AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
The use of a support medium to maximise the surface area of a…
Full specification wording
Catalysts (A-level only): The use of a support medium to maximise the surface area of a heterogeneous catalyst and minimise the cost.
Heterogeneous catalysis happens only on the surface, so metal buried inside a lump is wasted.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 3 of 7
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Heterogeneous catalysis happens only on the surface, so metal buried inside a lump is wasted. The fix is to spread a thin layer of the metal over an inert support with a very large surface area. A catalytic converter uses a ceramic honeycomb coated with platinum, palladium and rhodium; the honeycomb contributes nothing chemically but gives an enormous area over which the gases can meet the metal. Two benefits follow. The number of active sites per gram of metal rises sharply, so the reaction is faster for the same mass of catalyst. And because far less of an expensive metal is needed, the cost falls, which is the reason converters are affordable at all given the price of rhodium.
What earns the marks
- Stating that the support increases the surface area of the catalyst.
- Linking greater surface area to more active sites and so a faster rate.
- Stating that less of the expensive metal is needed, so reducing cost.
- A named example, such as the ceramic honeycomb in a catalytic converter.
Common mistakes and how to put them right
Saying the support itself catalyses the reaction, when it is chemically inert.
Explaining the cost saving by the support being cheap rather than by less metal being used.
Confusing the support medium with a promoter, or with the catalyst being poisoned.
Claiming a larger surface area lowers the activation energy, when it increases the number of sites.
Exam tips
- Give both reasons, surface area and cost, because questions usually credit each separately.
- Name a real support such as a ceramic honeycomb rather than saying something to hold it.
- Say the support is inert if asked whether it takes part in the reaction.
Try the lesson
The start of the Catalysts (A-level only): The use of a support medium to maximise the surface area of a heterogeneous catalyst and minimise the cost. lesson, made from checked teaching notes for this specification point.
Have a go first
Why is a heterogeneous catalyst often spread thinly over a support medium?
Listen
Heterogeneous catalysis happens only on the surface, so metal buried inside a lump is wasted. The fix is to spread a thin layer of the metal over an inert support with a very large surface area. A catalytic converter uses a ceramic honeycomb coated with platinum, palladium and rhodium; the honeycomb contributes nothing chemically but gives an enormous area over which the gases can meet the metal. Two benefits follow. The number of active sites per gram of metal rises sharply, so the reaction is faster for the same mass of catalyst. And because far less of an expensive metal is needed, the cost falls, which is the reason converters are affordable at all given the price of rhodium. A strong answer earns marks for these points. stating that the support increases the surface area of the catalyst. linking greater surface area to more active sites and so a faster rate. stating that less of the expensive metal is needed, so reducing cost. a named example, such as the ceramic honeycomb in a catalytic converter. Exam tip: Give both reasons, surface area and cost, because questions usually credit each separately.
Key words
Say what each word means before you turn the card.
support medium
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active sites
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catalytic converters
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See every definition
- support medium
- An inert material with a very large surface area over which a thin layer of metal is spread, contributing nothing chemically.
- active sites
- The places on the surface of a heterogeneous catalyst where the reaction happens, and their number per gram of metal rises sharply with a support.
- catalytic converters
- Devices using a ceramic honeycomb coated with platinum, palladium and rhodium, giving an enormous area over which the gases can meet the metal.
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