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Rates of Reaction

Deducing rate equations from initial rate data

  • 3 things to know
  • Unit A2 1

Overview

Recall questions on why the initial rate is used, reading the order for one reactant from a table when concentration is doubled or trebled, deducing a whole rate equation from two experiments, calculating the rate constant and its units, using the equation to predict a rate, and why only one concentration is changed at a time.

What you need to know

Answer each one before you reveal it.

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Key term

Why is the initial rate used rather than a rate measured later in the reaction?

Key term

Describe how the order with respect to one reactant is found from a table of initial rate data.

Key term

Doubling the concentration of a reactant doubles the initial rate. State the order with respect to that reactant.

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    Why is the initial rate used rather than a rate measured later in the reaction?

    Model answer

    At the very start the concentrations are known exactly and no products have built up, so the measured rate corresponds to the stated concentrations.

  2. 2

    Describe how the order with respect to one reactant is found from a table of initial rate data.

    Model answer

    Two experiments are compared in which only that reactant's concentration differs, and the factor by which the rate changes gives the order directly.

  3. 3

    Doubling the concentration of a reactant doubles the initial rate. State the order with respect to that reactant.

    Model answer

    The reaction is first order with respect to that reactant.

  4. 4

    Doubling the concentration of a reactant quadruples the initial rate. State the order with respect to that reactant.

    Model answer

    The reaction is second order with respect to that reactant.

  5. 5

    Doubling the concentration of a reactant leaves the initial rate unchanged. State the order with respect to that reactant.

    Model answer

    The reaction is zero order with respect to that reactant.

  6. 6

    In a reaction of A with B, doubling [A] at constant [B] quadruples the rate and doubling [B] at constant [A] doubles the rate. Write the rate equation.

    Model answer

    The rate equation is rate = k[A]²[B].

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