Cambridge O Level Chemistry · Syllabus 5070 · Chemical Reactions
Dynamic Equilibrium
What is Dynamic Equilibrium?
The state reached by a reversible reaction in a closed system when the forward and reverse reactions are proceeding at exactly equal rates, so that the concentrations of all reactants and products remain constant. Both reactions continue: nothing has stopped, and the concentrations need not be equal to one another.
This definition is part of the Chemical Reactions chapter in Cambridge O Level Chemistry.
Dynamic Equilibrium in context
A reversible reaction, shown with the symbol \(\rightleftharpoons\), can run in both directions at once, and in a closed system it reaches dynamic equilibrium when the forward and reverse rates are equal, so the concentrations of reactants and products stop changing. Changing temperature, pressure or concentration shifts the position of equilibrium, but a catalyst never does — it speeds up both directions equally, reaching the same equilibrium sooner rather than reaching a different one. The Haber and Contact processes each pick operating conditions that trade off rate, yield, safety and cost rather than chasing the highest possible yield alone. In redox terms, oxidation is loss of electrons and reduction is gain of electrons, so an oxidising agent is itself reduced and a reducing agent is itself oxidised.
Common mistakes with Dynamic Equilibrium
- 13. “At equilibrium the reaction has stopped.” Why temptingNothing observable changes. RepairBoth reactions continue at equal, non-zero rates. That is exactly what the word dynamic is doing in “dynamic equilibrium”. TransferWhat would happen to the colour of a sealed \(\mathrm{NO_2}\)/\(\mathrm{N_2O_4}\) tube if the forward reaction really stopped? It would keep changing until one substance ran out.