large K
  • Very large K reaction essentially complete
Very small K
  • Very small K reaction barely proceeds

Part 6 of 7Changing the equation

ChangeNew K
Reverse1/K
Multiply by cKc
Add equationsK1 × K2

Part 7 of 7More from the specification

1Reaction quotient comparison against Kc
  • Reaction quotient comparison against Kc comparing Q against Kc predicts the spontaneous shift direction in a reversible chemical system.
2Reaction quotient
  • The ratio of product to reactant concentrations at any moment.
3Reactants
  • The substances that react together at the start of a reaction.
4Coefficient
  • The number in front of a formula in a balanced equation.

Key terms

TermMeaning
KcEquilibrium constant using concentrations.
KpEquilibrium constant using partial pressures.
Reaction quotientQ, the expression at any time.
Extent of reactionIndicated by the size of K.

Avoid these mistakes

Avoid these mistakes
Including solids in K.
Using Q in place of K.
Confusing the rules for K with those for ΔH when scaling an equation.

Last updated:

3 & IGCSE —

Kc

  • : Kc! (ΔH < 0) Kc. (ΔH > 0) Kc.
  • : ( ).
  • : Kc .
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