collision theory

Higher and foundation tier

Surface Area and rates of reaction

Surface area has a big affect on reaction rates. Consider the reaction of a piece of metal such as magnesium and magnesium powder. The powder will react many many times faster than a strip of magnesium ribbon. This is due to the way the atoms are arranged in the solid strip of magnesium or indeed any solid and how the atoms are arranged in a powder. This is outlined below:
NOTE: breaking a solid into smaller and smaller pieces increases the surface area available to react.

Example 2- The reaction of marble chips with hydrochloric acid.

Consider the reaction between marble chips (calcium carbonate) and hydrochloric acid. Equations for these reactions are shown below:

calcium carbonate(s) + hydrochloric acid(aq) calcium chloride(aq) + carbon dioxide(g)+ water(l)
CaCO3 + 2HCl(aq) CaCl2(aq) + CO2(g) + H2O(l)

We can measure the rate of reaction by recording the mass loss over time. You can see in the diagram below that if both reactions are started at the same time then the flask with the smaller pieces, that is the calcium carbonate chips with the largest surface area reacts fastest and loses the most mass, 0.9g compared to 0.5g in the first conical flask.

diagram showing how surface area effects rate

surface area rate graph We can show the results for the two reaction on a graph. If we were to grind up the lumps into a fine powder and react it in a third conical flask then where do you think the line would appear on the graph?
The steeper the line the more gas is given off in a given time, that is the loss is mass is greatest and the rate of reaction will be fastest. A shallow line on the graph will indicate a slow reaction, a steep line a fast reaction.

Measuring the rate from volume of gas released

Since this reaction produces carbon dioxide gas you could of course measure the rate of reaction by measuring the volume of gas released in a given time. The more gas that is released in a given time the faster is the reaction taking place. There are a number of ways of doing this, see below:

measuring rate of reaction using a gas syringe

Using the method shown above you could measure the rate of reaction by measuring the volume of gas released every 30 seconds Obviously the larger the volume of gas released the faster the reaction. A typical set of results for this experiment as shown in the table below:

time/s 0 30 60 90 120 150 180 210 240
volume of gas/cm3 0 40 75 100 111 116 119 119 119

chalk acid reaction results A graph of these results is shown opposite. There are a few points you should note from the graph:

Key Points

Practice questions

Check your understanding - Questions on rates and surface area