AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
The orders with respect to reactants can provide information about the…
Full specification wording
Determination of rate equation (A-level only): The orders with respect to reactants can provide information about the mechanism of a reaction.
A multi step reaction goes no faster than its slowest step, the rate determining step, so only the species taking part in that step, or in steps before it, can influence the rate.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 2 of 4
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A multi step reaction goes no faster than its slowest step, the rate determining step, so only the species taking part in that step, or in steps before it, can influence the rate. That makes the rate equation a window into the mechanism. A species that appears to the first power contributes one molecule to the rate determining step, one to the second power contributes two, and a zero order species is involved only in a fast step after the rate determining one. Hydrolysis of halogenoalkanes is the standard illustration: a tertiary halogenoalkane gives rate = k[RBr], showing the slow step is the breaking of the carbon to halogen bond alone, while a primary one gives rate = k[RBr][OH⁻], with both species in the slow step.
What earns the marks
- Stating that the rate determining step is the slowest step in the mechanism.
- Identifying the species present in the rate determining step from those in the rate equation.
- Matching the order with respect to a species to the number of its molecules in that step.
- Deducing that a zero order species takes part only after the rate determining step.
Common mistakes and how to put them right
Claiming a zero order species plays no part in the reaction at all.
Assuming the rate determining step is the first step of every mechanism.
Proposing a mechanism whose steps do not add up to the overall balanced equation.
Ignoring the order when counting molecules, so a second order species is shown once in the slow step.
Treating the rate equation as proof of a single unique mechanism rather than as evidence consistent with one.
Exam tips
- Check that your proposed steps add to give the overall equation before you commit to a mechanism.
- Say consistent with rather than proves when using a rate equation to support a mechanism.
- Use the orders to count molecules in the slow step: order one means one molecule, order two means two.
Try the lesson
The start of the Determination of rate equation (A-level only): The orders with respect to reactants can provide information about the mechanism of a reaction. lesson, made from checked teaching notes for this specification point.
Have a go first
A reaction is zero order with respect to reactant Z. What does this suggest about the mechanism?
Listen
A multi step reaction goes no faster than its slowest step, the rate determining step, so only the species taking part in that step, or in steps before it, can influence the rate. That makes the rate equation a window into the mechanism. A species that appears to the first power contributes one molecule to the rate determining step, one to the second power contributes two, and a zero order species is involved only in a fast step after the rate determining one. Hydrolysis of halogenoalkanes is the standard illustration: a tertiary halogenoalkane gives rate = k[RBr], showing the slow step is the breaking of the carbon to halogen bond alone, while a primary one gives rate = k[RBr][OH⁻], with both species in the slow step. A strong answer earns marks for these points. stating that the rate determining step is the slowest step in the mechanism. identifying the species present in the rate determining step from those in the rate equation. matching the order with respect to a species to the number of its molecules in that step. deducing that a zero order species takes part only after the rate determining step. Exam tip: Check that your proposed steps add to give the overall equation before you commit to a mechanism.
Key words
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rate determining step
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multi step mechanisms
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orders and molecularity
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evidence for mechanism
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See every definition
- rate determining step
- The slowest step of a multi step reaction, which the reaction goes no faster than.
- multi step mechanisms
- Reaction pathways in which the overall change happens through more than one step, and the rate depends on the slowest step.
- orders and molecularity
- A species to the first power contributes one molecule to the rate determining step, one to the second power contributes two, and a zero order species is involved only in a fast step after it.
- evidence for mechanism
- The rate equation is a window into the mechanism, but it is evidence consistent with one rather than proof of a single unique mechanism.
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