Calculating and comparing rates

Calculating rates

In a typical rates experiment, the mass or volume of product is measured at regular time intervals. The results are usually recorded in a suitable table.

Time (mins)Volume of gas produced (cm3)
00
134
242
348
450
550

The results recorded here show that the reaction had finished by four minutes, as no more gas was produced after that.

The mean rate of reaction = 50 ÷ 4 = 12.5 cm3/min

However, the rate decreased during the reaction. The table shows how this happened.

MinuteVolume of gas (cm3)Rate of reaction (cm3/min)
First (0 to 1)34 – 0 = 3434 ÷ 1 = 34
Second (1 to 2)42 – 34 = 88 ÷ 1 = 8
Third (2 to 3)48 – 42 = 66 ÷ 1 = 6
Fourth (3 to 4)50 – 48 = 22 ÷ 1 = 2
Fifth (4 to 5)50 – 50 = 00 ÷ 1 = 0

Graphs

The rate of reaction can be analysed by plotting a graph of amount of product against time. The graph below shows this for two reactions.

A line-graph showing how a fast reaction rises sharply from zero before gradually levelling off. In comparison, a slow reaction rises less sharply but eventually finishes at the same level.

Compared to the slow reaction, the graph line for the faster reaction:

  • has a steeper gradient at the start
  • becomes horizontal sooner (showing that the rate of reaction is greater)
curriculum-key-fact
When plotting rate of reaction graphs, the curve of best fit is drawn as a free-hand smooth curve, NOT joined dot-to-dot with a ruler.

[Higher tier only]

You are expected to be able to calculate the rate of reaction at any time during a reaction by drawing a tangent to the curve at that time and then calculating the gradient of the tangent. An example is shown below.

Graph and equation showing how to calculate the rate of reaction at any time during a reaction by drawing a tangent to the curve.