The Combustion of Alchohols

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The Combustion of Alchohols

Chemistry Coursework

Introduction

In order to carry out this investigation, I first have to define the terms of the objectives so that an understanding is present and a clear process to acquire the answer can be devised.

What are alcohols?

Alcohols are a specific group of substances that are hydroxyl derivatives of hydrocarbons but more explicitly they are types of fuel. A fuel is a substance (e.g. methane) that readily undergoes combustion (the combustion of a substance involves its reaction with oxygen and the release of energy) and gives out large amounts of energy. The combustion of fuels is usually an exothermic reaction.

The distinction between alcohols and other fuels, such as methane, is that alcohols are a homologous series of compounds that contain the -OH group of atoms as the functional group. The functional group in a molecule is the group of atoms within the structure that determines the characteristic reactions of that substance. All the earlier, smaller alcohols have a neutral pH, and are colourless liquids that do not conduct electricity. The general chemical structure of an alcohol is as follows:

CnH(2n+1)OH

By far the most well known alcohol is ethanol, which often goes by the common name of alcohol itself.

Its chemical formula is C2H5OH. It is produced in industry by reacting ethene and steam together. They are first compressed to 60 atmospheres and passed over a catalyst (immobilised phosphoric(v) acid) at 300ºC.

300ºC, 60 atmospheres

ethene + steam -------› ethanol

phosphoric acid

C2H4(g) + H2O(g) --› C2H5OH(g)

It can also be produced via fermentation because ethanol and CO2 are the natural waste products of yeasts when they ferment sugar. However, ethanol is toxic to yeast so once the ethanol concentration has reached around 14% or the sugar runs out then the multiplying yeast die and the fermentation ends. The Babylonians and Egyptians discovered this fermentation and found the product of crushed grapes (also containing sugar) would be a drink with a kick.

yeast

glucose --› ethanol + carbon dioxide

enzymes

C6H12O6(aq) ---› 2C2H5OH(aq) + 2CO2(g)

This alcohol is not always used as a fuel for combustion but also has an array of different purposes in industry and everyday life. It is an important solvent and raw material in making many other organic chemicals. Many everyday items such as paints, glues, perfumes and aftershaves use ethanol as a solvent, and notoriously for alcoholic drinks. Ethanol can be safely consumed but must only be drunk in moderation. Mediocre amounts of alcohol seem to reduce coronary heart disease, however, heavy drinking can damage muscle tissue of the heart and eventually cause liver failure and death. Methanol is very toxic and even in small amounts can cause blindness and death.

Coming back to the idea of alcohols as a fuel; when fuels are combusted the reaction with oxygen produces two products - carbon dioxide and water as well as energy from the exothermic reaction. So:

fuel + oxygen --› carbon dioxide + water

and replacing the fuel with the alcohol gives:

alcohol(CnH2n+1OH) + oxygen(O2) --› carbon dioxide(CO2) + water(H2O)

What am I going to study?

The aim of the investigation is to work with the combustion of specific alcohols. However, just mindlessly burning fuels is not going to achieve anything extensive. I need to have something to study. Firstly I have to narrow down the investigation by stating which alcohols I am going to work with. Below are the alcohols I will use:

* Methanol (CH3OH) 1 carbon atom least carbon atoms

* Ethanol (C2H5OH) 2 carbon atoms

* Propanol (C3H7OH) 3 carbon atoms

* Butanol (C4H9OH) 4 carbon atoms

* Pentanol (C5H11OH) 5 carbon atoms most carbon atoms

These are the first five basic alcohols that share common properties such as being colourless, non-conductors and have a neutral pH. They are exclusively linked together by the pattern in the number of carbon atoms in each of the alcohols' molecules. As you can see from the chemical formulas of the alcohols they go in ascending order of number of carbon atoms - methanol starting with one up to pentanol, which has five carbon atoms in a molecule.

What I want to study and find out is that between the five alcohols, which alcohol is the best alcohol? But on what conditions and criteria can I determine which alcohol is the best?

Relating back to investigating combustion it would be logical to grade the alcohols by which one can produce the most energy. An experiment to find this, however, would have to be carefully planned because calculating energy output by means of a practical experiment can be endlessly flawed in a variety of ways. Also to find the best one would involve comparing one to another. This is not as straightforward as it sounds. It would not be viable to simply burn equal masses of each fuel and see which one release most energy. This is because the size and the mass of the molecule of each alcohol is different and so having the same mass of each alcohol would result in having a different number of moles of each, which are ultimately what are being reacted - the molecules. Therefore, each alcohol would have to be compared by the number of moles of each that are burnt.
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The best alcohol can now be defined as follows:

The best alcohol is that which is the most efficient; that which releases the most energy for the same amount of moles combusted.

I can calculate the energy released by the reaction by setting up an experiment where a volume of water is heated by the energy from the reaction. Then using the specific heat capacity of the water as well as calculating how many moles of the alcohol is burnt, I can get an estimate of the total energy released by the reaction.

Hypothesis

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