Investigating Factors Affecting the Heat of Combustion of Alcohols

Investigating Factors Affecting the Heat of Combustion of Alcohols

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Investigating Factors Affecting the Heat of Combustion of Alcohols

PLANNING SECTION


Introduction
------------

Alcohols are organic substances, and consist of Hydrogen, Oxygen and
Carbon. All alcohols are toxic but the amount that can be tolerated by
the human body varies for different alcohols. For example drinking
small amounts of Methanol can lead to blindness and even death.
Ethanol is the only alcohol that can be drunk safely and is found in
all alcoholic drinks. Throughout this investigation I am going to
investigate to different factors that affect the breakdown of an
alcohol.

[IMAGE]e.g. Methane (HCO) + Oxygen (O2) Carbon dioxide (CO2) + Water
(H2O)


Aim
---

I am going to investigate increasing the chain length and see what
effect there is on the heat of combustion.


Preliminary Work
----------------


The point of preliminary work is to find out what things that I should
vary and keep constant. For my preliminary work I used computer
software called Focus Science Investigation 2. This software enables
you to simulate experiments. This means that I am able to quickly
carry out experiments to help in planning for my investigation.
----------------------------------------------------------------------

Alcohol

Temperature Increase (oC)

Mass of burner before exp. (g)

Mass of burner after

exp. (g)

Change in mass (g)

Methanol

20

170.00

167.08

2.92

Ethanol

20

170.00

167.77

2.23

Propan-1-ol

20

170.00

168.03

1.97

Butan-1-ol

20

170.00

168.16

1.84

Pentan-1-ol

20

170.00

168.24

1.76

Having found these results I then worked out the combustion per mole
of alcohol.

Alcohol

Mass of water heated (g)

Heat evolved during reaction (J)

Change in mass of burner

(g)

Combustion of one mole of alcohol (kJ/mole)

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Related Searches

Methanol

100

8360

2.92

-91.6

Ethanol

100

8360

2.23

-172

Propan-1-ol

100

8360

1.97

-255

Butan-1-ol

100

8360

1.84

-336

Pentan-1-ol

100

8360

1.76

-418

Correct to 3s.f.

I have now plotted these results into a graph. This graph is labelled
G1 and can be seen on the back page. From this graph I can see that
there is a linear relationship between the heat of combustion of one
mole of alcohol and the number of CH2 sections.


Constants
---------

Throughout this experiment I am going to keep the following things
constant:

1. The time that the alcohols are left to burn for. I have decided to
keep this constant, as it means that I will be able to easily compare
the heat of combustion having found the rise in temperature and change
in mass. I also think that it will be easier to keep the time constant
than the temperature so this us also why I have decided to keep the
time constant.

2. The distance between the copper can and the wick of the spirit
burner. This is to try and minimise error, thus increasing the
accuracy by keeping it a fair experiment.

3. The position of the thermometer. This is essential, as the heat
from the spirit burner is most likely to be concentrated in one area.

4. The amount of water (200ml) to be kept the same throughout the
investigation. I have decided to keep this constant as the more water
you have the more accurate your results are, as there is less
evaporation however I will have to keep the constant time higher to
account to allow for this.


Variables
---------

Throughout this experiment I am going to vary the following things:

1. The alcohols which I am using. This is so I am able to see the
difference in the heat of combustion having increased the length of
the chain in the alcohol.


Apparatus
---------

* Spirit Burners

* Retort Stand

* Splint

* Lighter

* Metal Beaker (Copper Can)

* Measuring Cylinder

* Thermometer

* Wooden lid for metal beaker

* Measuring Balance

* Water

* Draft Excluder (3 sided)

Following alcohols:

· Methanol

· Ethanol

· Propan-1-ol

· Butan-1-ol

· Pentan-1-ol

I have realised that the use of a draft excluder would ensure that the
flame was more concentrated to the copper can. I also thought that it
was possible for heat to be lost, by having an open lid to the copper
can. This is why I have decided to use a wooden lid. There is a wooden
hole in the centre, which enable the thermometer to be placed through
and kept in the same place.


Diagram
-------

[IMAGE] Retort Stand

[IMAGE]




[IMAGE][IMAGE] Thermometer

[IMAGE] Lid for copper can

[IMAGE][IMAGE][IMAGE][IMAGE]

[IMAGE][IMAGE] Copper Can

[IMAGE]




[IMAGE][IMAGE]

[IMAGE] Water

[IMAGE][IMAGE]

[IMAGE] 5cm Draft Excluder (3 sided)

[IMAGE][IMAGE][IMAGE] Spirit Burner

[IMAGE][IMAGE] Alcohol

[IMAGE]


[IMAGE][IMAGE] Heat proof mat
-----------------------------


Method
------

· Set up diagram as seen above

· Using a measuring cylinder, measure and then pour 200cm2 of water
into the metal beaker

· Affix the lid to the metal beaker and record the temperature of the
water

· Fill spirit burner with alcohol and weigh it on measuring balance

· Place burner under metal beaker so that the wick is 5cm below beaker

· Place draft excluder (three sided) into position

· Light spirit burner, having removed the lid of the spirit burner

· Leave the spirit burner burning for 2 minutes (120secs)

· Quickly replace the lid of the burner

· Record the new temperature of the water

· Weigh the sprit burner again

To increase the accuracy of my results repeat three times for each
alcohol. I must then repeat with the following alcohols:

- Methanol

- Ethanol

- Propan-1-ol

- Butan-1-ol

- Pentan-1-ol

Prediction

My prediction is that

'The amount of energy released per mole of any certain alcohol is
linearly proportional to the number of CH2 sections in one of mole of
that certain alcohol.'


Scientific Reasoning for Prediction
-----------------------------------

I have made this prediction with help from my preliminary work that I
carried out using Focus Science Investigations 2. I carried out a
similair experiment to the one which I have now planned and will carry
out. In my preliminary work although I only carried each experiment
out once but I don't believe that I have anomalies. I am only changing
the quantities however I believe that there will be linear
relationship between the number of CH2 sections and the amount of
energy released per mole of a certain alcohol.

The same bonds will release the same amount of energy, this means that
each CH2 section releases the same amount of energy. However it will
not be directly proportional as it to go throguh zero on the graph. If
it was zero it would mean that no energy was given off from the
reaction.

OBTAINING EVIDENCE SECTION

Changes to Plan

There are no parts of the plan that I think that are necessary to
change, as I am satisfied with my plan.

Results

I have put my results into a table, which can be seen overleaf. I have
also calculated the enthalpy and added this column into the table. I
have put all my results to 3.s.f.

I have shown an example of how I worked out the combustion of heat.

I am going to take the first result for methanol.

Temperature change (oC) = 14

Change in mass of alcohol (g) = 1.10

Specific heat of water = 4.18

Heat evolved (J) = Temperature change (oC) x Mass of water heated (g)
x Specific heat of water

Heat evolved (Q) = 14 x 200 x 4.18

= 11704J

Q is the quantity of heat evolved on burning D m grams of methanol
(Mr=32)

The amount of heat released when 1g of methanol burns = Q / Dm x 32J

= 340480

DHc = DH for combustion of one mole of methanol = - 340kJ/mole

This is what I have done for each experiment.

ANALYSIS SECTION

Processing my results

I have already processed my results, which can be seen in the table in
the previous section. I have also drawn a graph, which can be seen
overleaf. This graph is labelled G2.


What my results show
--------------------

My results show a negative correlation, which thus shows a linear
relationship. The results steadily decreasing (excluding anomalous
results), constantly increasing. I would say that this was directly
proportional (i.e. the graph shows a straight line through the origin)
however this is not possible as some heat has to taking in or given
off in a reaction. In the combustion of alcohols heat is always given
off so therefore it is inappropriate and incorrect for this graph to
pass through the origin.

Comparison with Prediction

In my prediction I stated:

'The amount of energy released per mole of any certain alcohol is
linearly proportional to the number of CH2 sections in one of mole of
that certain alcohol.'

All my results (except Propan-1-ol) show exactly that this that there
is a relationship which is linearly proportional.

Scientific Explanation

Excluding anomalous results my graph can be explained scientifically
by exactly the same as in my planning section. This is that each CH2
chain gives off the same amount of energy. Therefore every time an
extra chain is added the amount of energy given off extra will be the
same. This therefore means, that will be a linear relationship.

EVALUATING EVIDENCE SECTION


The quality of my evidence
--------------------------

I think my evidence is of a high quality. I can tell this from looking
at my results. On the whole my results followed a clear trend, except
one result. The majority of my results are close to my line of best
fit, which shows that they are quite accurately.

I have also observed that except for my results for Propan-1ol all of
my repeats for the same alcohols were similar. This set of anomalous
results for Propan-1-ol are most likely to have been caused due to the
surface area of the wick or errors such as measuring the distance
between the wick and copper can accurately as having done in previous
experiments.

The largest difference between an average temperature and the values
used to make up the average is found in the alcohol Butan-1-ol. The
average heat of combustion of one mole of alcohol is -1090, with a
deviation of from this of -1010. This is a percentage difference of
7%. This is not bad given it is the furthest out of all my results
(excluding anomalies).

Sources of error

Any measurement is subject to a random error, so my temperatures and
masses will not be exact. However I carried out each the experiment
out three times for each alcohol, averaging the results which should
have reduced this error.

Each time I used the measuring cylinder, some of its contents would
have remained in the cylinder, meaning that not exactly 200ml could
have gone into the copper can.

Although I had a draft excluder, this didn't stop all the effect of a
draft on the flame. This means that the flame wasn't concentrated at
the same point throughout.

At the end of each experiment soot (carbon) could be seen on the base
of the copper can. This means that not all the energy from the alcohol
went to heat the water. Also if there were a gradual build up of soot
on the copper can, it would mean that it the thickness between the
water and the flame was increasing. This could again have affected the
results.

The spirit burners were all slightly different. This could not be
helped due to the equipment available to me. The surface area of the
wicks was slightly different which meant that on occasions the flame
was spread out of a larger area.

Possible ways to minimise errors

Taking more results before averaging them can reduce random errors
such as volume measurement and mass.

I could have used a more accurate measuring device such as a burette.

I could construct something out of tin foil that would cover the
copper can and the spirit burner, to help concentrate the flame even
more than it was previously.

There is little I can do to stop the soot forming, however at the end
of each experiment I could wipe off the soot which had formed with a
damp cloth to reduce the possibility of this effecting my results
adversely.

If I were caring this out again, I would just have to make sure that
the spirit burners' wick were all the same surface area.

Further Work

To provide further evidence about the link between concentration and
the enthalpy of alcohols I could do the following.

I could use different alcohols. Instead of investigating the affect of
increasing the length of the chain, I could investigate the position
of the O-H section. I could do this by using other alcohols such as
Propan-2-ol, Butan-2-ol and Pentan-2-ol comparing them to Propan-1-ol,
Butan-1ol and Pentan-1-ol.

I could calculate the heat of reaction, and then compare it with data
book values. This would allow me to calculate the percentage errors.

I could change the constants and variables in such as seeing the
effect by varying the distance between the spirit burner and copper
can or volume of water. I could even investigate this experiment with
another substance and not water and compare the results.

I could also look up more information from the Internet. There are
websites that have sample data that could be useful towards my
investigation.
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