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Combustion of alcohols is exothermic; energy is given out. Salter's
Chemical storylines says, " Different fuels have different enthalpy
changes". I will investigate how the carbon chain length of the
alcohol affects its enthalpy of combustion.
Chemistry for you page 184 says, "breaking bonds requires energy, it
is endothermic ". It also says " making new bonds gives out energy it
is exothermic". The difference between the two determines whether the
reaction is exothermic or endothermic. It also determines how much the
total enthalpy of combustion is this can be shown in energy level
Diagram 1 is an energy level diagram of an exothermic reaction. It
shows that more energy is given out making bonds than is used breaking
them. Combustion is an exothermic reaction.
Diagram 2 is an energy level diagram of an endothermic reaction. It
shows that more energy is used breaking bonds than is used to make
Number and Range
I plan to use five alcohols in this experiment. The alcohols I will
use are; methanol, ethanol, propanol, butanol and octanol.
It is important that I use five alcohols so that there are enough
results to spot any patterns. The experiments are to be done three
times so that an average can be taken this will make the results more
reliable because any anomalous results will have less affect on the
The first four of the alcohols listed above have carbon chains with
1,2,3 and 4
Carbons. I chose to use these alcohols with an increasing number of
carbon atoms in their carbon chains. The reason for this is I can
investigate whether the change in the enthalpy of combustion increases
by the same amount for every extra carbon in the carbon chain. I also
decided to use octanol it has
a carbon chain length of eight so it has twice as many carbons as
butanol. So I can see if the enthalpy of combustion is proportional to
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value of octanol to be roughly twice that of butanol.
The variables in this investigation are:
* Volume of water used
* Height of cooper can above flame
* Type of can
* Temperature the water is heated to
* Type of alcohol
To make this investigation a fair test all of the variables except the
one I am investigating, the type of alcohol need to be kept constant.
I will always use 100 cm3 of water for each experiment. I will use the
same size copper can for each experiment and I will wipe of the bottom
of the can after each experiment. This is because carbon builds up on
the bottom of the can and carbon is an insulator. This means the
carbon will stop heat reaching the water so it is important it is
removed. I will measure the distance between the can and spirit burner
and ensure it is always 7 cm. I will also need to make sure I always
measure it from the same place. Each time the experiment is done the
water will be vented to 50oC from room temperature. A draught my blow
the fame and heat away from the tin so I will make a draft excluder
out of card to put around the burner. I will also ensure the lids are
on the alcohol burners right up until they are lit as the alcohol may
evaporate and make the masses inaccurate.
Using calculations I will calculate the enthalpies of composition for
my chosen alcohols. Then I will do experiments to find the enthalpies
of combustion from my chosen alcohol. I will compare the two results
to help me reach my conclusion.
Methanol, ethanol, propanol, butanol, octanol
Alcohol burners (5)
The apparatus was set up as shown in the diagram with the draft shield
around the. 100 cm3 of water was measured using the measuring cylinder
and poured into the copper can. The initial mass of the alcohol burner
will be recorded and the initial temperature of the water will be
recorded. The distance between the copper can and the burner was
measured and made to be 7cm. When this has been done the burner can be
lit. When the temperature of the water reaches 50oC the flame on the
burner will be extinguished. The burner will be weighed and the mass
will be recorded.
I decided that I would burn the alcohol until the water reached 50oC
so that enough energy has been transferred for an accurate experiment.
The heat from the flame heats up the copper can, which conducts the
heat and energy to the water. Copper is a good conductor which is why
a copper can is used, most of the heat the copper can gets will be
transferred. The higher the enthalpy of combustion of the alcohol the
faster the water will be heated. This is because the more energy
that's released the more energy can be transferred to the water. The
energy causes the water molecules to vibrate producing heat, which
makes the water warmer. So the more energy given out the less time the
water takes to heat. So from the amount of alcohol burned to heat
water to 50oC the enthalpy of combustion can be calculated. 100cm3 of
water was used because it is an amount that is easy to measure
accurately and the more accuracy there is the more accurate the
results will be.
I think that this plan is likely to provide precise results because
the alcohols will be weighed to two decimal places and the thermometer
is accurate to one degree C. so the temperature can be estimated to
half a degree, which is reasonably precise. The experiment being
repeated improves the accuracy.
To make this experiment safe the following precautions will be taken:
· Goggles will be worn at all times.
· The experiment will be done standing up
· Heatproof mats will be put under the alcohol burners.
· Handle hot equipment with care.
· Avoid spillages because the alcohols are flammable.
· Keep alcohols I am not using away from naked flames.
This experiment is fairly safe and there is little risk if these
guidelines are followed,
I predict that the longer the carbon chains in the alcohol the larger
the enthalpy change or combustion will be for that reaction. This
means more energy is given out by alcohols with the most carbons.
With alcohols with small carbon chains only a small amount of energy
is need to break the bonds because there are fewer of them. This means
that less bond breaking energy is needed so there is a smaller amount
of energy used to break the bonds than if there was a bigger carbon
chain. However this also means that less energy is given out to make
bonds so they give out less energy than alcohols with longer carbon
chains. This can be shown with energy level diagrams.
Diagram 1 is an energy level diagram for the combustion of an alcohol
with a longer carbon chain. There is more energy needed to break bonds
but also more energy is given out to make bonds because there are more
atoms to be bonded.
The calculated enthalpy of combustion for methanol is - 658 kj/mol.
The calculated enthalpy of combustion for Octanol is - 4984 kj/mol.
With the combustion of Octanol more energy is given out than with
Methanol because the enthalpy is lower which means more heat is given
I predict that the difference between the enthalpies of combustion of
the alcohols will be approximately 618 kj/mol. Because that is the
difference between the enthalpies of combustion I have calculated for
these alcohols with carbon chains where one carbon and two hydrogen
atoms have been added the enthalpy will give out an extra 618 kj/mol.
I also think that alcohols enthalpies of combustion will be roughly
proportional to each other. Butanol has four carbons in its chain and
Octanol has eight.
So Octanol should give out roughly twice as much energy.
Octanols enthalpy calculated was -4984 and Butanols was -2512. If we
make these positive to show the energy given out half of ectanol is
2492 which is close to 2512 so they should be similar.
Chemistry for you
Salter's Chemical Ideas