How Fast? | OCR A-Level Chemistry A (H432)

How Fast?

107 questions
10 subtopics
Module 5: Physical chemistry and transition elements
Component 01 and Component 03

Kinetics beyond collision theory: rate equations, orders and the rate constant, reading order from concentration-time and rate-concentration graphs, what the rate equation reveals about the mechanism, and the Arrhenius equation..

It covers rate equations, order and the rate constant, deducing orders and finding the units of k, what the rate equation reveals about the mechanism, concentration-time graphs and continuous monitoring, half-life and the rate constant of a first-order reaction, rate-concentration graphs and the initial rates method, clock reactions, colorimetry and analysing kinetic graphs, the rate-determining step and testing a proposed mechanism, temperature, the rate constant and the Arrhenius equation and the ln k against 1/T graph and finding the activation energy.

Sample questions from How Fast?

Answer each one closed book first, then open the answer.

Rate equations, order and the rate constant

What is the role of the rate constant k in a rate equation?

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The rate constant k is the proportionality constant that relates the rate to the concentrations of reactants raised to their respective orders.
Deducing orders and finding the units of k

Why might the orders in a rate equation differ from the stoichiometric coefficients in the balanced equation?

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Because orders depend on the reaction mechanism, not the overall stoichiometry, and can only be determined experimentally.
What the rate equation reveals about the mechanism

Why might a reactant shown in the overall equation not appear in the rate equation?

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Because that reactant only participates in faster subsequent steps after the rate-determining step.
Concentration-time graphs and continuous monitoring

What does the gradient of the tangent to a concentration–time curve at any point represent?

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The instantaneous rate of reaction at that point.
Half-life and the rate constant of a first-order reaction

How can the half-life of a first-order reaction be determined from a concentration–time graph?

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By measuring the time taken for the concentration to fall to half its value; this time remains constant throughout the reaction.
Rate-concentration graphs and the initial rates method

Why does a second-order reaction produce a curved rate–concentration graph rather than a straight line?

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Because rate is proportional to concentration squared, so the relationship is non-linear (parabolic).
Clock reactions, colorimetry and analysing kinetic graphs

How does colorimetry allow reaction rates to be investigated?

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By measuring the absorbance of light by a coloured solution, which is proportional to concentration, allowing concentration changes to be tracked over time.
The rate-determining step and testing a proposed mechanism

A reaction is second order with respect to reactant A. What does this suggest about the mechanism?

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Two molecules of A are involved up to and including the rate-determining step.

The 10 subtopics

One subtopic is one session. Work down the list.

Subtopic	What it covers	Questions
Rate equations, order and the rate constant	Recall questions on the meaning of order, the overall order, the role of k, half-life, and how rate depends on concentration for each order.	11
Deducing orders and finding the units of k	Recall questions on reading order from initial rates data, the general form rate = k[A]ᵐ[B]ⁿ, why orders cannot be read off the balanced equation, and deriving the units of k.	11
What the rate equation reveals about the mechanism	Recall questions on which species appear in the rate equation and why one may not, and on the principle and advantage of the initial rates method.	8
Concentration-time graphs and continuous monitoring	Recall questions on the shapes for zero and first order, why a tangent rather than a chord is used, and what continuous monitoring offers.	12
Half-life and the rate constant of a first-order reaction	Recall questions on the constant half-life of a first-order reaction, reading it from a graph, and k = ln 2 / t½.	8
Rate-concentration graphs and the initial rates method	Recall questions on the shapes for zero, first and second order, and on what the gradient of a linear rate-concentration graph gives.	11
Clock reactions, colorimetry and analysing kinetic graphs	Recall questions on what a clock reaction approximates, how colorimetry follows a rate, and choosing scales, reading intercepts and drawing tangents.	10
The rate-determining step and testing a proposed mechanism	Recall questions on what the rate-determining step is, what the orders imply about it, and the two criteria a proposed mechanism must satisfy.	16
Temperature, the rate constant and the Arrhenius equation	Recall questions on why raising the temperature raises k, and on what Eₐ, A, R and T each represent in the Arrhenius equation.	10
The ln k against 1/T graph and finding the activation energy	Recall questions on the logarithmic form, which graph is plotted, and what its gradient and intercept give.	10

How Fast? is 107 of the 1,913 questions in the guide.Get the guide, £8

How the guide is worked

Answering a question from memory stores it far better than reading the answer again. The guide runs that as a fixed procedure on one subtopic at a time, about twenty minutes a session.

Step 1 · Closed book
Cover the answers. Work through one subtopic and write down what you can. Leave blanks where you have nothing.
Step 2 · Open book
Go back to the top. Read each printed answer and write it out in full, including the ones you had right.
Step 3 · Closed book again
Same questions, same order, from memory. The gap between pass one and pass three is the session result.

Read the full method, the return schedule and the research behind it.