The Effect of Surface Area
Introduction

Have you ever tried starting a fire with wood – perhaps when camping or on Bonfire Night? If you have, you will know that it is much easier to use thinly cut wood shavings than larger sticks to get the fire going. By the end of this unit you will be able to explain why this is the case, using the collision theory described in the unit Introducing Rates of Reaction.

Increasing surface area
We can investigate the effect of surface area on reaction rate by looking at the reaction between marble chips and dilute hydrochloric
acid
An acid is a substance that forms a solution with a pH value of less than 7. Acidic solutions contain an excess of hydrogen ions, H+(aq).
acid
:


CaCO3(s)  + 2 HCl(aq)     CaCl2(aq)  +  H2O(l)  +  CO2(g)

We can use two sets of marble chips: 5 g of large chips and 5 g of small chips. To measure the rate of reaction, we can monitor the loss in mass as the carbon dioxide gas escapes from the flask.

Which set of marble chips has the larger surface area?
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Look at the experiment below:

Figure 1.   Investigating the effect of surface area on rate of reaction.
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Which set of marble chips reacts faster?
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The results of this reaction are typical of reactions in general. We find that:

The greater the surface area of a solid reactant, the faster its rate of reaction
.

Explaining the effect of surface area
When we cut up a piece of a solid, we increase its surface area. By doing this, we expose more of its particles to attack by other
reactant
A reactant is a substance we start with before a chemical reaction.
reactant
particles. Therefore we increase the chance of collisions between reactant particles. Because the collisions become more frequent, the rate of reaction increases.

Predict which of the following would react faster with the oxygen gas in air?
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Look at magnesium ribbon and magnesium powder reacting when heated in air, shown in Fig.2 and Fig.3. See how long it takes for the reaction to finish.

Figure 2.   Magnesium ribbon reacting in air.
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Figure 3.   Magnesium powder reacting in air.
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Which reaction finished more quickly?
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What is the
product
A product is a substance formed in a chemical reaction.
product
formed in each reaction?
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The model in Fig.4 below helps you to imagine what happens when the surface area of a solid is increased in a reacting mixture:

Figure 4.   Effect of increasing surface area on the rate of a reaction.
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Summary


As we increase the surface area of the reactants, we increase the rate of reaction.

Increasing the surface area of a solid reactant exposes more of its particles to attack. This results in an increased chance of collisions between reactant particles, so there are more collisions in any given time and the rate of reaction increases.

Exercises
1. Zinc reacts with dilute sulfuric acid. Which gas is given off in the reaction?
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2. Which would react fastest with dilute sulfuric acid?
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3. A test was carried out to judge the effect of varying surface area on the rate of the reaction above, and the results plotted on the graph in Fig.5. Match the type of zinc to the curve on the graph.
  • A
    B
    C
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Figure 5.  
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4. Which type of zinc has the largest surface area?
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5. Name the product formed when iron is heated in air.
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6. Which would react faster when heated in air?
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Well done!
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