A catalyst is a substance that increases the rate of a reaction without being used up. A catalyst works by providing an alternative route with a lower activation energy.

Describe the steps of chemisorption on the surface of a solid catalyst.

Reactant molecules adsorb onto active sites and form weak bonds with the surface. Bonds within the reactants weaken and break, new bonds form, and the products desorb.

Why must a good heterogeneous catalyst adsorb reactants neither too strongly nor too weakly?

Too weak an adsorption holds reactants too loosely for their bonds to weaken. Too strong an adsorption stops the products desorbing, blocking the active sites.

Why are industrial solid catalysts used as fine powders, pellets or gauzes?

A finely divided form gives a very large surface area of active sites. More reactant molecules can adsorb at once, so the rate is higher.

CCEA A-Level Life and Health Sciences 0008

Experimental Techniques

Topic overview Standard solutions, titrations and qualitative tests Rates of reaction and organic preparations Physics measurements: oscillations, resistivity, lenses and critical angle Biological tests, colorimetry, chromatography and osmosis

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Physical Chemistry in Industrial Processes

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Analytical Chemistry TechniquesDouble Award

Topic overview Volumetric analysis in industry and healthcare Applied titrations of vinegar, indigestion tablets and bleach Redox titration of iron tablets and evaluating titrations Principles of colorimetry Colorimetric analysis of copper ore, aspirin and iron Thin-layer chromatography and its uses Visualising chromatograms and identifying painkillers and pigments Calorimetry and combustion Molar enthalpy and bond energy calculations Qualitative tests for cations and gases

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Physical Chemistry in Industrial Processes

Catalysts, chemisorption and catalytic converters

3 things to know
AS 3

Overview

How catalysts change the reaction profile, heterogeneous catalysts and chemisorption, catalyst surface area, catalytic converters and poisoning, the Haber and Contact catalysts, and the economics of catalysts.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Define a catalyst.

Key term

How does a catalyst change the reaction profile of a reaction?

Key term

What is a heterogeneous catalyst?

Questions from this subtopic

Each one with the model answer underneath.

1
Define a catalyst.
Model answer
A catalyst is a substance that increases the rate of a reaction without being used up. A catalyst works by providing an alternative route with a lower activation energy.
2
How does a catalyst change the reaction profile of a reaction?
Model answer
A catalyst lowers the peak of the profile, reducing the activation energy. The energies of the reactants and products, and so ΔH, stay the same.
3
What is a heterogeneous catalyst?
Model answer
A heterogeneous catalyst is in a different phase from the reactants. A solid catalyst acting on gases is the commonest industrial example.
4
Describe the steps of chemisorption on the surface of a solid catalyst.
Model answer
Reactant molecules adsorb onto active sites and form weak bonds with the surface. Bonds within the reactants weaken and break, new bonds form, and the products desorb.
5
Why must a good heterogeneous catalyst adsorb reactants neither too strongly nor too weakly?
Model answer
Too weak an adsorption holds reactants too loosely for their bonds to weaken. Too strong an adsorption stops the products desorbing, blocking the active sites.
6
Why are industrial solid catalysts used as fine powders, pellets or gauzes?
Model answer
A finely divided form gives a very large surface area of active sites. More reactant molecules can adsorb at once, so the rate is higher.

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