Reaction Kinetics
Rate equations, mechanisms, activation energy and catalysis.
Before you begin
Rate equations, mechanisms, activation energy and catalysis.
Learning goals
- Rate Equations, Orders, Rate Constant
- Initial Rates Method
- Concentration–Time Graphs and Half-life
- Activation Energy and Boltzmann Distribution
- Mechanisms and Rate-determining Step
- Catalysis and Enzymes
Syllabus statements covered
- explain and use the terms: rate of reaction; rate equation; order of reaction; rate constant; half-life of a reaction; rate-determining step; activation energy; catalysis
- construct and use rate equations of the form rate = k[A]m[B]n (limited to simple cases of single-step reactions and of multi-step processes with a rate-determining step, for which m and n are 0, 1 or 2), including: — calculating an initial rate using concentration data [integrated forms of rate equations are not required]
- construct and use rate equations of the form rate = k[A]m[B]n (limited to simple cases of single-step reactions and of multi-step processes with a rate-determining step, for which m and n are 0, 1 or 2), including: — deducing the order of a reaction by the initial rates method
- calculate a rate constant using the initial rates method
- devise a suitable experimental technique for studying the rate of a reaction, from given information
- construct and use rate equations of the form rate = k[A]m[B]n (limited to simple cases of single-step reactions and of multi-step processes with a rate-determining step, for which m and n are 0, 1 or 2), including: — justifying, for zero- and first-order reactions, the order of reaction from concentration-time graphs
- show understanding that the half-life of a first-order reaction is independent of concentration
- use the half-life of a first-order reaction in calculations
- construct and use rate equations of the form rate = k[A]m[B]n (limited to simple cases of single-step reactions and of multi-step processes with a rate-determining step, for which m and n are 0, 1 or 2), including: — verifying that a suggested reaction mechanism is consistent with the observed kinetics
- construct and use rate equations of the form rate = k[A]m[B]n (limited to simple cases of single-step reactions and of multi-step processes with a rate-determining step, for which m and n are 0, 1 or 2), including: — predicting the order that would result from a given reaction mechanism
- explain qualitatively, in terms of frequency of collisions, the effect of concentration changes on the rate of a reaction
- show understanding, including reference to the Boltzmann distribution, of what is meant by the term activation energy
- explain qualitatively, in terms both of the Boltzmann distribution and of collision frequency, the effect of temperature change on a rate constant (and hence, on the rate) of a reaction
- explain that, in the presence of a catalyst, a reaction has a different mechanism, i.e. one of lower activation energy, giving a larger rate constant
- interpret this catalytic effect on a rate constant in terms of the Boltzmann distribution
- outline the different modes of action of homogeneous and heterogeneous catalysis, including: — the Haber process
- outline the different modes of action of homogeneous and heterogeneous catalysis, including: — the catalytic removal of oxides of nitrogen in the exhaust gases from car engines (see also Section 11.4)
- outline the different modes of action of homogeneous and heterogeneous catalysis, including: — the catalytic role of atmospheric oxides of nitrogen in the oxidation of atmospheric sulfur dioxide
- outline the different modes of action of homogeneous and heterogeneous catalysis, including: — catalytic role of Fe2+ in the I–/S2O82– reaction
- describe enzymes as protein molecules that act as biological catalysts with high specificity (in the reactions that they catalyse and in their choice of substrates as exemplified by the lock-and-key model), temperature sensitivity and pH sensitivity [knowledge of the levels of structure of proteins and the details of the denaturation process are not required]
Lessons
Work through them in order.
- Rate Equations, Orders, Rate ConstantWrite rate equations and work out the units of the rate constant.
- Initial Rates MethodCompare experiments to deduce reaction orders.
- Concentration–Time Graphs and Half-lifeUse concentration–time graphs and half-life to identify first-order reactions.
- Activation Energy and Boltzmann DistributionExplain temperature effects and catalyst action.
- Mechanisms and Rate-determining StepLink the slow step of a proposed mechanism to the observed rate equation.
- Catalysis and EnzymesCompare heterogeneous and homogeneous catalysts, and explain how enzymes depend on conditions.
Practise and check
Recommended nextReaction Kinetics topic check
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Topic reference
This topic builds one chain: experiment, rate equation, mechanism, explanation. The lessons deduce rate equations from data, link them to mechanisms, then explain temperature and catalyst effects.
Be comfortable with:
- Solution Concentration and Dilution: concentration units and n = cV.
- Speed of Reaction and Collision Theory: O Level rate basics.
- Algebra with concentration and rate ratios.
Quick Reference
| If the question asks… | Start with… | What to show |
|---|---|---|
| “find the order” (initial rates) | compare experiments where only one concentration changes | show ratios, then state the order clearly |
| “write the rate equation” | use your orders | Rate = k[A]^m[B]ⁿ (define k) |
| “find units of k” | write rate units and concentration units | divide by concentration powers (use overall order) |
| “is it first order?” | compare successive halving intervals on concentration–time data | “constant half-life” wording |
| “link rate law to mechanism” | identify the rate-determining step | use the stated fast equilibrium where needed to remove an intermediate |
What You Must Memorise
- Rate equation form: rate = k[A]^m[B]ⁿ (orders are found experimentally).
- Order meaning: doubling [A] changes rate by 2^m; zero order means no rate change when concentration changes.
- Initial rates ratio (when only [A] changes): rate₂/rate₁ = ([A]₂/[A]₁)^m.
- First-order signature required here: constant half-life from concentration–time data.
- Boundary: integrated rate equations, logarithmic plots, the Arrhenius equation and steady-state derivations are not required by 9476.
- Units anchor: rate typically in mol dm⁻³ s⁻¹; units of k depend on overall order.
- Catalyst language: alternative pathway with lower Eₐ → larger fraction with E ≥ Eₐ.
Common Exam Traps
- Assuming orders equal stoichiometric coefficients (not true unless stated elementary).
- Comparing experiments where more than one concentration changes (invalid comparison).
- Deducing an order but not stating the rate equation (or missing the constant k).
- Forgetting that units of k depend on overall order.
- Concentration–time: assuming half-life is constant for any order (it’s constant for first order only).
- Mechanisms: using the overall equation coefficients to “guess” the rate law.
- Catalyst explanations that don’t link to lower Eₐ / more successful collisions.
- Importing logarithmic plots, integrated equations or Arrhenius calculations when the question only requires concentration–time, half-life or Boltzmann reasoning.