Dynamic equilibrium
Predict a change in equilibrium composition without confusing yield and reaction rate.
What you will learn
- Describe dynamic equilibrium in a closed system.
- Predict temperature effects for an exothermic forward reaction.
Equal rates, not stopped reactions
At dynamic equilibrium in a closed system, forward and reverse reactions continue at equal rates. The amounts of reactants and products stay constant over time, but they need not be equal to each other.
Disturbing an equilibrium changes the conditions. The system then approaches a new equilibrium composition. A catalyst speeds the approach by increasing both forward and reverse rates; it does not change the equilibrium composition at a fixed temperature.
Temperature changes the preferred direction
If the forward reaction is exothermic, heating favours the endothermic reverse direction and lowers the equilibrium product yield. Cooling favours the exothermic direction. A higher temperature may nevertheless make the reactions occur faster, so rate and final yield can move in different directions.
Worked example
An exothermic synthesis is heated after reaching equilibrium. Explain the change in product yield.
Show the worked solution
- The reverse reaction is endothermic.
- Heating favours that direction relative to the forward reaction.
- More product is converted back into reactants until a new equilibrium is reached.
Answer The equilibrium product yield decreases.
Common mistakes
- Equal rates do not mean equal concentrations.
- A catalyst changes the time to reach equilibrium, not the equilibrium yield.
What can you explain now?
Does adding a catalyst change the equilibrium constant at fixed temperature?
Compare with the explanation
No
It alters the rates of both directions but does not alter the equilibrium constant for that temperature.
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