5.1.1 How fast?

This sub-topic is divided as follows;

Orders, rate equations and rate constants

(a,b) Orders, rate equations and rate constants

  • explanation and use of the terms: rate of reaction, order, overall order, rate constant, half-life, rate-determining step.
  • deduction of:

(i) orders from experimental data

(ii) a rate equation from orders of the form: rate = k[A]m[B]n, where m and n are 0, 1 or 2

{Learners are expected to interpret initial rates data to determine orders with respect to reactants.}

{Integrated forms of rate equations are not required}

(c) Using the rate equation

  • calculation of the rate constant, k, and related quantities, from a rate equation including determination of units

(d,e,f,g) Rate graphs and orders

(d) from a concentration–time graph:

(i) deduction of the order (0 or 1) with respect to a reactant from the shape of the graph

(ii) calculation of reaction rates from the measurement of gradients

{Concentration–time graphs can be plotted from continuous measurements taken during the course of a reaction (continuous monitoring).}

(e) from a concentration–time graph of a first order reaction, measurement of constant half-life, t1/2

{Learners should be aware of the constancy of half-life for a first order reaction.}

(f) for a first order reaction, determination of the rate constant, k, from the constant half-life, t1/2, using the relationship: k = ln 2/t1/2

{Learners will not be required to derive this equation from the exponential relationship between concentration and time, [A] = [A0]e–kt.}

(g) from a rate–concentration graph:

(i) deduction of the order (0, 1 or 2) with respect to a reactant from the shape of the graph

(ii) determination of rate constant for a first order reaction from the gradient

{Rate–concentration data can be obtained from initial rate investigations of separate experiments using different concentrations of one reactant}

{Clock reactions are an approximation of this method where the time measured is such that the reaction has not proceeded too far}

(h) the techniques and procedures used to investigate reaction rates

  • the techniques and procedures used to investigate reaction rates by the initial rates method and by continuous monitoring, including use of colorimetry

(i) Rate-determining step

for a multi-step reaction, prediction of,

(i) a rate equation that is consistent with the rate-determining step

(ii) possible steps in a reaction mechanism from the rate equation and the balanced equation for the overall reaction

(j,k) Effect of temperature on rate constants

(j) a qualitative explanation of the effect of temperature change on the rate of a reaction and hence the rate constant

(k) the Arrhenius equation:

(i) the exponential relationship between the rate constant, k and temperature, T given by the Arrhenius equation,

k = Ae–Ea/RT

(ii) determination of Ea and A graphically using: ln k = –Ea/RT + ln A derived from the Arrhenius equation.

{Ea = activation energy, A = pre-exponential factor, R = gas constant (provided on the Data Sheet)}

{Explanation of A is not required.}

{Equations provided on the Data Sheet.}