An Investigation into the Enthalpies of the Combustion of Alcohols

An Investigation into the Enthalpies of the Combustion of Alcohols

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Planning

My aim for this experiment is to see the energy produced from

different alcohols. This investigation involves burning alcohol in the

air. ‘GCSE Chemistry’ by B.Earl and L.D.R Wilford says that "alcohols

form, another homologous series, with the general formula Cn H2n+1OH

". The alcohol reacts with the oxygen in the air to form the products

water and carbon dioxide.

This reaction is exothermic, as heat is given out. This is because the

amount reactant energy is more than the product energy the difference

between this is ΔH, therefore some energy has been given out in the

form of heat. The energy is given out when forming the bonds between

the new water and carbon dioxide molecules. This can be shown in an

energy level diagram: Reaction co-ordinate ΔH is the heat content,

which is the enthalpy, which is negative in exothermic reactions as

the diagram shows that energy is 'lost' as heat. Enthalpy is defined

as the energy of reaction, or the heat energy associated with a

chemical change. For any reaction carried out directly at a constant

pressure, the heat flow is exactly equal to the difference between

enthalpy of products and that of the reactants.

To measure the heat energy given off, we must use this energy to heat

something, this will be water. This is assuming that all the heat

produced by combustion of the alcohol will equal the amount of heat

absorbed by the water (q). So I will measure the amount of energy

required to do so. This can be worked out by using the formula: q =

mass x specific heat capacity x temperature rise or q = MCΔH. Where q

is the quantity of heat. The specific heat capacity is the amount of

energy required to heat the substance, and is calculated using the

formula: q =energy supplied/ Mass x Temperature, where q is the

enthalpy, c is the specific heat capacity and ΔH is the temperature

rise. I chose to use water as it is safe, easily obtainable, and has a

constant, reliable specific heat capacity of 4.2J/°C.

The bonds which are made in a exothermic reaction are forces of

attraction between the atoms or ions in a substance. These can be of

two types: covalent, in which the atoms share electrons. Examples of

this are water and carbon dioxide, whi...

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temperature reading to heat the 100cm3 of water.

The procedure of the experiment was good enough of finding out the

energy gained from the individual alcohols, but changes could have

been made to make the reliability of the results more accurate. This

could have been done by using a better and more accurate thermometer,

such as an electric thermometer, which have gave me a much more pin

point set of results.

If I were to do this experiment again I would make a number of

improvements or changes, I could use a Bomb Calorimeter which

submerges the reaction inside an insulated container of water. An

electrical heating device starts the reaction inside a sealed reaction

vessel and the temperature rise of the water which surrounds it is

measured. Bomb calorimeters are often used to find the calorific value

of foods.

Looking at the experiment I did I thought we could have extended the

results we achieved by using different flame lengths, this would have

told us how much the alcohol uses its energy when the flame is

increased. I could have also changed the amount of water in the copper

calorimeter, or even change the concentration of alcohol to see the

effect.