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4.2.3Extracting Metals and Equilibria

The Position of Equilibrium

5 min read 6 parts Lesson 10 of 10 Unit quiz

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Section 1 of 6Starting ideas (HT only)

equilibrium from reactants
equilibrium from reactants
equilibrium from products
equilibrium from products
Dynamic equilibrium
  • Dynamic equilibrium In a closed system a reversible reaction reaches equilibrium when the forward and backward reactions occur at the same rate.
  • The amounts of reactants and products then stay constant.
Position of equilibrium
  • Position of equilibrium The position tells you whether there is more product (lying to the right) or more reactant (lying to the left) in the mixture.
Le Chatelier's principle
  • Le Chatelier's principle If a condition of a system at equilibrium is changed, the system shifts to oppose the change.
  • You need to use the idea to make predictions, rather than to remember the name.

Section 2 of 6Temperature

temperature equilibrium shift
temperature equilibrium shift
ChangeExothermic forward reactionEndothermic forward reaction
Temperature increasedShifts left, so less productShifts right, so more product
Temperature decreasedShifts right, so more productShifts left, so less product
ReasonThe system opposes a rise by favouring the endothermic directionThe system opposes a fall by favouring the exothermic direction

Section 3 of 6Pressure (gases only)

pressure equilibrium shift
pressure equilibrium shift
1Count gas molecules
  • Count gas molecules Compare the number of gas molecules in the balanced equation on each side.
2Increase pressure
  • Increase pressure The equilibrium shifts towards the side with fewer gas molecules to reduce the pressure.
3Decrease pressure
  • Decrease pressure The equilibrium shifts towards the side with more gas molecules.
4Equal numbers
  • Equal numbers If both sides have the same number of gas molecules, pressure has no effect on the position.

Section 4 of 6Concentration

reactant concentration equilibrium shift
reactant concentration equilibrium shift
cyclopropane concentration rate data
cyclopropane concentration rate data
concentration cell emf table
concentration cell emf table
Changing a concentration at equilibrium
Add reactant
  • Add reactant The system uses up the extra reactant by shifting right, forming more product.
Remove product
  • Remove product The system replaces it by shifting right, so more reactant is converted.
Add product
  • Add product The system shifts left, forming more reactant.
Remove reactant
  • Remove reactant The system shifts left to try to replace the reactant.

Section 5 of 6Key terms

TermMeaning
EquilibriumA state in a closed system where the forward and reverse reaction rates are equal.
Position of equilibriumThe relative amounts of reactants and products present at equilibrium.
Le Chatelier's principleA system at equilibrium responds to a change by shifting to oppose it.
Reversible reactionA reaction in which products can react to re-form the reactants.
Closed systemA system in which no substances enter or leave.

Section 6 of 6Exam technique

1
Saying a catalyst changes the position of equilibrium; it speeds up both directions equally, so only the time to reach equilibrium changes.
2
Applying pressure changes to solids and liquids; only gases count.
3
Reversing the temperature rule; increasing temperature favours the endothermic direction.
4
Writing that the reaction 'stops' at equilibrium; it continues in both directions at equal rates.
3 & IGCSE —

Kc

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