My aim is to make an accurate comparison of the enthalpy changes of different alcohols and explain how and why the enthalpy change is affected by its molecular structure.

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Chemistry Coursework

Aim:

My aim is to make an accurate comparison of the enthalpy changes of different alcohols and explain how and why the enthalpy change is affected by its molecular structure.

Background Information:

Alcohols are organic compounds in which the functional group is the -OH group, which is also called the hydroxyl group. Many belong to a homologous series which has the general formula CnH2n+1OH. In alcohols, a hydrogen atom is bonded to an oxygen atom. The oxygen atom in the hydroxyl group is in turn bonded to a carbon atom. Alcohols make good fuels because they burn easily and they release a lot of heat energy, which can be harnessed to power machines

        In this experiment I will be burning fuels, therefore the enthalpy change I will be measuring will be that of combustion, which s a measure of the energy transferred when 1 mole of the fuel burns completely. When an alcohol burns, it reacts with oxygen to form carbon dioxide and water. If there is a restricted supply of oxygen, it will lead to incomplete combustion, and will produce carbon monoxide instead of carbon dioxide.

This is an exothermic reaction because heat energy is given out to the surroundings. Many reactions like this are accompanied by a temperature rise; the more bonds there is the more energy needed to break them. However some reactions are accompanied by a fall in temperature. Theses are known as endothermic reactions because heat energy is taken in from the surrounding; more energy is released as more bonds are broken. Reactions are exothermic because more energy is released in making new bonds than used in break the bonds.

To work out the enthalpy change in each of the alcohols, I will use the formula mc∆t, where:

 m= mass of the substance heated,

c= specific heat capacity of the substance

 ∆T= change in temperature in the substance.

In this case I will heat water, so in my calculations I will use the mass of water I use, and its specific heat capacity which is 4.2j-1k-1.   

        Alcohols belong to the general family of compounds whose molecules are based on chains of carbon atoms. These substances differ from all other organic in that their molecules contain at least one oxygen atom joined to a carbon atom solely by a single bond. In alcohols this oxygen is part of a hydroxyl group, that is, the oxygen atom is joined to the hydrogen atom as well as the carbon atom- C-OH. Generally, the oxygen bearing carbon atoms in alcohols are part of the hydrocarbon chains, series of carbon atoms to which hydrogen atoms are attached. Examinations of these structures suggest that alcohols can be considered derivatives of water, in which a hydrocarbon chain has replaced a hydrogen atom.

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Combustion:

        When a hydrocarbon is burnt is a plentiful supply of air it reacts with the oxygen in the air to form carbon dioxide and hydrogen. This is an example of combustion:

Hydrocarbon + oxygen  carbon dioxide + water

For e.g. when methane (natural gas) is burnt:

CH4 (g) + 2O2  CO2 (g) 2H2O (l)

        

Where the energy does comes from?

When a fuel is burnt the reaction can be considered to take place in two stages. In the first stage covalent bonds between the atoms in the fuel molecules and ...

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