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Equilibrium

Equilibrium — AP CHEMISTRY Revision Notes & Topical Questions

5 min read 8 parts Lesson 3 of 19 1 quiz

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Lesson summary

Equilibrium — AP CHEMISTRY notes: Describe reversible reactions and dynamic equilibrium, and recognise equilibrium from rate and concentration data.FeatureIrreversibleReversibleDirectionOne direction onlyBoth directionsEnd pointStops when a reactant is used upReaches dynamic equi…

  • Closed system
  • Definition
  • Composition
  • Concentration or pressure against time
  • Rate against time
  • Particle diagrams

Section 1 of 8Reversible and irreversible reactions

FeatureIrreversibleReversible
DirectionOne direction onlyBoth directions
End pointStops when a reactant is used upReaches dynamic equilibrium
Arrow→⇌
ExampleCH4 + 2O2 → CO2 + 2H2OH2O(l) ⇌ H2O(g)

Section 2 of 8Reversible processes

1Physical
  • Physical evaporation and condensation; CO2(g) ⇌ CO2(aq); NaCl(s) ⇌ Na+(aq) + Cl−(aq)
2Acid-base
  • HCN + NH3 ⇌ NH4+ + CN−, a proton transfer
3Redox
  • Cd(s) + 2Ag+(aq) ⇌ 2Ag(s) + Cd2+(aq), an electron transfer
4Closed system
  • Closed system reactions involving gases are reversible only in a closed system

Section 3 of 8Dynamic equilibrium

Definition
  • Definition the state where the forward and reverse rates are equal and concentrations or partial pressures stay constant
Dynamic
  • Dynamic both reactions continue even though no change is observed
Composition
  • Composition reactants and products are both present, and their amounts may be greater than, less than or equal to each other

Section 4 of 8Recognising equilibrium on graphs

Recognising equilibrium on graphs
Concentration or pressure against time
  • Concentration or pressure against time lines become horizontal at equilibrium
Rate against time
  • Rate against time the forward and reverse rates become equal
Particle diagrams
  • Particle diagrams the numbers of particles of each species stay constant
Data table
  • Data table equilibrium was established no later than the first time the value stops changing

Section 5 of 8Evidence needed

Constant amounts
  • Constant amounts the concentrations stay constant after a certain time
Limit of data
  • Limit of data if no readings were taken between two times, equilibrium was established somewhere in that interval

Section 6 of 8More from the specification

Molecule
  • Two or more atoms held together by covalent bonds.
Period
  • A row of the Periodic Table, equal to the number of occupied shells.

Section 7 of 8Key terms

TermMeaning
Reversible reactionReaction that can proceed in both directions.
Dynamic equilibriumState with equal forward and reverse rates and constant concentrations.
Irreversible reactionReaction that goes to completion in one direction.
Closed systemSystem in which matter cannot enter or leave.

Section 8 of 8Exam technique

Avoid these mistakes
Thinking the reaction stops at equilibrium — both reactions continue at equal rates.
Thinking concentrations are equal at equilibrium — they are constant, not necessarily equal.
Thinking gas reactions are reversible in open containers — a closed system is required.

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