I am going to investigate the difference in enthalpy of combustion for a number of alcohols, the enthalpy of combustion being the'enthalpy change when one mole of any substance is completely burnt in oxygenunder the stated conditions'.

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I am going to investigate the difference in enthalpy of combustion for a number of alcohols, the enthalpy of combustion being the 'enthalpy change when one mole of any substance is completely burnt in oxygen under the stated conditions'. I will be attempting to find how the number of carbon atoms the alcohol contains effects the enthalpy change that occurs during the combustion of the alcohol.

Method

I plan to measure the enthalpy change by burning the alcohol, using a spirit burner, I will then use the heat produced during the combustion of the alcohol to heat 100ml of water that will be situated in a copper calorimeter directly above the burning alcohol. The calorimeter is made of copper as copper has a high thermal conduction value, this basically means that it is a good conductor of heat so a lot of the heat the copper receives will be passed on to the water which I am then able to measure, by using a calculation that it takes 4.2J of energy to heat 1g of water by 1 c

I plan to compare the enthalpy changes of combustion of different alcohol´s. As I know that combusting different substances produces differing amounts of energy, I plan to find out which alcohol, out of methanol, ethanol, propanol and butanol, produces the most energy when burned in air. I will do this by heating 100ml of water by 50?C and using the calculation of it takes 4.2j of energy to heat 1g of water by 1?C. To do this I will need:

During the experiment I will be taking a number of measurements, I will firstly take the initial temperature of the water and initial mass of the alcohol I will then burn the alcohol until an increase in temperature of 20oc has occurred in the water I will then reweigh the alcohol.

The measurements

* Mass of alcohol burned (g)

* Temperature increase (oc)

will tell me what mass of alcohol is used during combustion to cause the temperature increase of 20oc in the water, I can then work out the energy released per mole and compare these values and see which has the highest enthalpy of combustion. I will need to repeat my experiment a number of times and take an average so I am sure of an accurate result.

The set up of the apparatus that I plan to use is shown below

The set up of the apparatus as you can see is very simple, the calorimeter, which contains the 100ml of water, is held directly above the spirit burner by a retort stand and clamp. The calorimeter has a mercury thermometer in it, which are very accurate, this will be used to measure the water temperature. I have decided that the calorimeter should be held 1cm above the top of the flame produced by the burning alcohols as so to keep the experiment fair, this being as apposed to having it at a random height. I have also decided that the size of the wick from which the alcohol burns from should be constant on all the spirit burners, so to keep the experiment as fair as possible so I will adjust them accordingly so they are all the same length. I have decided that the length should be should be one cm, I will do this so that all the alcohols burn from the same surface area, this will mean that I will also have to use wicks of the same thickness. The experiments will be taking place in a laboratory so this means that the environment each experiment takes place in should be pretty constant i.e. room temperature etc, this will also help improve my results.

Prediction

I am expecting that the alcohols with a greater number of carbon atoms within the molecule to have a higher enthalpy of combustion than the ones with less.

For any reaction to take place bonds must be broken and made, bond breaking requires energy while bond making releases energy. Bonds between different atoms require or release different amounts of energy when broken or made because they are different in strength. By looking at the equation for the reaction and more importantly looking at the bonds that are being broken and made, it is possible to work out an estimation for the amount of energy that will be released in the reaction. The estimation is worked out by applying the average bond enthalpies, an example for doing this is shown below for methanol

Methanol (CH3OH)

The balanced equation for the combustion of methanol is

CH3OH(l) + 1.5 O2(g) CO2(g) + 2H2O(l)

Below is the type and number of bonds within each mole of reactants and products, they are shown with the amount of energy, measured in kilo joules per mole (DH/KJ mol-1) released or required for the particular bond

Methanol Oxygen Carbon Dioxide Water

CH3OH 1.5 O2 CO2 2H2O

3 C__H 413 1.5O=O 497 2C=O 740 4O__H 463

C__O 360

O__H 463

2062 745.5 1480 1852

The total energy required to break The total energy released in the forming the bonds in the reactants is of the bonds in the products is

2807.5 DH/KJ mol-1 3332 DH/KJ mol-1

the difference between the reactants and products is

-524.5 DH/KJ mol-1

From above, there are 2807.5 kJ mol-1 of energy absorbed initially by the reaction when the bonds are broken. Then 3332KJ mol-1 of energy is released by the reaction when the new bond are formed, overall this leaves a difference of 524.5 kJ mol-1 between the reactants and products, this energy is released by the reaction in the form of heat energy.

The value above for the energy released by the alcohol is only an approximation for the combustion of methanol this is because firstly the bond enthalpies vary slightly from one molecule to another and so the values used are only an average. The values given for the bond enthalpies also assume that the reactants and products are in a gaseous state but as you can see from the equation they are clearly not, with the water and the alcohol's both being in a liquid state.

The alcohols that I plan to use in my investigation are methanol, ethanol, propanol and butanol. The estimation for the enthalpy of combustion, using the bond enthalpies are worked out below for each of the alcohols.

Methanol CH3HO(l) + 1.5O2 (g) CO2(g) + 2H2O(l)

3 C__H 413 1.5O=O 497 2C=O 740 4O__H 463

C__O 369

O__H 436

2062 745.5 1480 1852

2807.5 DH/KJ mol-1 3332 DH/KJ mol-1

-524.5 DH/KJ mol-1

Ethanol C2H5HO(l) + 3O2 (g) 2CO2(g) + 3H2O(l)
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C__C 346 3O=O 497 4C=O 740 6O__H 463

5 C__H 413

C__O 369

O__H 436

3234 1491 2960 2778

4723 DH/KJ mol-1 5738 DH/KJ mol-1

-1013 DH/KJ mol-1

Propanol C3H7HO(l) + 4.5O2 (g) 3CO2(g) + 4H2O(l)

2 C__C 346 4.5O=O 497 6C=O 740 8O__H 463

7 C__H 413

C__O 369

O__H 436

4406 2236.5 4440 3704

6642.5 DH/KJ mol-1 8144 DH/KJ mol-1

-1501.5 DH/KJ mol-1

Butanol C4H9HO(l) + 6O2 (g) 4CO2(g) + 5H2O(l)

3 C__C 346 ...

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