Heat of Combustion

Chemistry – Heats of combustion. To determine the heats of combustion of selected alkanols and to use this information to determine the heat of combustion for a longer chain alkanols.

Formula
ΔH=-mCΔT Heat of combustion
ΔH/n = molar heat of combustion.

Materials
-100 degrees thermometer
-glass stirring rod
-aluminium beaker
-electronic balance
-bosshead and clamp
-retort stand

Method
1) Set up the apparatus as shown above.
2) Measure the weight of aluminium can
3) Use the electronic balance to calculate the mass of water as 100 g
4) Pour the water in the aluminium can
5) Record the initial temperature of water, initial mass of the spirit burner using an electronic balance.
6) Adjust the height of the clamp so that the tip of the flame touches the aluminium can.
6) Light the alkanol for 3min.
7) Record the final mass of the burner, and final temperature.
8) Repeat the experiment for all 3 alkanols.

Validity:
Invalid, as we assumed complete combustion occurs when incomplete may occur, not all of the heat is transferring to the can, the energy was lost to the atmosphere.
Cause of differences in Theoretical and practical value
Energy loss to environment, equipment, incomplete combustion

Reliability:
Not reliable as practical has not been repeated with the same method and variables at least a minimum of 3 times.

Variables
Independent: The three alkanols
Dependent: The change in temperature of water, and change in mass of the corresponding spirit burner.
Controlled: Perform in SLC and use the same apparatus, height of flame above aluminium can, same amount of water.
Accuracy:
The experimental design is inaccurate due to the large loss of heat to the environment. It can be improved through:
-Use more accurate equipment, eg electronic balance that measures more sig figs minimise heat lost:
1) Use container that absorbs less heat for example copper
2) Positioning the beaker so that the blue part of the flame touches the bottom of the container
3) Ensuring experiment is performed in a well-ventilated area to provide sufficient oxygen for combustion
4) Stir water to ensure even distribution of heat.

Safety precautions
- Toxic fumes may be given off which is hazardous to your eyes use safety glasses and perform in fume cupboard to avoid inhaling.
- Alkanols are highly volatile, keep away from naked flames.

Miscellaneous:
Molar heat of combustion is the heat energy, in joules or kilojoules, released by the combustion of 1 mole of fuel.

Ethanol is mainly mixed as fuel to supplement petrol supplies. In the United States, ethanol is manufactured mainly by fermentation of corn and sugar cane residues. Sugar cane and wheat is fermented in Australia to produce ethanol. However, in Australia there is increasing acceptance of ethanol/petrol blends. The large scale production of ethanol is considered uneconomic, due to the huge cost of distillation and difficulty of removing all water from alcohol.

Disadvantages of using ethanol Statement | Description | Explanation | Ethanol has a lower heat of combustion than petrol | Combustion of ethanol produces less energy than combustion of same amount of octane. | Cars can travel further with the same amount of octane than ethanol thus ethanol is more expensive. | Existing car engines need to be modified if >10% ethanol is used in the fuel | It is difficult to remove all water during distillation of fermented biomass | Water causes corrosion to engines and fuel lines. | Large areas of land need | Land is needed to grow crops to make ethanol | This is a problem because land is also needed to grow food crops leading towards deforestation. | Lots of energy is used in the production of ethanol | Ethanol has to be distilled from the fermentation mix | This is a problem as distillation is energy intensive- energy is obtained by burning fossil fuels and also costs are high, making ethanol expensive. | Statement | Description | Explanation | Ethanol is produced from renewable resourse | Ethanol is produed through the fermentation of biomass whereas other fuels are produced from petroleum | Since ethanol is produced through the fermentation of biomass, It is a renewable resourse as they can be regrown | Ethanol burns more completely cleanly than other fossil fuels | Carbon monoxides is reduced by 25-30% and little or no carbon is produced less is deposited in the car and spark plugs last longer | The oxygen in ethanol molecule ensures that less oxygen is needed to allow complete combustion of the fuel
Ethanol is an excellent solvent, dissolving deposits built up in the engine | Carbon dioxide neutral | Net CO2 dioxide stays constant. The amount of CO2 used to produce crops for ethanol production is equivalent to the CO2 produced | Carbon dioxide is produced as ethanol burns. This is used in photosynthesis to make crops which is later converted to ethanol | 10 percent ethanol can be safely added to petrol | Ethanol acts as a petrol extender without damage to engines | No engine modification is required if only 10 percent is added. Thus making petrol supply last longer |

Advantages of using ethanol