Kinetics
Reaction Rates — AP CHEMISTRY Revision Notes & Topical Questions
5 min read 10 parts Lesson 3 of 14 Unit quiz
Lesson summary
Reaction Rates — AP CHEMISTRY notes: Measure reaction rate, relate rate to stoichiometry, determine rate laws and orders, use integrated rate laws and half-life, and explain catalysis.Rate of reaction = change in concentration of a reactant or product per unit time, in M s−1.By c…
- Instantaneous rate
- For aA + bB → cC + dD, rate
- Pressure of gases
- Temperature
- Surface area of solids
- Zero order
Section 1 of 10Measuring rate
Rate of reaction
Rate of reaction change in concentration of a reactant or product per unit time, in M s−1.
By convention the rate is positive.
- For a product, rate = Δ[product]/Δt.
- For a reactant, rate = −Δ[reactant]/Δt.
On a concentration-time graph the steeper the gradient the faster the rate.
Instantaneous rate
Instantaneous rate the gradient of a tangent drawn to the curve at that time.
Reactant graphs need the sign changed so the rate is positive.
Section 2 of 10Stoichiometry and rate
1For aA + bB → cC + dD, rate
−(1/a)Δ[A]/Δt = −(1/b)Δ[B]/Δt = (1/c)Δ[C]/Δt = (1/d)Δ[D]/Δt.
2
This gives one positive rate that is independent of the substance used to measure it.
Section 3 of 10Factors affecting rate
Concentration
- Concentration more particles in the same volume give more successful collisions.
Pressure of gases
- Pressure of gases particles occupy less space, so collisions are more frequent.
Temperature
- Temperature particles have more kinetic energy, so more collisions are successful.
Surface area of solids
- Surface area of solids more particles are available to collide.
Catalyst
- Catalyst an alternative path with lower activation energy.
These factors change only the rate, not the amount of product or reactant consumed.
Section 4 of 10Rate law and order
1
Rate law
- Rate = k[A]m[B]n, where m and n are orders found only by experiment.
2
Intermediates do not appear in rate equations.
3
Zero order
- Zero order changing the concentration has no effect on rate, so it is left out.
4
First order
- First order rate is proportional to concentration, so doubling the concentration doubles the rate.
5
Second order
- Second order doubling the concentration increases the rate by a factor of 4.
6
Overall order
Overall order sum of the individual orders.
7
Method of initial rates
- Method of initial rates compare experiments where one concentration changes at a time and use order = change in rate.
- (change in concentration)
8
Units of k follow from the overall order: M s−1 (zero), s−1 (first) and M−1 s−1 (second).
Section 5 of 10Concentration-time graphs
1Zero order
- [A] against t is a straight line going down with slope −k.
2First order
- [A] against t is a curve that falls and levels out.
3Second order
- [A] against t falls more steeply, then levels out.
| Order | Integrated law | Straight-line plot | Slope | Intercept |
|---|---|---|---|---|
| Zero | [A]t = −kt + [A]0 | [A] against t | −k | [A]0 |
| First | ln[A]t = −kt + ln[A]0 | ln[A] against t | −k | ln[A]0 |
| Second | 1/[A]t = kt + 1/[A]0 | 1/[A] against t | k | 1/[A]0 |
Concentration-time graphs
The plot that gives a straight line identifies the order.
The integrated laws for first and second order are given in the AP equations sheet.
Section 6 of 10Half-life
1Half-life (t1/2)
Half-life (t1/2) time for the concentration of a reactant to fall to half its initial value.
2First order
- First order t1/2 = 0.693/k, constant throughout the reaction and independent of initial concentration.
3
Radioactive decay is first order; uranium-238 has a very long half-life so it decays slowly.
4
Zero and second order half-lives depend on initial concentration (zero: [A]0/2k; second: 1/k[A]0); the AP course description specifically covers only first-order half-life.
Section 7 of 10Catalysts
1
A catalyst increases the rate without being consumed overall.
2
It provides an alternative mechanism with lower activation energy, so a greater fraction of collisions are effective and k increases.
3
It may also improve orientation, increasing the frequency factor A.
4
A catalyst does not change ΔH, the energies of reactants and products, the kinetic energy of molecules, the yield or the equilibrium position.
5
It speeds forward and reverse reactions equally.
6
In a mechanism a catalyst is consumed in an early step and regenerated in a later one, and it can appear in the rate law.
Section 8 of 10More from the specification
More from the specification
Initial rate determination from tangents
- Initial rate determination from tangents calculating the gradient of concentration curves provides a graphical means to evaluate initial chemical reaction rates.
Decomposition
- A reaction where one compound breaks down into simpler substances.
Hydrogen
- A gas that gives a squeaky pop with a lighted splint.
Section 9 of 10Key terms
| Term | Meaning |
|---|---|
| Reaction rate | Change in concentration per unit time. |
| Rate law | Equation relating rate to reactant concentrations. |
| Order of reaction | Power of a concentration in the rate law. |
| Rate constant | Proportionality constant k in the rate law. |
| Half-life | Time for a reactant concentration to halve. |
| Integrated rate law | Equation relating concentration to time. |
Section 10 of 10Exam technique
1
Giving the rate of a reactant as a negative value — the rate is positive by convention
2
Writing orders from the balanced overall equation — they must come from experiment
3
Saying temperature changes the order — it changes k only
4
Saying a catalyst changes the yield or ΔH — it only lowers the activation energy
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