Rates of reaction - kinetics (A2)
Read our general notes on Risk Assessment
How do you calculate the units of the rate constant? I realise it is dependent on the reaction being studied and whether concentration or pressure is being measured, but I do not know the general rule for determining it.
Igloo writes
The Nuffield Chemistry Students’ Book doesn’t appear to show you how to determine rate constant units, so your query is a highly relevant one, since Nuffield examination questions often ask for them to be quoted.
For First Order reactions
These follow the rule
Rate = k [A]
Rate is conventionally measured in units of mol dm-3 s-1
and the concentration of A, i.e. [A], is measured in units of mol dm-3, so, substituting these units into the formula,
mol dm-3 s-1= units of k x mol dm-3
Therefore, units of k = mol dm-3 s-1/ mol dm-3 = s-1 (or, more generally, time-1)
For Second Order reactions
These follow the rule
Rate =k [A] 2 , or perhaps, Rate = k [A] [B], so, substituting these units into either formula, we get
mol dm-3 s-1= units of k x mol dm-3 x mol dm-3
So, units of k = mol dm-3 s-1/ (mol dm-3 x mol dm-3) = mol-1dm3 s-1,
more usually re-arranged into dm3 mol-1s-1 (or, more generally, dm3 mol-1time-1)
I’ll leave you to determine the units of the rate constant for higher order reactions, and for reactions following changes in pressure rather than of concentration. Simply carry out algebra calculations similar to those above.
I’m not going to give you “the general rule”, as it is always possible to do quick calculations such as those above. If you don’t like algebra (!) perhaps it would be a good idea to memorize the relevant units above.
Risk assessment
Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from these hazards, and then decide appropriate control measures to reduce the risks. You must have these approved by those in authority in your school or college laboratory. Do not rely on what is said on this website.
For further guidance see our tutorial on Risk Assessment.
back to Rates of reaction - kinetics (A2)
Rate this page or react
Share your views on this page, 6 ratings so far
, rated at 
updated: 22 August 2004
