1.1. Three of the factors that can influence the rate of a chemical reaction are:

• physical state of the reactants

• light

• the presence of a catalyst

The first recorded dust explosion was in a flour mill in Italy in 1785. Flour contains carbohydrates. Explosions are very fast exothermic reactions.

i) Use the collision theory to explain why the reaction between the particles of flour and the oxygen in the air is very fast. 

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 ii) Write a word equation for this exothermic reaction. 

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1.2. The decomposition of silver(I) bromide is the basis of film photography. The equation for this decomposition is:

This reaction is photochemical.

A piece of white paper was coated with silver(I) bromide and the following experiment was carried out.

Explain the results.

1.3. The fermentation of glucose is catalysed by enzymes from yeast. Yeast is added to aqueous glucose, the solution starts to bubble and becomes cloudy as more yeast cells are formed.

C₆H₁₂O₆ (aq) → 2C₂H₅OH (aq) + 2CO₂ (g)

The reaction is exothermic.

Eventually the fermentation stops when the concentration of ethanol is about 12%.

 i) What is an enzyme? 

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 ii) Pasteur said that fermentation was respiration in the absence of air. Suggest a definition of respiration. 

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 iii) On a large scale, the reaction mixture is cooled. Suggest a reason why this is necessary. 

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 iv) Why does the fermentation stop? Suggest two reasons. 

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v) When the fermentation stops, there is a mixture of dilute aqueous ethanol and yeast. Suggest a technique which could be used to remove the cloudiness due to the yeast. 

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 vi) Name a technique which will separate the ethanol from the ethanol/water mixture. 

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