Chemical Reactions, Energetics, Rates and Equilibrium
Chemical reactions, energetics, rates and equilibrium
This domain tests exothermic and endothermic changes, activation energy, collision theory, factors affecting rate, and reversible reactions. GCE O-Level questions often ask you to explain rate changes using collisions rather than simply naming a factor.
Core rules
- Exothermic reactions release heat to the surroundings; endothermic reactions absorb heat. Bond breaking is endothermic; bond making is exothermic.
- The activation energy is the minimum energy needed for a collision to cause reaction. A catalyst lowers activation energy and provides an alternative pathway.
- Rate increases with higher temperature, higher concentration or pressure, larger surface area and a catalyst. These factors increase the frequency or energy of successful collisions.
- At equilibrium, forward and reverse rates are equal and concentrations remain constant. The equilibrium is dynamic, not static.
- Le Chatelier's principle: if a change is imposed, the system shifts to oppose it. Increasing pressure favours the side with fewer gas molecules; increasing temperature favours the endothermic direction.
Worked example
For N2(g) + 3H2(g) <=> 2NH3(g), predict the effect of increasing pressure at constant temperature.
Reasoning: there are 4 moles of gas on the left and 2 moles on the right. Increasing pressure favours the direction producing fewer gas molecules, so the equilibrium shifts to the right. More ammonia is formed. A catalyst would speed up both directions equally and reduce the time to reach equilibrium, but it would not change the equilibrium yield.
Traps that cost marks
A catalyst does not change the enthalpy change or equilibrium position. Equilibrium does not mean equal concentrations of reactants and products. Increasing temperature increases both rates but shifts equilibrium in the endothermic direction. Rate and yield are different ideas. Do not say particles have more energy at equilibrium unless temperature changes.
Key points
- Exothermic releases heat; endothermic absorbs heat.
- Catalysts lower activation energy without changing enthalpy change.
- Rate factors affect collision frequency or success.
- Dynamic equilibrium has equal forward and reverse rates.
- Le Chatelier predicts the direction of equilibrium shift.