Surface area and rate
In this topic you will:
- investigate the rate of reaction when the surface area of one of the reactants is changed
- carry out an investigation using the reaction between calcium carbonate and dilute hydrochloric acid
- consider how changing the shape of a material can affect the surface area.
Key words
- surface area
The effect of changing the surface area
When you put magnesium ribbon in a Bunsen flame it reacts very quickly, and burns with a bright white flame to form magnesium oxide. However, if you place a large block of magnesium in the Bunsen flame it does not burn. If you place magnesium powder in the Bunsen flame it burns much faster than the ribbon.
Why does this happen?
Think about what is happening as the magnesium reacts with oxygen in the air. Only the magnesium atoms on the surface can make contact with the oxygen and react with it. In the block of magnesium, most of the atoms are inside the block, away from the oxygen. In the magnesium ribbon, more of the atoms are on the surface and react. Magnesium ribbon has a larger total surface area than a magnesium block of the same mass. An equivalent mass of magnesium powder has an even larger total surface area and because it has the most atoms available to react, the reaction is even quicker.
Small pieces of solids always react faster than larger pieces.
Each time you cut a solid into smaller pieces, you increase the total surface area.
For example, these slices of bread have a total surface area greater than the loaf of bread.
Common Mistake
Students often assume that mass is the only factor affecting reaction rate. Remember that surface area has a major impact, especially for solids reacting with gases or liquids.
Think Like a Scientist
Investigation: Burning Iron
Objective: To compare how iron nail, iron wool, and iron filings react when heated in air, and observe differences related to surface area.
You will need:
Safety glasses, tongs, Bunsen burner, heatproof mat, spatula, iron nail, iron wool, iron filings.
Safety:
Wear safety glasses and take care not to touch anything hot.
Method:
Step 1: Grip the iron nail with tongs and hold it in the flame of a Bunsen burner.
Step 2: Hold the iron wool in tongs and heat it in the flame.
Step 3: Use a spatula to gently sprinkle a few iron filings into the flame and observe the reaction.
Show Answer
- The iron nail glows dull red and reacts slowly.
- The iron wool burns more rapidly with bright sparks.
- The iron filings produce intense sparks and burn almost instantly.
Show Answer
Increasing surface area increases the rate of reaction, as more particles are exposed and available to react with oxygen at once.
Show Answer
The greater the surface area, the more frequent and widespread the collisions with oxygen, leading to faster combustion.
Think Like a Scientist
Investigation: Investigating the Effect of Surface Area on the Rate of Reaction
In this investigation, you will explore how changing the size of calcium carbonate (marble) chips affects the rate of reaction with hydrochloric acid. You can measure the rate using one of three methods:
- Gas syringe
- Top pan balance (mass loss)
- Gas over water (measuring volume of gas collected)
Before starting the experiment, answer these questions:
Show Answer
Answers will vary – any of the three methods is valid if used consistently.
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The reaction with small chips should be faster due to their larger surface area.
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To ensure a fair test by changing only the surface area (independent variable).
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Dependent: rate of reaction (gas produced / mass lost)
Independent: surface area of calcium carbonate (chip size)
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Hydrochloric acid, conical flask, measuring cylinder, timer, gas syringe / balance / water trough, marble chips (2 sizes).
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Time (s) | Gas Volume (cm³) / Mass Loss (g)
Use separate columns for large and small chips.
Method:
Step 1: Assemble your apparatus.
Step 2: Add 5 g of large marble chips to 25 cm³ of hydrochloric acid in a conical flask.
Step 3: Start the timer and measure the gas produced or mass loss every 30 seconds.
Step 4: Repeat using 5 g of small marble chips.
Step 5: Plot both sets of results on the same graph.
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The line for small chips should be steeper (faster reaction).
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The small chips due to larger surface area.
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The rate increases because more particles are exposed for collisions.
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Yes, if smaller chips produced gas faster / greater mass loss.
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The reaction would be even faster due to the maximum surface area.