Compare the enthalpy changes of combustion of different alcohols

Compare the enthalpy changes of combustion of different alcohols. To find the enthalpy changes of combustion of different alcohols and draw conclusions on how these values are affected by their molecular structure. I shall experiment with a minimum of five alcohols. Each alcohol I use will be from the homologous series of primary alcohols, e.g. propan-1-ol or pentan-1-ol. By adding a hydroxyl group on the first carbon each time this ensures that there is only one variable between my 5 alcohols, the carbon chain length.

Background Theory and Prediction:

Energy changes occur whenever a chemical reaction takes place. A reaction that gives out energy and heats the surroundings is called an exothermic reaction; a reaction that takes in energy and cools the surroundings is called an endothermic reaction. When finding the enthalpy changes of combustion of different alcohols, the chemical reactions that take place will be exothermic. This is because H is always –ve and heat is given out to the surroundings.

The standard enthalpy change of combustion, Hc is the enthalpy change that occurs when 1 mole of a fuel is burned completely in oxygen. In theory, the fuel needs to be burned under standard conditions, 1 atmosphere pressure and 298K. However, in practice this is impossible so I will burn the fuel in the normal way and make the necessary adjustments to allow for non-standard conditions. To measure the enthalpy change we use this relationship:

Energy transferred=cmT

Where c is the specific heat capacity of water (4.2J/g) and m is the mass of water used.

I predict that as the length of chain of carbons increase in an alcohol, the enthalpy change of combustion will increase e.g. heptan-1-ol will give out more energy than methan-1-ol. I predict this because as more bonds need to be broken and more bonds need to be formed. The intermolecular forces tend to be bigger as the chain increases. I will use the complete combustion of propan-1-ol and butan-1-ol as examples to explain my prediction.

Propan-1-ol                                                Butan-1-ol

C3H7OH + 4.5O2 → 3CO2 + 4H2O                        C4H9OH + 6O2 → 4CO2 + 5H2O

Bonds Broken for 1 mole:                                Bonds Broken for 1 mole

7x (C-H)                                                9x(C-H)

1x (C-O)                                                1x (C-O)

1x (O-H)                                                1x (O-H)

2x (C-C)                                                3x (C-C)

4.5x (O=O)                                                6x (O=O)

Bonds Formed for 1 mole                                Bonds Formed for one mole

6x (C=O)                                                8x (C=O)

8x (O-H)                                                10x (O-H)

Hc= -2021 kJ/mol-1        per mole                        Hc= -2676 kJ/mol-1        per mole

This shows that the enthalpy change of combustion has increased by 655kJ/mole due to the extra carbon atom being added to propan-1-ol to form butan-1-ol. The difference between the two alcohols is that there are 2 more C-H bonds, 1 more C-C bond and 1.5 more O=O bonds to be broken. There are also 2 C=O bonds and 2 O-H bonds to be formed. The double bonds between O=O and C=O need much more energy to break them because they have a lot more energy. This also makes the total enthalpy change higher as the number bonds that need to be broken and formed increases.

I predict that as the carbon chain increases then the enthalpy change of combustion will also increase. I can use the enthalpy change of alkanes to find a relationship with the enthalpy change of alcohols. I think that if the enthalpy change of combustion applies to alkanes then it will apply to alcohols also.

Alcohols have similar properties to their respective alkanes. Alcohols are obtained from alkanes by replacing a hydrogen atom with a hydroxyl group (-OH).

Propane                                                        Propan-1-ol

Enthalpy change of                                                Enthalpy change of combustion= -2220kJ/mol-1                                        combustion= -2021kJ/mol-1

Octane                                                        Octan-1-ol

Enthalpy change of                                                 Enthalpy change of

combustion= -5470kJ/mol-1                                        combustion= -5294kJ/mol-1

From this analysis we can see that the alcohols have slightly less enthalpy change of combustion than their equivalent alkanes. The reason for this is due to the added hydroxyl group (-OH), this means that one of the bonds of oxygen has already been made so there is less energy needed as there are less bonds to be broken. This is the same for both Propane/Propanol and Octane/Octanol. There is a slightly larger difference in enthalpy change of combustion for the Octane alkane and alcohol than there is with the Propane Alkane and alcohol (the difference between Hc for Propane and Propanol is 199kJ/mol-1, for Octane and Octanol it is 176kJ/mol-1). This is only a slight difference. I will draw a graph to show the relationship of enthalpy change of combustion for alcohols and alkanes.

From the graph, we can see that alcohols and alkanes both follow the same trend that as the number of carbons increases in the chain, so does the enthalpy change of combustion.

List of Alcohols to choose from:

• Methan-1-ol
• Ethan-1-ol
• Propan-1-ol
• Propan-2-ol (I will not use this because it is not a primary alcohol. It has different chemical properties to the other alcohols although it has the same molecular formula as propan-1-ol.
• Butan-1-ol
• Pentan-1-ol

In a primary alcohol, the carbon which carries the (-OH) group is only attached to one alkyl group. An alkyl group is a group such as CH3 methyl or CH3CH2 ethyl. The structural diagram of a primary alcohol is as follows.

Equipment

• A minimum of 5 spirit burners containing the various alcohols that I will chose from. They must be primary alcohols so there is only one variable that will change between them.
• 1 copper with at least 500 ml capacity for the water, used as a calorimeter.
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