Calculating the DHc of the combustion of alcohols.

Sam Forshaw

Calculating the ΔHc of the

combustion of alcohols.

Aim:

To work out the ΔHc of the alcohols: ethanol, methanol, pentan-1-ol, butan-1-ol and propan-1-ol by using calorimetry. The idea in this experiment is that if you burn a substance to heat water there is a link between the rise in temperature of the water and the amount of energy the substance contains. For this you need to know the amount of the substance you have burned and the volume and mass of the water you are heating.

For this experimant I wil use a copper container for the water, place an aluminium lid on top (to prevent heat loss) and a spirit lamp containing the alcohol that I will be burning. There are however problems with this experiment. Not all of the heat from the reaction will go into the water, much of this energy will be lost to the surroundings and the copper container and aluminium also absorb some of the heat and rise in temperature also. Both of these factors greatly reduce the amount of heat energy produced by the combustion of the alcohol that ...

This is a preview of the whole essay

The standard definition for enthalpy change is ‘Enthalpy change is the amount of heat energy taken in or given out during any change in a system, provided the system is kept at constant pressure’. Standard enthalpy changes (ΔHθ) are these that occur under standard conditions of 100kPa, 298K and 1 mol dm-3 and is defined as: ‘The enthalpy change for a reaction under standard conditions at 298K and 100kPa, with all substances in their standard state, is called the standard enthalpy change of reaction’.

Standard Enthalpy change of combustion ΔHcθ is the one I am interested in for this experiment. This is defined as ‘The enthalpy change when 1 mole of a substance undergoes complete combustion in excess oxygen, under standard conditions of 298K and 100kPa’. Standard enthalpy of combustion is usually negative. Combustion means completely burned in oxygen. The specific heat capacity of a substance is the temperature of one gram of the substance by 1K this is measured in Jg-1K-1 eg, the specific heat capacity of water is 4200 Jg-1K-1. The energy transferred to a body/substance can be calculated by using the equation:

Energy transferred = mass x specific x temperature change

heat

capacity

Experimental methods for determining the enthalpy changes in combustion reactions such as this often and typically give values for the ΔHc lower than the actual values due to the amount of energy lost during the experiment.