9701 · 26.1
Simple rate equations, orders of reaction and rate constants flashcards
Revision flashcards for Cambridge 9701 Simple rate equations, orders of reaction and rate constants (syllabus 26.1). Flip, recall, then mark a real past-paper question.
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What is the 'rate of reaction'?
The change in concentration of a reactant or product per unit time. Its units are typically mol dm⁻³ s⁻¹.
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What is a 'rate equation'?
An equation that links the rate of reaction to the concentrations of the reactants raised to certain powers. For a reaction A + B → C, it takes the form: Rate = k[A]^m[B]^n.
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What is the 'order of reaction' with respect to a reactant?
The power to which the concentration of that reactant is raised in the rate equation. It indicates how the reactant's concentration affects the rate.
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How is the 'order of reaction' determined?
Experimentally only. It cannot be determined from the stoichiometry of the balanced chemical equation.
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What is the 'overall order' of a reaction?
The sum of the individual orders of reaction for all reactants in the rate equation. For Rate = k[A]^m[B]^n, the overall order is m + n.
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What is the 'rate constant', k?
The constant of proportionality in the rate equation. It is constant for a given reaction at a specific temperature.
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What is the only factor that significantly changes the value of the rate constant, k?
Temperature. The rate constant, k, increases as temperature increases.
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How do you determine the units for the rate constant, k?
Rearrange the rate equation to make k the subject and substitute the units for rate (mol dm⁻³ s⁻¹) and concentration (mol dm⁻³). Then cancel and simplify.
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What are the units of k for a second-order reaction?
dm³ mol⁻¹ s⁻¹
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What is 'half-life' ($t_{1/2}$)?
The time taken for the concentration of a reactant to decrease to half its initial value.
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What is a key feature of the half-life for a first-order reaction?
It is constant and independent of the initial concentration of the reactant.
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What does it mean if a reactant is 'zero-order'?
The concentration of that reactant has no effect on the rate of reaction. The rate is constant with respect to that reactant.