Use the language of reaction kinetics
Define rate equation, order, rate constant, half-life, rate-determining step and intermediate.
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 26. Rate laws are empirical unless an elementary step is explicitly stated. Written practice is self-checked
Before you begin
- Use AS collision theory, concentration, graph gradients, and balanced reaction equations.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Where do reaction orders in a rate equation normally come from?
What you will learn
- Distinguish rate equation from balanced chemical equation
- Define order and overall order
- Use rate constant, half-life, rate-determining step and intermediate precisely
Kinetic terms describe measured rates and mechanisms
A rate equation relates reaction rate to reactant concentrations, for example rate = k[A]ᵐ[B]ⁿ. The order with respect to A is m, the order with respect to B is n, and the overall order is m+n. These orders are determined experimentally; they are not generally read from the balanced equation.
The rate constant k is the proportionality constant for a stated temperature. Half-life is the time for a reactant concentration to fall to half a specified value. The rate-determining step is the slow step in a proposed mechanism that controls the overall rate.
An intermediate is formed then used
An intermediate is produced in one step of a mechanism and consumed in a later step, so it cancels when the steps are added and is absent from the overall equation. A catalyst is used in one step and regenerated in a later step.
Use a complete mechanism and the rate equation to distinguish these roles; a species appearing in only one equation is not automatically a catalyst or intermediate.
Pause and explain
What happens to an intermediate when mechanism steps are added?
Worked example
In rate = k[A]²[B], identify the orders, overall order and rate constant.
Show the worked solution
- Read the power of [A].
- Read the power of [B].
- Add the orders for the overall order.
Answer The order with respect to A is 2, with respect to B is 1, and the overall order is 3. k is the rate constant at the stated temperature.
Try a topic test
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
For rate = k[A][B]², state the order in A, order in B and overall order.
Give me a hint
Read each exponent and add them.
Compare my reasoning
- A has exponent 1.
- B has exponent 2.
- Add 1+2.
Order in A = 1, order in B = 2, overall order = 3.
Look for these in your work
- Individual orders read
- Overall order summed
- Rate equation interpreted
A mechanism contains A → I followed by I + B → P. Explain why I can be an intermediate.
Give me a hint
Check whether I is produced and then consumed.
Compare my reasoning
- I is formed in the first step.
- I is used in the second step.
- Adding the steps cancels I.
I is an intermediate because it is formed then consumed and cancels from the overall equation.
Look for these in your work
- Formation step identified
- Consumption step identified
- Cancellation explained
A learner says a first-order rate constant is the same as the reaction rate. Correct the statement.
Give me a hint
Compare the units and role of k with the rate.
Compare my reasoning
- Rate is the measured change in concentration per time.
- k is a proportionality constant in a rate equation.
- Their numerical values and units need not match.
The rate is a concentration change per unit time; k relates that rate to reactant concentration through the rate equation. Their units depend on overall order, so k is not generally the rate itself.
Look for these in your work
- Rate defined
- k role stated
- Units distinguished
Common mistakes
- Calling an overall equation coefficient a reaction order
- Confusing a rate constant with reaction rate
- Calling a consumed intermediate a catalyst
- Assuming every mechanism step has the same rate
What can you explain now?
Where do reaction orders in a rate equation normally come from?
Compare with the explanation
Experimental rate data
Orders are measured from how rate changes with concentration.
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Make it stick
Which kinetic terms describe concentration dependence, timing and the species in a proposed mechanism?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.