rates of reaction

Higher and foundation tier

Calculating rates of reaction

Most of the time rates of reaction are calculated from graphs. Remember when working out the rate of the reaction you will be calculating how quickly one of the reactants is used up or how quickly one of the products appears. Most of the time it measuring how quickly a gas is produced or how long a particular colour change takes to happen.

Example 1
Magnesium reacts with hydrochloric acid according to the equations below:

magnesium(s) + hydrochloric acid(aq) magnesium chloride(aq) + hydrogen(g)
Mg(s) + 2HCl(aq) MgCl2(g) + H2(g)
The student carrying out this reaction will measure the rate by measuring the volume of hydrogen gas collected in the gas syringe every 30 seconds until the reaction stops. The acid is in excess, this will sure all the magnesium reacts. The apparatus used is shown below.

using a gas syringe to measure rates

The results from the experiment are shown below:

Time/s 0 30 60 90 120 150 180 210
volume of gas/ml 0 14 25 32 37 39 40 40

graph of results Below is a graph of the results.
To begin with the rate of reaction is fast because there are lots of reactant particles available to collide and react to form the products. However as time passes the gradient of the graph becomes more shallow which means the rate of production of hydrogen gas is falling. The graph levels or flattens off at a time of 175 seconds. This means the reaction has stopped, no more hydrogen gas is being released and the reaction has finished. Over the 175 seconds that the reaction took to finish 40 ml of hydrogen gas was released. You can see that the graph slope or gradient is steep to begin with but gradually the slope decreases as the volume of hydrogen released every 30s goes down.
1. To calculate the average rate of reaction use the formula:

rate = volume of gas released ÷ time/s

So volume of gas released was 40ml or 40cm3 and the time taken was 175s
Rate= 40ml ÷ 175s = 0.23 ml/s

0.23ml of gas was released on average every second.

Note: to get the units for rate you just divide the units for volume and time.
That is ml÷ s, or ml/s, which reads as ml per second

2. To calculate the rate of production of gas between two given times you carry out a similar calculation e.g.2 was the rate average of reaction in the first 30s?
So from the graph at 30s volume of gas released was 14ml.
So between 0 and 30 seconds 14ml of gas was released. So to calculate the rate of reaction use the same formula as above:

rate = volume of gas released ÷ time/s
So by simply putting the numbers into the equation we get: 14ml ÷ 30s = 0.47ml/s

using tangents to calculate rates 3. Calculating instantaneous rates (rate of reaction at one point or time in a reaction). To calculate the rate at any one point you need to draw a tangent to the line at that point.
To calculate the rate of the reaction at a time of 120s- draw a tangent to the graph at time=120. The tangent should touch the curve but never cross it, aim to get an equal area either side of the tangent. Then draw the tangent using a ruler. I have marked 2 points which are on the tangent, the points (30, 30) and (140, 38) The two y co-ordinates from these points are 38 and 30. The two x co-ordinates are (140 and 30). The gradient is worked out by :
Gradient= change in y ÷ change in x
Change in y = (38-30) = 8
Change in x= (140-30) = 110
So the gradient is = 8 ÷ 110 = 0.07ml/s. Not very fast at all, the reaction has almost finished at this point!!!