Heat of Combustion

A series of free IGCSE Chemistry Activities and Experiments (Cambridge IGCSE Chemistry).

Heat of Combustion
This activity compares the energy given out by liquid fuels by measuring the mass of each fuel that will heat a given volume of water to a given temperature. The fuels are burnt in a spirit burner which is weighed before and after heating the water.

In general, the heat of combustion depends on the number of carbon atom. The higher the number of carbon atoms, the higher the heat of combustion. Also, the heat of combustion is greater for the hydrocarbons than for the alcohols.

Method
First draw up a table to record the temperature change and the masses of the spirit burner before and after heating the water in the calorimeter.

1. Put 25 cm³ of water into the calorimeter or beaker.
2. Measure the temperature of the water.
3. Weigh the spirit burner containing methanol.
4. Light the burner. Stir the water with the thermometer and keep checking the temperature.
5. When the temperature is just about to reach 30 °C, put out the flame.
6. Weigh the spirit burner again, immediately (before any fuel is lost by evaporation).

Calculate the temperature rise per gram of methanol burnt.

Questions

1. Heat losses are always a consideration in this type of experiment. How can these be reduced?
2. What factors need to be controlled to make sure the comparison between fuels is reliable?

Answers

• Show Answers
  1. A shield could be put around the area of the lab bench where the experiment was being carried out to reduce draughts.
     Polystyrene cup can be used instead of beaker to reduce heat loss.
  2. The volume of water in the beaker. The distance between the flame and the beaker. The level of wick in the burner.

Energy Changes and Calorimetry
Enthalpy Changes
All substances contain chemical energy, called enthalpy. It is measured in Joules. When reactions happen, energy is given out or taken in - these are enthalpy changes.

Endothermic and exothermic reactions
In exothermic reaction, chemical energy (enthalpy) is being turned into heat energy which is transferred to the surroundings, so the temperature we measure increases.
In endothermic reaction, heat energy is taken from the surroundings and converted into chemical energy (enthalpy), so the temperature decreases, or we have to heat the reaction constantly to make it work.

When a reaction takes place, bonds in the reactants are broken, and new bonds formed in the products.
The energy required to break all the necessary bonds in the reactants is called the activation energy.
In an exothermic reaction less energy is used to break the bonds in the reactants than is released when the bonds in the products are formed.
In an exothermic reaction more energy is used to break the bonds in the reactants than is released when the bonds in the products are formed.

A catalyst increases the rate of reaction without being used up itself. It does this by lowering the activation energy for the reaction, by providing an alternative pathway. On the catalyst surface the reactants have their bonds weakened, so that less energy is needed to break these bonds.

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