Comparing The Enthalpy Change Of Combustion Of Different Alcohols

Comparing The Enthalpy Change Of Combustion Of Different Alcohols

The aim of my experiment is to investigate the enthalpy of combustion

of a range of alcohols. The standard enthalpy of combustion is the

enthalpy change that occurs when 1 mole of a fuel is burned completely

in oxygen under standard conditions – 1 atmosphere pressure and 298K.

All combustion reactions are exothermic which is why I am expecting

all the values for the enthalpy change of combustion to always be

negative.

Prediction

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I predict that the greater the number of carbon atoms there are in the

alcohol molecule, the greater the enthalpy change of combustion it

will have. This is because in a chemical reaction energy is needed in

order to break bonds and energy is released when bonds are made. If I

were to look at these two values; the energy required to break the

bonds of the reactants and the energy released when the bonds of the

product are made then I am able to work out the enthalpy change of

combustion of the alcohol. In order to do this we need to know the

bong enthalpies of the bonds in the process. The quantity of energy

needed to break a particular bond in a molecule is called the bond

enthalpy. Below is a table containing the bond enthalpies of the bonds

that occur in the combustion of an alcohol.

Bonds

We use the average bond enthalpies as the exact value of a bond

enthalpy depends on the particular compound in which the bond is

found.

GRAPH

DH1

By looking at the equation for the reaction that occurs when an

alcohol burns, we realise that the reaction involves both breaking

bonds and making new ones. From the equation of the process we can

tell how many of the different number of bonds are broken and made and

so work out the enthalpy change of combustion. We can work out the

enthalpy of combustion of methanol by using the bond enthalpy values:

CH3OH + 1.5O2

CO2 + 2H2O

We can use the enthalpy cycle above to work out the value for the

enthalpy change of combustion of methanol, represented by DH1. The

calculation is done as shown below:

DH2 = enthalpy change when bonds are broken

= 3 ´ E(C-H) + 1 ´ E(C-O) + 1 ´ E(O-H) + 1.5 ´ E(O=O)

= 3(413) + 358 + 464 + 1.5(498)

= 2808 kJ mol-1

DH3 = enthalpy change when bonds are made