Investigating the Enthalpy Change of Combustion for Various Alcohols

Investigating the Enthalpy Change of Combustion for Various Alcohols

P l a n n i n g

Aim: To investigate the enthalpy change of combustion for various

alcohols

Alcohols will be burnt to heat up water. The aim is to find out how

much energy is produced when burning the following alcohols : ethanol,

propanol and butanol. Alcohols react with oxygen in the air to form

carbon dioxide, water and energy is liberated (exothermic reaction),

because the reactants' energy is higher than that of the products'.

The alcohols form a series of compounds which are related to each

other. This is called a homologous series and has the formula : Cn H

2n+1 OH

In this investigation the alcohols used were: ethanol, propanol and

butanol.

Ethanol : C2H5OH

Propanol : C3H7OH

Butanol : C4H9OH

Combustion Reactions:

Ethanol :

[IMAGE]C2H5OH + 3O2 2CO2 + 3H2O + heat

Propanol :

[IMAGE]C3H7OH + 4.5O2 3CO2 + 4H2O + heat

Butanol:

[IMAGE]C4H9OH + 6O2 4CO2 + 5H2O + heat

Heat (joules) = T x mass (g) x specific heat capacity

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Heat energy = rise in temperature x mass of water (g) x 4.18

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The specific heat capacity is the number of joules required to raise

the temperature of 1g of water by 1oC

(1g of H2O raised by 1oC = 4.18 joules)

The combustion of the alcohols is not exactly as shown in the equation

because some carbon is deposited on the calorimeter base ie.

[IMAGE]C2H5OH + O2 2C + 3H2O

Additionally, some carbon monoxide is produced ie.

[IMAGE]C2H5OH + 2O2 2CO + 3H2O

Therefore the equation provided for the complete combustion is an over

simplification.

Prediction:

When bonds are broken, energy is absorbed by the system (endothermic

process); and when bonds are formed, energy is released to the

external environment (exothermic process). When alcohols are burnt,

the reaction is always overall exothermic. Therefore more energy is