Unit 4: Application Exercises

Today’s topic is about energy.

These are the application questions cool


State whether these processes are endothermic or exothermic. Give a reason for each.

Burning a charcoal briquette.

Burning a charcoal briquette is an exothermic process because heat is generated.

Water evaporating from your skin

This is an endothermic process as heat is absorbed from the skin and used to break the inter molecular bond between the water molecules so that water can evaporate.

Ice melting 

Ice melting is an endothermic process as energy is absorbed to break inter molecular bonds between water molecues.


Chemical explosions are very exothermic reactions. Describe the relative bond strengths in the reactants and products that would make for a good explosion.

An energy level diagram showing an exothermic reaction

From the figure above we can see that the energy level of the reactants are higher than that of the products. Bond breaking is an endothermic process while bond forming is a exothermic process. Since the overall reaction is exothermic the energy released by bond forming would be greater than energy absorbed by bond breaking. Hence the bond strength of the products would be stronger.


How might you explain the differences between temperature and heat to a friend? Use some practical, everyday examples

In scientific terms, heat is the total measure of the motion of molecules in a substance while temperature is the average energy of molecular motion in a substance. However this doesn’t really tell you the difference

Consider a small cup of hot water and a large tub of hot water. Both have the same temperature. However the large tub of water has more heat. The large tub of water contains the molecules with the same energy as the small cup but since the large tub has more molecules it has more energy and hence more heat.

In short temperature is not energy but a measure of it. Heat is energy.


A premium gasoline available at most stations has an octane rating of 98. What does that tell you about:

a) the knocking characteristics of this gasoline?

b) whether the fuel contains oxygenates

An octane rating of 98 means that that the fuel acts similarly to a fuel that is 98% resistant to knocking.

The octane rating does not tell us any information about whether the fuel contains oxygenates.