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:

##### 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.

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:

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

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

• In the first 30 seconds of the reaction 40 ml or 40 cm3 of gas was released.
• In the next 30 seconds the volume of gas went from 40cm3 to 75cm3, so 35 cm3 of gas was released.
• In the next 30 seconds the volume of gas went from 75cm3 to 100cm3, so 25 cm3 of gas was released.
• The curve levels off and is flat when 119cm3 of gas has been released. The fact that the curve is flat tells us that no more product is being made and that the reaction has stopped.
• You can carry on and calculate the amount of gas that was released over the next 30 s and continue it for all the results given. It is clear that the rate of production of gas is falling, that is the rate of the reaction is slowing down. This is obvious from the graph, the slope of the line will give an indication of the rate of reaction, the line to begin with has a steep gradient, indicating a fast reaction but the gradient of the line decreases as time passes. The reactants will be getting used up as the reaction proceeds, so there will be less chemicals available to react and so less gas will be produced.

### Key Points

• To increase the surface area of a solid you can grind it up using a mortar and pestle.
• Small chips have a lrager surfaer area than large chunks of a solid materials. Powders have a very large surface area.
• The larger the surface area the faster the reaction is likely to be. This is because more of the reacting particles will be able to collide successfully and so form the products.