Rate equations and orders of reactionEdexcel International A Level Chemistry: Flashcards
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What is the rate equation for a reaction with orders m and n in A and B?
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- What is the rate equation for a reaction with orders m and n in A and B?
- rate = k[A]ᵐ[B]ⁿ
- Define the order of reaction with respect to a substance.
- The power to which its concentration is raised in the rate equation. It shows how the rate depends on that concentration.
- What is the overall order of a reaction?
- The sum of the orders with respect to each substance in the rate equation.
- Can the orders be deduced from the balanced equation?
- No. They can only be found from experimental data.
- What does the rate constant k represent?
- The constant of proportionality in the rate equation. It is constant at a given temperature.
- What are the units of k for an overall second-order reaction (mol dm⁻³, s)?
- dm³ mol⁻¹ s⁻¹
- What are the units of k for a first-order reaction?
- s⁻¹
- What is the effect on rate of doubling the concentration of a second-order reactant?
- The rate increases by a factor of four.
- How can you tell first order from a concentration–time graph?
- The half-life is constant.
- What does a rate–concentration graph look like for a first-order reactant?
- A straight line through the origin.
- What is the rate-determining step?
- The slowest step in the mechanism, which controls the overall rate of reaction.
- What is the difference between a homogeneous and a heterogeneous catalyst?
- A homogeneous catalyst is in the same phase as the reactants. A heterogeneous catalyst is in a different phase.
Exam questions on Rate equations and orders of reaction
- A technician studies the gas-phase reaction 2NO(g) + 2H₂(g) → N₂(g) + 2H₂O(g) at constant temperature. Experiments show that the rate equation is rate = k[NO]²[H₂].The concentration of NO is doubled and the concentration of H₂ is halved, with the temperature unchanged. Deduce the factor by which the rate changes.2 marks
- Peroxodisulfate ions oxidise iodide ions: S₂O₈²⁻(aq) + 2I⁻(aq) → 2SO₄²⁻(aq) + I₂(aq). A student measures the initial rate of this reaction in three experiments at constant temperature. Experiment 1: [S₂O₈²⁻] = 0.020 mol dm⁻³, [I⁻] = 0.040 mol dm⁻³, initial rate = 2.4 × 10⁻⁶ mol dm⁻³ s⁻¹. Experiment 2: [S₂O₈²⁻] = 0.040 mol dm⁻³, [I⁻] = 0.040 mol dm⁻³, initial rate = 4.8 × 10⁻⁶ mol dm⁻³ s⁻¹. Experiment 3: [S₂O₈²⁻] = 0.020 mol dm⁻³, [I⁻] = 0.080 mol dm⁻³, initial rate = 4.8 × 10⁻⁶ mol dm⁻³ s⁻¹.Calculate the initial rate of the reaction at the same temperature when [S₂O₈²⁻] = 0.050 mol dm⁻³ and [I⁻] = 0.060 mol dm⁻³.2 marks
- A reactant R decomposes in solution at constant temperature. Its concentration is 0.800 mol dm⁻³ at the start, 0.400 mol dm⁻³ after 120 s, 0.200 mol dm⁻³ after 240 s and 0.100 mol dm⁻³ after 360 s.Deduce the order of reaction with respect to R and justify your answer.3 marks
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).