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Rate equations (A-level only)
This A-level-only topic makes reaction-rate analysis quantitative.
9
Objectives
45
Flashcards
45
Questions
90 min
Study time
AQAA LevelChemistryPhysical chemistry
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Syllabus checklist
What you need to know
9 objective pages available
Rate equations and orders (A-level only)5 objectives
- Deduce orders of reaction from initial rate data.
- Write rate equations using experimentally determined orders.
- Calculate rate constants and their units.
- Use half-life evidence for first-order reactions.
- Interpret concentration-time graphs.
Mechanisms and rate-determining steps (A-level only)4 objectives
- Explain the rate-determining step in a mechanism.
- Use a proposed mechanism to predict a rate equation.
- Use a rate equation to assess whether a proposed mechanism is consistent with data.
- Distinguish overall equation from rate-determining step.
Key terms
initial rate dataorder of reactionrate equationrate constantunits of rate constanthalf-lifefirst-order reactionconcentration-time graphreaction orderrate-determining stepmechanismoverall equation
Exam tips
- Understanding Initial Rate Data: When analyzing initial rate data, carefully identify the changes in concentration and the corresponding changes in reaction rate to deduce the order of reaction for each reactant.
- Writing Rate Equations: To write a rate equation, identify the rate-determining step and the orders of reaction from experimental data.
Common mistakes
- Misunderstanding Reaction Orders: To fix this, students should focus on how the initial rate data is used to determine the order of reaction, which may not directly correspond to the coefficients in the balanced equation.
- Incorrect Rate Equation Formation: To write the correct rate equation, identify the order of each reactant based on experimental data, not the coefficients. For example, if the rate law is rate = k[A]^2[B]^1, this indicates that the reaction is second order with respect to A and first order with respect to B. Ensure to use the correct orders derived from initial rate data.
Practice preview
- In an experiment, the initial rate of reaction doubles when the concentration of reactant A is doubled. What is the order of reaction with respect to A?
- If the initial rate of a reaction is found to be proportional to the square of the concentration of reactant B, what is the order of reaction with respect to B?
- In a reaction where the initial rate is observed to remain constant despite changes in the concentration of reactant C, what can be deduced about the order of reaction with respect to C?
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