3.1.9.1 Rate equations
The rate of a chemical reaction is related to the
concentration of reactants by a rate equation of the form:
Rate = k[A]m [B]n
where m and n are the orders of reaction with respect to
reactants A and B and k is the rate constant.
The orders m and n are restricted to the values 0, 1, and 2.
The rate constant k varies with temperature as shown by
the equation:
k = Ae–Ea/RT
where A is a constant, known as the Arrhenius constant,
Ea is the activation energy and T is the temperature in K.
Students should be able to:
•• define the terms order of reaction and rate constant
•• perform calculations using the rate equation
•• explain the qualitative effect of changes in temperature
on the rate constant k
•• perform calculations using the equation k = Ae–Ea/RT
•• understand that the equation k = Ae–Ea/RT can be
rearranged into the form ln k = –Ea /RT + ln A and know
how to use this rearranged equation with experimental
data to plot a straight line graph with slope –Ea/R
These equations and the gas constant, R, will be given
when required.
3.1.9.2 Determination of rate equation
The rate equation is an experimentally determined
relationship.
The orders with respect to reactants can provide
information about the mechanism of a reaction.
Students should be able to:
•• use concentration–time graphs to deduce the rate of a
reaction
•• use initial concentration–time data to deduce the initial
rate of a reaction
•• use rate–concentration data or graphs to deduce the
order (0, 1 or 2) with respect to a reactant
•• derive the rate equation for a reaction from the orders
with respect to each of the reactants
•• use the orders with respect to reactants to provide
information about the rate determining/limiting step of a
reaction.
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