Dynamic chemical equilibrium
Distinguish equal reaction rates from equal concentrations.
Before you begin
- Reversible reactions
- Reaction rates
Explain the dynamic balance
In a closed system, a reversible reaction can reach a state where the forward and reverse reactions occur at equal rates. Reactant and product concentrations then remain constant over time, but they need not be equal to each other. Reactions continue at the particle level, which is why the equilibrium is called dynamic.
Predict the response to a disturbance
Changing concentration, pressure or temperature can alter the equilibrium composition. For gaseous equilibria, increasing pressure by reducing volume favours the side with fewer gas molecules when the numbers differ. Increasing temperature favours the endothermic direction. A catalyst speeds both directions and helps the system reach equilibrium sooner without changing its equilibrium composition at a fixed temperature.
Worked example
For N₂(g) + 3H₂(g) ⇌ 2NH₃(g), predict the effect of increased pressure at constant temperature by reducing volume.
Show the worked solution
- Count gaseous coefficients: four on the left and two on the right.
- A higher pressure favours the side with fewer gas particles.
- The composition shifts towards ammonia; this is a qualitative prediction.
The equilibrium favours more ammonia.
Try it yourself
At equilibrium, which quantities must be equal?
Common mistakes
- A pressure change has no such gas-number preference when both sides have equal gaseous coefficients.
What can you explain now?
Does a catalyst increase the equilibrium yield at a fixed temperature? Explain.
Compare with the explanation
No; it speeds attainment of equilibrium without changing its composition.
It lowers the activation barrier for both directions. The balance is reached more quickly, but the equilibrium position remains the same.
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