This sub-topic is divided as follows;
Orders, rate equations and rate constants
(a,b) Orders, rate equations and rate constants
(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}
(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
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.}