Calculating the rate of a reaction
Using the results of experiments like these, the average rate of the reaction can be calculated. No matter which quantity is measured during the course of a reaction, the average rate of reaction can be calculated using the equation below.
This equation is sometimes written as \(rate=\frac{\Delta quantity}{\Delta t}\)
The change in measurable quantity can be read from a table of results or from a graph produced from results.
As the rate is changing throughout the reaction, we are calculating the average rate over a given time period.
For example, the graph below could be used to calculate the average rate over any period of time
Using this graph, we can calculate the average rate between 30 seconds and 120 seconds.
\(average\,rate = \frac{{change\,in\,mass}}{{change\,in\,time}}\)
\(average\,rate = \frac{{3.1 - 1.4}}{{120 - 30}}\)
\(average\,rate = \frac{{1.7}}{{90}}\)
\(average\,rate = 0.0189g\,s_{}^{ - 1}\)
The unit that rate is measured in depends on the measurable quantity. Since a change in mass is measured in grams and a change in time in seconds in this example, the unit of rate would be grams per second (g s-1). Similarly, if a change in concentration is measured (in mol l-1), then rate will have the unit moles per litre per second (mol l-1 s-1) or a change in volume measured in cubic centimetres, centimetres cubed per second (cm3 s-1).