C2 Topic 5 - Chemical Reactions

Question 1

Explain what is meant by an endothermic reaction.

Answer 1

An endothermic reaction is one which overall takes in energy from the surroundings in the form of heat, shown by a fall in temperature.

The reactants require more energy to break their bonds than the products release to make new ones.

Examples include photosynthesis and ammonium nitrate dissolving in water.

Question 2

What are the four factors that affect the rate of a reaction?

Answer 2

Temperature
Concentration (or pressure for gases)
Catalyst
Surface area of a solid (or size of particle)

Question 3

What is a catalyst?

Answer 3

A catalyst is a substance which changes the speed of a reaction, without being used up in the reaction.

Question 4

Explain collision theory with each factor for rate of reaction.

Answer 4

Temperature:
Higher temperature -> more energy -> more kinetic energy -> particles move around faster and more -> higher probability of frequent collisions

Concentration:
Higher concentration -> more particles in a smaller volume -> less room to move without hitting other reactants -> collisions are more likely

Surface area:
Larger surface area -> more area for reactants to collide -> more frequent collisions

Catalyst:
A solid catalyst gives a surface for the reactants to stick to and react on. They increase the number of successful reactions by lowering the amount of activation energy needed to react.

Question 5

Explain what is meant by an exothermic reaction.

Answer 5

An exothermic reaction is one which overall gives out more energy to the surroundings in the form of heat, shown by a temperature rise.

The products release more energy making new bonds that the reactants used to break them.

Examples include burning fuels and explosions.