Investigating How the Relative Molecular Mass Changes in Heat Combustion of an Alcohol:

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Investigating How the Relative Molecular Mass Changes in Heat Combustion of an Alcohol



As alcohol burns in air it gives out energy as heat and light. I am going to investigate how the energy output of an alcohol in combustion changes, with increased relative molecular mass, or RMM. RMM is the sum of the atomic masses of every atom in the molecule. Using the alcohols: Methanol, Ethanol, Propan-1-ol, Butan-1-ol and Pentan-1-ol, I will plan, and complete an experiment that tests the prediction below.


Prediction And Theory

In the combustion of alcohols in air, the alcohol reacts with oxygen molecules, to create carbon dioxide and water. Many bonds are broken in the process using up energy. At the same time, the atoms reforming into the new molecules of carbon dioxide and water give out energy. In the combustion of alcohols, the energy created, when forming bonds will always be more that what is lost, when breaking bonds, this gives us excess energy. This energy is given out primarily as heat, but also as light and sound. As energy is given out it is called an exothermic reaction. If the opposite were true, it would be an endothermic reaction. It is never possible to calculate exact energy change by experimentation due to inaccuracies and energy waste, so we use bond energy calculations give the exact theoretical energy change.

Bond energy calculations show that the higher the RMM the more energy will be produced for the same weight of fuel (RMM is the sum of the atomic masses of every atom in the molecule). This is because as the RMM increases there are more atoms and therefore, more bonds to be broken and then made. As, when burning alcohols, this process gives out energy, the more bonds go through this process, ie as the RMM increases the more energy should be released. The calculations also suggest that for every carbon atom you add to the chain of an alcohol the energy out should increase by

618 Kj/mol. I predict then, that as the RMM goes up then the energy change will get increasingly more negative i.e. more energy is given off. The RMM will be proportional to the final energy created as both should increase by the same number each time, (RMM by 14 as one C and 2 H atoms are added, and the energy out by 618KJ/mol). This will therefore result in a straight-line on the graph. The bond energy calculations show how much energy should be released, accounting for experimental inaccuracies however, I expect the experimental output to be considerably less.

Proposed Method

I am going to test how the energy output per mole in the combustion of alcohols with increasing RMM. I need, therefore, to be able to measure the energy given out in combustion and then divide that by the amount of moles used. As the majority of energy given out is in the form of heat energy, I will attempt to measure the heat energy given off. If I place a beaker full of water over the burning alcohol then some of the energy given off in combustion will be transferred to this water. By measuring the rise in temperature of the water it is possible to calculate, how much energy was transferred to the water from the combustion of an alcohol.

Energy released (J)=Specific heat capacity (J) x Mass Of Water (ml) x Rise in Temp ( C)

The specific heat capacity is the amount of energy it takes to raise the temperature of 1ml of a specific substance by 1 C. I chose to heat water as it has a reliable s.h.c., of

4.18 J, and because it is safe and cheap. The mass of water will stay constant at 200ml, and we plan to stop heating when the temperature has risen by 40 C. This means that in every case, the energy used (J) = 4.18 x 200 x 40

= 33440 J

= 33.44 KJ

To give us the energy output per mole, we then need to find out how many moles...
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