5.1.6Physical Chemistry and Transition Elements
Temperature and Rate Constants — OCR A Level Chemistry A H432 Revision Notes & Topical Questions
5 min read 5 parts Lesson 7 of 40
Part 1 of 5What the Rate Constant Is
Definition
- The rate constant k is the proportionality constant in the rate equation and is specific to one reaction at one temperature.
What changes it
- Only temperature and the presence of a catalyst change k;
- altering a concentration changes the rate but leaves k the same.
Units
- The units depend on the overall order, found by rearranging the rate equation and cancelling the concentration units.
Size of k
- A larger k at a given temperature means a faster reaction, so k allows reactions to be compared fairly.
| Overall order | Rate equation example | Units of k |
|---|---|---|
| 0 | rate = k | mol dm⁻³ s⁻¹ |
| 1 | rate = k[A] | s⁻¹ |
| 2 | rate = k[A][B] | dm³ mol⁻¹ s⁻¹ |
| 3 | rate = k[A][B]² | dm⁶ mol⁻² s⁻¹ |
Part 2 of 5Why Heating Increases k
The explanation uses the distribution of molecular energies at a given temperature.
1
At any temperature the particles have a spread of kinetic energies, drawn as a Maxwell–Boltzmann curve with the area underneath equal to the total number of particles.
2
Only collisions with energy equal to or greater than the activation energy Eₐ can lead to reaction.
3
Raising the temperature moves the peak to a higher energy and flattens the curve, while the total area stays the same.
4
A much larger proportion of particles now lies beyond Eₐ, so the fraction of successful collisions rises sharply.
5
Particles also collide more often at higher temperature, but this is a small effect compared with the increase in the fraction above Eₐ.
Part 3 of 5Catalysts and Rate Constants
Catalysts and Rate Constants
Lower activation energy
- A catalyst provides an alternative route with a smaller Eₐ,
- so a larger fraction of collisions are successful at the same temperature.
Unchanged distribution
- The Maxwell–Boltzmann curve stays the same shape; only the Eₐ marker moves to the left.
Effect on k
- Because more particles exceed the lower Eₐ, the rate constant increases without any change in temperature.
Not consumed
- The catalyst is regenerated, so it does not change the position of equilibrium or the overall enthalpy change.
Part 4 of 5Worked Example
Question
- For the rate equation rate = k[A][B]², the rate is 2.4 × 10⁻³ mol dm⁻³ s⁻¹ when [A] = 0.20 mol dm⁻³ and [B] = 0.30 mol dm⁻³.
- Calculate k and its units.
- Rearrangek = rate ÷ ([A][B]²).
- [B]² = 0.30² = 0.090 mol² dm⁻⁶, so [A][B]² = 0.20 × 0.090 = 0.018 mol³ dm⁻⁹.
- k = 2.4 × 10⁻³ ÷ 0.018 = 0.133.
- Units(mol dm⁻³ s⁻¹) ÷ (mol³ dm⁻⁹) = dm⁶ mol⁻² s⁻¹.
Answer k = 0.13 dm⁶ mol⁻² s⁻¹ (2 s.f.).
Part 5 of 5Exam Checklist & Tips
5.1.6 – Key Points for the Exam
- Rate constant The constant k in the rate equation, which depends on temperature and catalyst but not on concentration.
- Activation energy The minimum energy that colliding particles need for a reaction to occur.
- Maxwell–Boltzmann distribution A curve showing how many particles have each energy at a given temperature.
- Overall order The sum of the orders with respect to every species in the rate equation.
Common Mistakes
- Saying that higher temperature works only because collisions are more frequent;
- the key effect is more particles exceeding Eₐ.
- Drawing a Maxwell–Boltzmann curve for a higher temperature with a larger area;
- the total area must remain constant.
- Giving k without units or with units that do not match the overall order.
- Stating that adding more reactant increases k; it increases the rate but k stays constant.
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