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

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

Planning

Introduction

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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.

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Prediction And Theory

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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