Chapter 15 · Chemical Equilibrium
15.2The Rate of a Chemical Reaction
10 min · two checks
Predict
Why do reactions speed up when you raise the temperature or the concentration?
The idea
- Explain collision theory.
- Predict the effect of concentration and temperature on rate.
A reaction in solution or gas happens when particles collide in the right orientation and with at least the activation energy, the minimum energy needed to reach the rearranged state. Most collisions fail. Rate is how fast a reactant is used up or a product appears, often in mol/(L·s). Raise the concentration and you raise the number of collisions, so the rate rises. Raise the temperature and you do something more dramatic: the fraction of collisions that clear the activation-energy bar grows quickly. A rule of thumb for many reactions is a rough doubling of rate for each 10 °C, but that is a rule of thumb, not a law.
Catalysts, saved for the end of the chapter, speed a reaction by offering a path with lower activation energy. They do not get used up and they do not change the equilibrium position. Crushing a solid raises rate by raising surface area. These are kinetic facts. Equilibrium, next, is about how far a reaction goes, not how fast. A reaction can be fast and still favor reactants, or slow and eventually favor products.
Keep these
- Reactions require successful collisions, not just any contact.
- Higher concentration and higher temperature increase rate.
- Rate (how fast) is different from equilibrium (how far).