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3.3.2Stoichiometry

Dynamic Equilibrium — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 4 parts Lesson 21 of 28 Unit quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision
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Section 1 of 4Features of Equilibrium

3.3.2 Dynamic Equilibrium
Closed system
  • Equilibrium can only be reached in a closed system, where no reactant or product can escape from the vessel.
Equal rates
  • At equilibrium the rate of the forward reaction is equal to the rate of the reverse reaction.
Constant concentrations
  • Once equilibrium is reached, the concentrations of reactants and products stop changing, though they need not be equal.
Dynamic
  • Equilibrium is dynamic because molecules keep reacting in both directions, and the balance is kept by constant change.
Constant conditions
  • The equilibrium holds only while conditions such as temperature and pressure stay the same.

Section 2 of 4Reaching Equilibrium in the Haber Process

1
At the start only nitrogen and hydrogen are present, so the forward reaction is fastest because reactant concentrations are highest.
2
As ammonia builds up, the reverse reaction speeds up because its rate depends on the ammonia concentration.
3
The forward rate falls as reactants are used up, while the reverse rate rises, until the two rates become equal.
4
From this point the concentrations of N₂, H₂ and NH₃ remain constant, and equilibrium is established in the closed vessel.

Section 3 of 4Reading a Rate Graph

Reading a Rate Graph
Start
  • The forward rate is highest and the reverse rate is zero, because no product is present at the start.
Approach
  • The forward rate decreases as reactants are used up, while the reverse rate increases as products build up.
Equilibrium point
  • The two rate lines meet and then stay level, showing that the forward and reverse rates are equal.
Concentrations
  • Reactant and product concentrations become constant at the equilibrium point.
Catalysts
  • A catalyst helps equilibrium to be reached faster but does not change the final concentrations.

Section 4 of 4Exam Checklist & Tips

3.3.2 – Key Points for the Exam

  • Dynamic equilibrium A state in a closed system in which the forward and reverse reactions occur at equal rates.
  • Closed system A system in which no substances can enter or leave.
  • Forward reaction The reaction in the direction written from left to right in the equation.
  • Reverse reaction The reaction in the opposite direction to the forward reaction.
  • Haber process The industrial manufacture of ammonia from nitrogen and hydrogen.

Common Mistakes

  • Saying equilibrium means the reaction has stopped; both reactions are still happening.
  • Describing equilibrium in an open system, where gases can escape from the container.
  • Claiming that reactant and product concentrations must be equal at equilibrium.
  • Assuming that the rates are equal at all times, rather than only once equilibrium has been reached.

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