Summary — What This Chapter Covers
- Rate of reaction is the change in concentration (or amount) of a reactant or product per unit time.
- Effective collisions have energy ≥ EA (and the right orientation); non-effective collisions don't lead to reaction.
- Higher concentration or pressure: more frequent collisions, so more frequent effective collisions.
- Activation energy, EA: the minimum energy required for a collision to be effective.
- The Boltzmann distribution shows how molecular energies are spread; at higher temperature more molecules have E ≥ EA.
- A catalyst provides a different mechanism with a lower activation energy and is not used up. Homogeneous: same phase; heterogeneous: different phase.
1. Rate of Reaction
Collision theory
Rate, collisions and effectiveness
- Rate of reaction: the change in concentration of a reactant or product per unit time (units mol dm⁻³ s⁻¹), or the change in amount, mass or gas volume per unit time.
- Frequency of collisions: the number of collisions between reactant particles per unit time.
- Effective collision: a collision with energy at least equal to the activation energy (and suitable orientation) that leads to reaction.
- Non-effective collision: a collision in which the particles bounce apart without reacting, because they have less than EA (or the wrong orientation).
Concentration and pressure
Increasing the concentration of a solution, or the pressure of a gas, puts more particles in the same volume. Collisions become more frequent, so the frequency of effective collisions increases and the rate increases. The proportion of collisions that are effective does not change.
Calculating rate
From experimental data
Rate
rate = change in concentration (or amount) ÷ time taken instantaneous rate = gradient of tangent to the curve
The rate at a particular moment is the gradient of the tangent to a concentration–time (or volume–time) graph at that moment. The rate is greatest at the start and falls as reactants are used up.
Worked Example
The concentration of a reactant falls from 0.100 to 0.080 mol dm⁻³ in the first 40 s. Calculate the average rate.
Worked Example
Marble chips react with HCl. 48.0 cm³ of CO₂ is collected in the first 60 s. Find the average rate in cm³ s⁻¹ and in mol s⁻¹ (molar gas volume 24.0 dm³ mol⁻¹).
Practice Question
A tangent drawn at t = 20 s on a volume–time graph passes through (0 s, 10 cm³) and (50 s, 60 cm³). What is the rate at 20 s?
2. Temperature and Activation Energy
Activation energy
The energy barrier
Activation energy, EA, is the minimum energy required for a collision to be effective.
The Boltzmann distribution
How energies are spread
Boltzmann distribution
area under the curve
A
Effect of temperature
higher temperature
A
much larger
- Main reason
A
- Smaller reason
Memorise