# Comparing the Enthalpy Changes of Combustion of Different Alcohols

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Comparing the enthalpy changes of combustion of different alcohols

Planning

What are alcohols?
==================

Alcohol is the common family name for the hydrocarbon group alkanols.
They are part of a homologous series. At least one of the hydrogen groups in the molecule is replaced by an OH group.

GRAPH

Propane
Propan-2-ol

They are all organic compounds.

The general formulas for the alcohols are:

CnH(2n+1)OH

Where n represents a number.

The first and simplest member of the alkanols family is methanol. Its molecular formula is CH3OH.

You can now see that each member of the alcohol family has a different number of carbons in its structure. They increase by one carbon atom and two
…show more content…
So as the number of carbon atoms increases the enthalpy of combustion will become more negative. I think this because as you add a H-C-H bond you increase the energy required to break the bonds by 826KJ/Mol.
This is worked out by the average bond enthalpy of the C-H bond, which is 413kJmol-1 being multiplied by two because there are two extra C-H bonds in the molecule.

Using this I will predict the enthalpy change of combustion value for methanol. To do this I will use the equation

CH3OH(l) + 1½O2 O(g) † CO2(g) + 2H2O(l)

In order to make this easier to work out and for people to understand
I will multiply everything by 2 so that I don’t have any values in the equation. This gives the new equation of;

2CH3OH(l) + 3O2 O(g) † 2CO2(g) + 4H2O(l)

On the left hand side of this equation you have

2 x (3 x C-H(413))

2 x (1 x C-O(358))

2 x (1 x O-H(464))

3 x (1 x O=O(498))

In total the total energy required to break the bonds is

(6 x 413 kJmol-1) + (2 x 358 kJmol-1) + (2 x 464 kJmol-1) + (3 x 498 kJmol-1)
=

2478 kJmol-1 + 716 kJmol-1 + 928 kJmol-1 + 1494 kJmol-1 = 5616kJmol-1

On the right hand side of the equation

2 x (2 x C=O(-805 kJmol-1))

4 x (2 x O-H(-464 kJmol-1))

In total the total energy required to form the bonds is

(4 x -805 kJmol-1) + (8 x -464 kJmol-1)

-3220 kJmol-1 + -3712

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