How fast, by what pathway, and how far?
Reaction rate describes how quickly reactants disappear or products form. Catalysis changes the pathway and rate. Chemical equilibrium describes the composition reached when forward and reverse reaction rates become equal in a closed system.
Learning goals: use collision theory; explain effects of concentration, gas pressure, temperature, surface area and catalysts; distinguish rate from equilibrium position; describe dynamic equilibrium and qualitative shifts.
1. Collision theory
Particles must collide with sufficient energy and suitable orientation. The minimum barrier is activation energy. Rate increases when the number of effective collisions per second increases. A simple experimental rate can be measured as change in product amount divided by time, or by tracking gas volume, mass, color or concentration.
2. Factors affecting rate
| Change | Particle explanation |
|---|---|
| Higher concentration | more particles per volume, so collisions occur more often |
| Higher gas pressure | gas particles are closer, increasing collision frequency |
| Higher temperature | particles move faster; more collisions exceed activation energy |
| Greater solid surface area | more solid particles are exposed to collisions |
| Add a catalyst | an alternative pathway has lower activation energy |