# Surface Area and rates of reaction

Surface area has a big effect on the rate of a chemical reaction. Consider the reaction of a metal such as magnesium with an acid. A strip of magnesium will fizz quickly in an acid but if magnesium powder is placed in the same test tube of acid a very violent reaction will take place. 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.

The surface area is as its name suggests how many of the atoms are exposed at the surface of the solid. In the diagram above most of the atoms in the solid are not exposed at the surface, they are buried or are underneath other atoms within the solid structure. In order for the atoms in a solid to react they will have to collide with other particles, however this will NOT be possible for the atoms buried deep within the solid structure. However if the solid is crushed or broken up into a powder then it is possible for nearly all the atoms to collide with each other and so have the potential to react to form a new substance. This is outlined in the image below:

In the image above the green atoms can only react with the brown atoms on the surface of the solid- they cannot collide with the brown atoms deep within the solid structure. However if solid is crushed then it is possible for the green atoms and the brown atoms to mix freely and collide. The powder has a much larger surface area in contact with the green atoms. The more we break-up a solid into smaller and smaller pieces the more we increase its surface area.

## 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)

The rate of reaction of this reaction could be worked out by recording how much carbon dioxide gas is released in a given time. However it can also easily be worked out by recording the mass loss over time. Carbon dioxide is a relatively "heavy gas" and you can see in the diagram below that if both reactions are started at the same time then the flask with the smaller pieces of calcium carbonate; 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.

Since the reaction of marble chips and hydrochloric acid releases carbon dioxide gas; which is a heavy gas; it will be possible to measure the rate of this reaction by recording the loss in mass say every 15 seconds as the reaction is occurring. The faster the reaction the more carbon dioxide gas will be released and the greater the loss in mass from the conical flask as the gas escapes into the air. If a student were to draw a graph of the results from this experiment what do you think it would show?

The graph opposite shows a typical set of results. The gradient or slope of the line will indicate the loss in mass; the steeper the line the more mass is lost or the more carbon dioxide gas is lost and the faster the reaction taking place. You can see that the small marble chips react faster than the marble chip "lumps". 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. So if a powder were to be used the slope of the line would be almost vertical and the reaction would likely be over very very quickly.

### 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. You would need to have at least five different consistencies of marble chips; ranging from large lumps to a fine powder. If an equal mass of each of the five different consistencies of marble chips were added to the same volume hydrochloric acid with a concentration of 0.5M you could record the volume of carbon dioxide gas released every 10 seconds from the scale on the gas syringe. Although I suspect that as the surface area of the marble chips increases the reaction may finish very quickly! Though you could slow down the reaction by reducing the concentration of the hydrochloric acid or by lowering its temperature. A possible set-up for this experiment is shown below although there are a number of ways of carrying out this experiment.

A typical set of results for this experiment is shown in the table below. Here the student recorded the volume of the carbon dioxide gas every 30 seconds.

 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 products are 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 fewer chemicals available to react and so less gas will be produced.

### Key Points

• To increase the surface area of a solid you can break it up into smaller pieces or grind it up using a mortar and pestle.
• The smaller the chips or pieces of a solid the larger surface area. Powders have a very large surface area; large lumps have small surface areas relative to their volume.
• 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.