Reaction Rate, Reversibility, and Chemical Equilibrium
Kinetics asks how fast composition changes; equilibrium describes the stable composition reached when forward and reverse rates match.
Learning goals
- Calculate average reaction rate from concentration and time.
- Explain rate factors with collision theory and activation energy.
- Describe dynamic equilibrium in a closed system.
- Write and interpret equilibrium expressions.
- Predict responses to concentration, pressure, and temperature changes.
1. Measuring reaction rate
Average disappearance rate of reactant A is −Δ[A]/Δt; average appearance rate of product P is Δ[P]/Δt. Units are commonly mol·L−1·s−1. For aA → pP, a single reaction rate divides each concentration change by its stoichiometric coefficient: r = −(1/a)Δ[A]/Δt = (1/p)Δ[P]/Δt.
Rate normally changes during a reaction because concentrations change. An instantaneous rate is the slope of a tangent to a concentration–time curve.
2. Collision theory and activation energy
Particles react only when collisions have sufficient energy and suitable orientation. The minimum barrier is activation energy, Ea. Rate increases when effective collisions become more frequent.
| Change | Particle explanation |
|---|---|
| higher concentration or gas pressure | more collisions per time |
| higher temperature | faster particles and a larger fraction above Ea |
| greater solid surface area | more exposed collision sites |
| catalyst | alternative pathway with lower Ea |
A catalyst accelerates forward and reverse reactions and is regenerated overall. It does not change ΔH, equilibrium constant, or equilibrium composition.