Can the orders be deduced from the balanced equation?

No. They can only be found by experiment.

What does zero order with respect to a reactant mean?

Changing its concentration has no effect on the rate.

What happens to the rate if [A] is doubled and the reaction is second order in A?

The rate increases by a factor of 4.

How do you find the rate at a point on a concentration–time graph?

Draw a tangent to the curve at that point; the rate is the magnitude of its gradient.

What shape is a rate–concentration graph for a first order reactant?

A straight line through the origin.

In an initial rates experiment, why is one concentration changed at a time?

So that any change in rate can be attributed to that one concentration.

Why is 1/time used in a clock reaction?

The same amount of change happens each time, so the rate is proportional to 1/time.

Why is a sample quenched when continuous monitoring uses titration?

To stop the reaction so that the composition does not change while the sample is analysed.

How is k found from rate data?

Rearrange the rate equation and substitute the rate and concentrations from one experiment.

Determining rate equations experimentallyAQA A-Level Chemistry: Flashcards

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What is a rate equation?

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What is a rate equation?: An experimentally determined expression for the rate in terms of reactant concentrations: rate = k[A]ᵐ[B]ⁿ.
Can the orders be deduced from the balanced equation?: No. They can only be found by experiment.
What does zero order with respect to a reactant mean?: Changing its concentration has no effect on the rate.
What happens to the rate if [A] is doubled and the reaction is second order in A?: The rate increases by a factor of 4.
How do you find the rate at a point on a concentration–time graph?: Draw a tangent to the curve at that point; the rate is the magnitude of its gradient.
What shape is a rate–concentration graph for a first order reactant?: A straight line through the origin.
What shape is a rate–concentration graph for a zero order reactant?: A horizontal straight line.
How does a concentration–time curve show a first order reaction?: The half-life is constant.
What are the units of k for a second order overall reaction?: dm³ mol⁻¹ s⁻¹.
What are the units of k for a first order overall reaction?: s⁻¹.
In an initial rates experiment, why is one concentration changed at a time?: So that any change in rate can be attributed to that one concentration.
Why is 1/time used in a clock reaction?: The same amount of change happens each time, so the rate is proportional to 1/time.
Why is a sample quenched when continuous monitoring uses titration?: To stop the reaction so that the composition does not change while the sample is analysed.
How is k found from rate data?: Rearrange the rate equation and substitute the rate and concentrations from one experiment.

Exam questions on Determining rate equations experimentally

The gas-phase reaction 2NO + 2H₂ → N₂ + 2H₂O was studied at constant temperature. Experiment 1: [NO] 0.0020 mol dm⁻³, [H₂] 0.0040 mol dm⁻³, initial rate 8.0 × 10⁻⁷ mol dm⁻³ s⁻¹. Experiment 2: [NO] 0.0040, [H₂] 0.0040 (both mol dm⁻³), initial rate 3.2 × 10⁻⁶ mol dm⁻³ s⁻¹. Experiment 3: [NO] 0.0020, [H₂] 0.0080 (both mol dm⁻³), initial rate 1.6 × 10⁻⁶ mol dm⁻³ s⁻¹.
Calculate the value of the rate constant, k, using Experiment 1, and state its units.2 marks
A student studies the decomposition of hydrogen peroxide, 2H₂O₂ → 2H₂O + O₂, using a catalyst, at constant temperature. She works out the concentration of H₂O₂ remaining at intervals: 0.800 mol dm⁻³ at 0 s, 0.400 mol dm⁻³ at 150 s, 0.200 mol dm⁻³ at 300 s and 0.100 mol dm⁻³ at 450 s. A tangent drawn to her smooth concentration–time curve at t = 0 passes through the point (0 s, 0.800 mol dm⁻³) and meets the time axis at 215 s.
The student states that the reaction is first order with respect to H₂O₂. Use the concentration data to justify her conclusion.2 marks
In the reaction S₂O₃²⁻(aq) + 2H⁺(aq) → S(s) + SO₂(g) + H₂O(l), a student mixes sodium thiosulfate solution, hydrochloric acid and water in a flask placed over a cross marked on paper. She records the time for the cross to disappear when viewed from above because of the sulfur formed. In three runs the total volume and the hydrochloric acid concentration are identical. The thiosulfate concentrations are 0.100, 0.050 and 0.025 mol dm⁻³ and the times are 25 s, 50 s and 100 s respectively.
Describe how the student should vary the concentration of thiosulfate ions so that the experiment is a fair test, and how the results can be used to show the order with respect to thiosulfate.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).