An Investigation into the Enthalpies of the Combustion of Alcohols.

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An Investigation into the Enthalpies of the Combustion of Alcohols

Planning

        My aim for this experiment is to see the energy produced from different alcohols. This investigation involves burning alcohol in the air. ‘GCSE Chemistry’ by B.Earl and L.D.R Wilford says that "alcohols form, another homologous series, with the general formula Cn H2n+1OH ". The alcohol reacts with the oxygen in the air to form the products water and carbon dioxide.

        This reaction is exothermic, as heat is given out. This is because the amount reactant energy is more than the product energy the difference between this is ΔH, therefore some energy has been given out in the form of heat. The energy is given out when forming the bonds between the new water and carbon dioxide molecules. This can be shown in an energy level diagram: Reaction co-ordinate ΔH is the heat content, which is the enthalpy, which is negative in exothermic reactions as the diagram shows that energy is 'lost' as heat. Enthalpy is defined as the energy of reaction, or the heat energy associated with a chemical change. For any reaction carried out directly at a constant pressure, the heat flow is exactly equal to the difference between enthalpy of products and that of the reactants.

 

        To measure the heat energy given off, we must use this energy to heat something, this will be water. This is assuming that all the heat produced by combustion of the alcohol will equal the amount of heat absorbed by the water (q). So I will measure the amount of energy required to do so. This can be worked out by using the formula:     q = mass x specific heat capacity x temperature rise or q = MCΔH.  Where q is the quantity of heat. The specific heat capacity is the amount of energy required to heat the substance, and is calculated using the formula: q =energy supplied/ Mass x Temperature, where q is the enthalpy, c is the specific heat capacity and ΔH is the temperature rise. I chose to use water as it is safe, easily obtainable, and has a constant, reliable specific heat capacity of 4.2J/°C.

        The bonds which are made in a exothermic reaction are forces of attraction between the atoms or ions in a substance. These can be of two types: covalent, in which the atoms share electrons. Examples of this are water and carbon dioxide, which has a double covalent bond because it shares two pairs of electrons are shared. The other type of bonds is ionic, where a metal is involved. This is where electrons are transferred from one ion to another, so there is an electrostatic force between the ions.

Variables involved:

For this experiment I would expect to have 3 different variables, Independent, Dependent, and fixed variables, which all helps me to plan and explain the experiment thoroughly.

The Independent variable, the thing that I will be changing in this experiment into the combustion of a range of alcohols, they will be four different alcohols, Methanol, Ethanol, Propanol, and Pentanol, which will help me to investigate combustion of different alcohols. I will measure these individual alcohols by mass which will be measured in grams, I'll be making two readings of each alcohol before the experiment and after, which would tell me the mass lost and how much heat is produced.

Dependent Variable, the things that I'll be measuring would be the temperature (oC) increase at the start and the final resting point once combustion has been stopped of each different alcohol being lit to heat the water in the copper calorimeter, and the mass before and after the experiment of alcohols.

Fixed Variables, the things that I have chosen to keep the same in each individual experiment to keep it a fair test, is to keep the same amount of water being heated, the same copper calorimeter, as it may vary in conductivity, the height of the calorimeter, and the same amount of time for each experiment. This would help me to compare and analyse the results of the individual experiments.

Scientific explanation of what would happen:

        When each alcohol is burned to give out energy to heat the copper flask I would expect each of the following would happen to each alcohol I have listed below. Looking at all the alcohol's I have noticed that they're all alkanes, which means that they all have single bonds. This means that the alcohol's are made up of chains of carbon atoms with single covalent bonds between them, which are called saturated hydrocarbons because they have no spare bonds left meaning they won't form polymers. All four alcohols will have complete combustion, which relates to this equation below.

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Hydrocarbon + Oxygen                    carbon dioxide + Water + (energy)

Hypothesis:

        My prediction for this experiment on investigating the energy produced by different alcohols is that, the longer the hydrocarbons in the chain of the alcohols, the more heat it will produce. After the alcohols have been lit by the splint it will give out oxygen and water as well as heat energy to heat the water, which will crack the hydrocarbons into simpler molecules and then will combust later, making this experiment an exothermic reaction. Therefore the long chains of hydrocarbons ...

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