Comparing the Enthalpy Changes of Combustion of Different Alcohols

Comparing the Enthalpy Changes of Combustion of Different Alcohols

Aim:

I will find the enthalpy change of combustion of a number of alcohols

so that I can investigate how and why enthalpy change is affected by

molecular structure of the alcohol.

Outline:

I will use Methanol, Ethanol, Propanol and Butanol in the experiment.

I will use these four because they should give me clear results, and

they range from short chained to long-chained hydrocarbons, so

patterns should be easy to spot in the conclusion.

Variables:

My independent variables are

- The type of alcohol (I will use ethanol, methanol, propanol and

butanol)

My dependant variables are

- the specific heating capacity of water

- Mass of water in grams

- Change in temperature of water

My control variables are

- The change in temperature of water- 20..C

- The mass of water. If this were changed then all the results would

be wrong, because it would take different amounts of energy to heat

different volumes of water

- The distance from the bottom of the calorimeter to the top of the

wick. Otherwise different amounts of energy could escape each time

making it an unfair test.

Plan:

From carrying out the follow experiment I will be able to find out the

enthalpy change of each fuel I am testing. The enthalpy change of

combustion of a fuel is the measure of energy transferred when one

mole of fuel burns completely. I will use the fact that 42 Joules of

energy are required to raise the temperature of 1 gram of water by 1

C. Enthalpy change varies according to the following conditions –

temperature, pressure and concentration of solutions. Therefore I will

use a standard pressure of I atmosphere, a standard concentration of 1

mol dm….3 and finally a specified temperature of…. I will use the

equation enthalpy change = cm…..T

c- Specific heating capacity of water

m- Mass of water in grams

…T- Change in temperature of water

I will then compare the enthalpy changes and the types of molecular

structures and so I will able to out why enthalpy change is affected

by the molecular structure of the alcohol

Prediction:

I predict that as the number of carbons in the alcohol is increased,

the enthalpy of combustion also increases. The enthalpy change of

combustion of alcohols increases as the length of the chain increases.

This can be explained if we compare one alcohol with the next in the

series. The chain length increases with an addition of a CH2 group.

When the alcohol is combusted what actually happens is the covalent

bonds are broken, and new bond are formed.