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An Investigation Into The Enthalpies Of The Combustion Of Alcohols

5145 words - 21 pages

An Investigation into the Enthalpies of the Combustion of Alcohols


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,...

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