Comparing the Enthalpy Changes of Combustion of Different Alcohols

Comparing the Enthalpy Changes of Combustion of Different Alcohols

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Comparing the Enthalpy Changes of Combustion of Different Alcohols
Introduction

In this investigation I will set out to find the differences in the
enthalpies of combustion of 6 different alcohols. These are methanol,
ethanol, butan-2-ol, propan-1-ol, propan-2-ol and 2Methylpropan-1-ol.
I can then find the variation between straight chain and branched
molecules and isomers.


Planning
--------


Method
------

Carrying Out The Experiment

To carry out the experiment, I will fill my calorimeter with 75g of
water. It is important that this is accurate, as it is needed to find
the enthalpy change. The calorimeter will be held up by a clamp,
connected by a boss to a clamp stand so that heat can be applied
without having to hold the calorimeter up myself. The calorimeter
always needs to be the same distance away from the heatproof mat at
the bottom. The calorimeter is made of copper and is a good conductor
which means that most of the heat the copper absorbs will be passed
onto the water inside it.I will then need another clamp stand, clamp
and boss to set up a draft-extrusion system. This will be 3 heatproof
mats which will surround the calorimeter, ensuring that as much of the
heat as possible goes to the calorimeter and not its surroundings. I
will then need to record the starting temperature, as this is needed
to find the change in temperature. Now, I take the mass of the spirit
burner I have so that I can calculate the amount of alcohol that was
used during the combustion. I can now light it and let it burn. I will
aim for a temperature change of 20°C. Once it reaches that temperature
change, I will need to pull away the spirit burner and then record the
final temperature of the water and also weigh the final mass of the
fuel. I can then find out the change in temperature, which is the
final temperature - the initial temperature. The mass of alcohol used
can be calculated by the initial mass of alcohol used - the final mass

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Related Searches

of alcohol. This will need to be repeated 3 times for each alcohol so
there will be 18 tests overall. This enables me to obtain the most
accurate results.

Specified Instructions for Experiment

Instruction

Amount

Change in temperature

20°C

Distance of calorimeter from base of experiment

12cm

Mass of water

75g


What to do in steps
-------------------

* Put apparatus together, ensuring the calorimeter is 12cm from the
base of the experiment

* Measure out water ensuring that it is 75ml or 75g. Record the
initial mass of the alcohol

* Take the reading for the initial temperature

* Apply lit spirit burner to the base of the calorimeter. Aim to
finish on a temperature 20°C away from the initial temperature

* Once finished, record the final temperature of the water and the
final mass of the alcohol, calculating the differences


Diagram
-------

[IMAGE]



Apparatus to be used
====================


Apparatus


Reason For Use

Spirit burner

Will contain the fuel and be used to heat up the water

Calorimeter

Used to hold the water when being heated

Balance

Needed to measure the amount of water being used and to measure the
change in weight of the alcohols before and after burning

Clamp Stand x2

Will be used to hold the calorimeter and heatproof mats in place

Heatproof mats x4

The mats will be needed to create a draft extrusion system so that as
much heat as possible gets to the calorimeter

Clamp x2

Needed to hold heatproof mats in place as well as the calorimeter for
heating

Boss x2

Used to join the clamp to the clamp stand

Thermometer

Will measure the temperature change of the water before and after
heating



Substances to be used
=====================

Substance

Formula

State

ÈAlcoholsÈ

Methanol

CH3OH

l

Ethanol

C2H5OH

l

Propan-1-ol

Isomers

C3H7OH

l

Propan-2-ol

C3H7OH

l

Butan-2-ol

Isomers

C4H9OH

l

2Methylpropan-1-ol

C4H9OH

l

Water

H2O

l



Obtaining Accurate Results
==========================

The formula to find out the enthalpy change of combustion of an
alcohol is:

rHc = cmrT

As I am finding rH, it is important that c, m and rT are accurate so
that I can produce an accurate value for the enthalpy change of
combustion. I need to find out the energy change per mole of an
alcohol and I need to find the mass of alcohol used to calculate this
so I need to be accurate when finding the amount of alcohol used,
also.

Specific Heat Capacity (c)

The specific heat capacity of water is already known, however to gain
the most accurate results possible, the number needs to be as precise
as possible. To do this, the value should be kept as 4.17 and not
rounded up to 4.2, as 4.17 is a more accurate value and this will
result in a more accurate enthalpy change of combustion.

Mass of Water (m)

The mass of water is a fixed amount which I have decided upon as 75ml
(75g). However, when carrying out the experiment it is important that
the mass of water used is accurate. For instance, if the mass of water
in the calorimeter is less than 75g, the change in temperature will be
bigger as there is less water to heat. If the mass is bigger than 75g,
then there will be a smaller change in temperature because there is
more water to heat. Either way, this is inaccurate so it is important
to measure out the water with care so that it is as accurate as
possible.

Change in Temperature (rT)

To calculate this, I need to take two different readings - one at the
start and one at the end of the experiment. These need to be as
accurate as possible as they affect the final value for the enthalpy
change.



The Calorimeter
===============

When being heated, the water will not take in all of the heat applied
because the calorimeter will absorb it first. Because of this and to
gain more accurate results, I will need to find out how much heat the
calorimeter has absorbed and add it on to how much heat the water will
have absorbed. The calorimeter is made of copper. This is because
copper has a low specific heat capacity so most of the heat it absorbs
will be passed onto the water and it also has high thermal
conductivity which again means the water will absorb more heat.



Calorimeter Testing
===================

To obtain more accurate results, I have decided to calculate how much
energy the calorimeter is absorbing so that I can then add on this
value to the amount of energy the water absorbed to gain a more likely
value for the enthalpy change of combustion. To do this,


The Process

I needed to find the heat capacity of the calorimeter that I was using
in the experiment. To do this, I mixed together equal amounts of both
hot and cold water in the calorimeter. I used 40g of both hot and cold
water each. I then recorded the final temperature of the water which.


Results

Test

Temp of Cold Water (°C)

Mass of Cold Water (g)

Temp of Hot Water (°C)

Mass of Hot Water (g)

Temp After 30s (°C)

1

21

40

56

40

37

2

21

40

61

40

37

3

22

40

46

40

33

Ave

21.3

40

54.3

40

35.7



Finding the Heat Capacity
=========================

Using rH=cmrT

(40 x 4.17 x (54.3 - 35.7) = (40 x 4.17 x (35.7 - 21.3)

= (3102.5 - 2101.9)

= 700.6 for rT of 14.4°C

Heat Capacity = 700.6/14.4

= 48.7 J/°C

I then need to multiply this number by the temperature change in each
experiment and add it on to the total energy absorbed by the water. I
can then multiply this by the amount of moles used and calculate the
enthalpy change of combustion.


Pilot Test
----------

I carried out a preliminary investigation. This gave me the chance to
try out my experiment and see how well it worked. From this, I could
then find any errors or problems and alter them to ensure that they do
not occur in the actual investigation. I could then also see if all
the values I was testing were correct or if I should change them.

I heated up 100g of water with an approximate temperature change of
25°C. I used the alcohol ethanol for the pilot test. The spirit burner
was kept 15cm away from the calorimeter.



Results
=======

Test

Mass of Ethanol Used (g)

Temp Change (°C)

Mass of Water (g)

Heat Capacity of Water (Jg-1°C-1)

cmrT (kJmol-1)

1/Moles Combusted

rHc (kJmol-1)

1

0.94

25.3

100

4.17

10.6

51.1

539.1

2

0.98

25.0

100

4.17

10.4

49.0

510.8

3

0.97

25.2

100

4.17

10.5

49.5

520.2



Modifications From the Pilot
============================

By carrying out the pilot test, I could find any problems and could
then decide on any modifications I wanted to make to the pilot to make
the actual experiment better when being carried out



Modification
============



Reason
======

Water from 100g to 70g

It took a long time for the water to increase by 25°C and it will be
quicker if a smaller amount of water is used, as there is less water
for the spirit burner to heat. Because of this, I have reduced the
mass of water by 30g.

Change in temperature from 25°C to 20°C

When nearing the end of 25°C, the experiment begins to slow and
valuable time is lost. This made me decide to reduce the increase in
temperature by 5°C so that each individual test would be carried out
much quicker and the experiment wouldn't slow down because of a
smaller temperature change.

Distance of calorimeter and base reduced to 12cm

Because the calorimeter was so far away from the spirit burner, the
water wasn't absorbing enough heat. So, I reduced the distance by 3cm
so that more water would be absorbed, and I could get a more realistic
value for the enthalpy change of combustion.



Theory
======

rHc

This is the energy change at standard temperature (298K) and pressure
(1 atmosphere). A mole is 6 x 1023 atoms of the chemical. The mass of
one mole is equal to the relative atomic mass of the alcohol in grams.
For instance, ethanol (C2H5OH) has a relative atomic mass of 45 so one
mole is 45g of ethanol. rHc will always be negative as combustion is
an exothermic reaction and heat is given out. I will need to find rH
and to do so, I will use this formula.

rH = cmrT

m= mass of water (g)

c = specific heat capacity of water (Jg-1°C-1)

= 4.17

rT = change in temperature (°C)

To find the energy change per mole of alcohol, the number of moles of
fuel will be needed. This means that I will have to measure the amount
of fuel I used so that I can calculate the amount of energy released
per mole. To do this I first find out how many moles were in the fuel
I used. To do this I divide the mass of fuel used by its molecular
mass. Then to find out rHc, I need to divide 1 by the number of moles
and then multiply that answer by the energy produced. The value needs
to be in kJ so I should then divide the answer by 1000 to give rHc

Example of a combustion reaction

[IMAGE]C2H5OH + 3O2 2CO2 + 3H2O

This is the combustion of ethane. The ethane is burned in oxygen. The
two react and form the products carbon dioxide and water. All of the
alcohols will do this.



What is an alcohol?
===================

Alcohols are organic molecules made up of hydrogen, carbon and oxygen
atoms. The general formula for alcohols is CnH2n-1OH where 'n' is the
number of carbon atoms. Alcohols are named as substituted alkanes by
removing the 'e' on the end of the name of an alkane and replacing it
with 'ol'. For instance, methane becomes methanol and ethane to
ethanol. The boiling points of alcohols increase with the molecular
size of the substance. Alcohols are good solvents and can dissolve in
a large number of organic molecules. Alcohols of a low molecular mass
are miscible in water. This is due to the hydrogen bonding so longer
chain molecules become less miscible as the mass increases. The
hydrogen bonds are found between the oxygen and hydrogen in the
hydroxyl group (the -OH attached to a carbon atom). The numbers in the
names of the alcohols refer to the position of the -OH group on the
carbon chain.



Uses of an Alcohol
==================

Alcohols can be used as solvents for various varnishes and paints in
the manufacture of aldehydes, ketones and esters. Methanol and
ethanol, as well as ethane-1,2-diol are used in anti-freeze mixtures.
Large quantities of ethanol are consumed in the form of alcoholic
beverages.



Structure of Isomers
====================

I will be investigating the difference between two sets of isomers:
propan-1-ol and propan-2-ol, as well as butan-2-ol and
2methylpropan-1-ol. The two have different enthalpies and other values
such as boiling and melting points. This is because of the forces of
attraction between the molecules. Propan-1-ol is a straight chain
molecule, as shown below and so, when grouped with other propan-1-ol
atoms, it is very near to the other atoms because of the strong force.
Propan-2-ol is branched and so when it is grouped with other
propan-2-ol atoms, they cannot get as close to each other as the
branches are in the way. This means it has a smaller force of
attraction between the molecules and it makes the molecules easier to
break.

[IMAGE]



Why Is It Exothermic?
=====================

An exothermic reaction is one which gives out energy. The enthalpy of
a reaction can be found by the following calculation.

rH = Hproducts - Hreactants



For a reaction to have a negative enthalpy, the reactants must be more
than the products. In other words, the reactants must require more
energy to have their bonds broken than the products need to have their
bonds made. This table shows the amount of bonds broken and the amount
of bonds made in the combustion of ethanol.
======================================================================



Bonds Broken
============



Bonds Made
==========

C-H (x5)

C=O (x2)

C-O

H-O (x6)

O-H

O=O (x3)

More bonds are broken than are made and the bonds which are broken
have higher enthalpies than that of the ones which are made. Because
of this, there is a big difference between the value of the reactants
and the products, so rH will be negative.

[IMAGE]

This graph shows that the enthalpy decreases once the bonds of the
reactants are broken and energy is given out. This makes it an
exothermic reaction and so rH will be negative because of this.

rHc Using Bond Enthalpies

To calculate the value for each enthalpy, the add up all of the values
for the bonds broken in the reactants and then do the same for the
products made in the reaction. The enthalpy is the calculated by
subtracting the value for the reactants from the products. As this is
exothermic, the enthalpy of each alcohol will be negative. The example
shown below is the estimated value for the enthalpy change of
combustion of propan-1-ol, using bond enthalpies.

Reactants

Products

Propan-1-ol

Oxygen

Carbon Dioxide

Water

C3H7OH

4.5O2

3CO2

4H2O

C-H (x7)

O=O (x4.5)

C=O (x6)

H-O (x8)

C-O

H-O

C-C (x2)

4407

2242.4

4830

3712

6649.4

8542

The difference between these 2 values is 1892.6 so the value for the
enthalpy of combustion of propan-1-ol could be around -1892.6kJmol-1.
However, this is based on average bond enthalpies so this value isn't'
accurate. The actual value is -2021kJmol-1.



Prediction
==========

As a rule, the more carbon atoms there are in the molecular structure
of an alcohol, the more energy that will be released during its
combustion. This is because when new bonds are produced, energy is
released. So breaking more of the C-H bonds forms more of the new
bonds which results in more energy being released in the process.

Because of this I expect butan-2-ol and 2methylpropan-1-ol to have
high enthalpies of combustion whereas methanol and ethanol will have
smaller values and the two propanol isomers will be in between these.
I have the actual values of the enthalpies of combustion for these
alcohols so I should work towards these but I shouldn't expect to get
my values very close to the actual ones as it very difficult to
replicate the way the experiment would have been carried out.

All combustion reactions are exothermic. This means I can expect that
the rHc will be negative. This is because during a reaction the
reactants are losing energy which is used to heat its surroundings
such as the calorimeter and the air. Because of this the products end
up with less energy compared to what the reactants had because the
surrounds receive a lot of it. A majority of the energy was wasted
when the calorimeter was heated and I hope to find out how much was
wasted so that I can add this on to the enthalpy of combustion that I
have calculated so that I have more accurate results.

By looking at the equation for the reaction of each alcohol, I can see
which bonds are being broken and which are being made, meaning I can
then estimate a value as an estimate for the amount of energy which
will be released in each reaction.

The general rule will be that as the mass number of an alcohol
increases, so will the enthalpy change of combustion. For the isomers,
I expect propan-1-ol to have a higher enthalpy than that of
propan-2-ol. This is because propan-1-ol is a straight chain molecule
and propan-2-ol is branched. The same will happen between butan-2-ol
and 2methylpropan-1-ol because 2methylpropan-1-ol has many branches
and so has weak forces of attraction so I expect it to have a lower
enthalpy change of combustion than that of butan-2-ol.

Methanol

[IMAGE]CH3OH + 1.5O2 CO2 + 2H2O

Ethanol

[IMAGE]C2H5OH + 3O2 2CO2 + 3H2O

Propan-1-ol and Propan-2-ol

[IMAGE]C3H7OH + 4.5O2 3CO2 + 4H2O

Butan-2-ol and 2methylpropan-1-ol

[IMAGE]C4H9OH + 6O2 4CO2 + 5H2O

Enthalpies of Combustion rHC

Alcohol

rHC (kJ mol-1)

Methanol

-726

Ethanol

-1367

Propan-1-ol

-2021

Propan-2-ol

-2006

Butan-2-ol

Unobtainable

2Methylpropan-1-ol

-2644

Structural Formulae

[IMAGE]Methanol

[IMAGE]Ethanol

[IMAGE]Butan-2-ol

[IMAGE]Propan-1-ol

[IMAGE]Propan-2-ol

[IMAGE]2Methylpropan-1-ol


Risk Assessment
---------------


Precautions

* Wear goggles

* Tuck in anything loose, such as ties to avoid contact with fire

* When measuring out alcohol, ensure this is done inside a fume
cupboard

* Avoid any flames near alcohol being stored or measured

* Ensure all spills are cleaned up immediately

* Do not handle hot apparatus. Allow it time to cool


Spirit Burner

Care needs to be taken with the spirit burner, as it is obviously very
hot and very dangerous. The fuel burner mustn't be tipped upside down
as a bigger flame will be produced, as more alcohol will be ignited.
The top of the burner should be cleaned often, also as alcohol may
seep out of the wick and onto the lid so when lit will produce and
bigger flame and will be more harmful.


The Alcohols
------------

All alcohols should be handled in a fume cupboard.

If alcohols get in the eyes, they should be irrigated with water for
10 minutes at least.

If contact is made between the alcohol and skin, it should be washed
off immediately with large amounts of water with soap.

Alcohol

Risk Assessment

Methanol

This is highly flammable and volatile so it will ignite easily in the
spirit burner. It is also toxic and so it should not be inhaled,
ingested, nor should it be absorbed in the skin.

Ethanol

Highly flammable. Therefore care must be taken when it is in the fuel
burner as too much could ignite. This is mildly toxic and can also be
irritant.

Propan-1-ol and Propan-2-ol

These alcohols are highly flammable also, and so the same precautions
should be taken when using them. They are mildly toxic if absorbed by
the skin. Also, they should not be inhaled or ingested.

Butan-2-ol

This is flammable so it must be used carefully with the spirit burner.
It is also harmful, so contact with the alcohol should be avoided.

2Methylpropan-1-ol

This substance is flammable; therefore the user needs to be careful
when using it in the fuel burner. It is also harmful so no contact
should be made with it. It shouldn't be inhaled or ingested.


Analysis

My graphs show that the enthalpies of combustion show an increase in
overall energy release as the sizes of the alcohols increase. This is
because of the increase in the number of carbon atoms. The graph
showing mass number v enthalpy shows that as the relative molecular
mass of an alcohol increases, so does its enthalpy change of
combustion with it. My results obtained are some way off from the
actual values but this is because I could not carry out the experiment
under such strict regulations. My draft extrusion system wouldn't have
worked well enough to ensure that all of the heat was kept in and it
couldn't be carried out at the standard 298K or 25°C.

The Isomers

Propan-1-ol and Propan-2-ol

My graph shows little difference between the two alcohols. However,
propan-1-ol did have the higher enthalpy change of combustion,
according to my average results. This is as I predicted. This is right
as the actual values have only 15kJmol-1 between them and my results
show a difference of 18.96kJmol-1, which shows the similarity between
my own results and the actual results which I aimed towards.
Propan-1-ol is a straight chain molecule and more energy is needed to
break its bonds because it has a stronger force of attraction than
propan-2-ol because that is a branched alcohol.

These results are as I predicted, that as the mass number increases,
so to does the enthalpy change of combustion.

Butan-2-ol and 2methylpropan-1-ol

As expected, butan-2-ol had a higher average enthalpy change of
combustion than 2methylpropan-1-ol. This is because of the forces of
attraction. 2methylpropan-1-ol has many branches and so it is easier
to break its bonds and so requires less energy than is needed to break
the bonds of butan-2-ol.

To Summarise

Branched molecules have lower enthalpies of combustion than straight
chain molecules. This is because when grouped together, the branched
molecules are not as close up as the straight chain ones so they have
weaker forces of attraction and their bonds can be broken more easily
and so they require less energy to be broken. The straight chain
molecules have less room between them and the forces of attraction are
much stronger than those of the branched molecules. This makes it much
harder to break the bonds and so it requires more energy, resulting in
a higher enthalpy change of combustion. This shows that what I
expected to see when the isomers were combusted has occurred.

Evaluation

Percentage Errors

I have calculated percentage errors in my experiment to find out how
far out my results could have been because of errors made during the
experiment. I have taken my third test for propan-1-ol to find the
percentage error for.

Measuring Cylinder

70cm3 +/- 2cm3 = 98cm3 - 102cm3

Difference of 4cm3

% Error = 4/100 x 100 = 4%

Thermomenter

+/- 0.05°C

If 20°C = 19.95 - 10.05

Difference of 0.01°C

% Error = 0.1/20 x 100 = 0.5%

Balance

+/- 0.1g

If 50g = 40.9 - 50.1

Difference of 0.2g

% Error = 0.2/50 x 100 = 0.4%

Total Percentage Error = 4 + 0.5 + 0.4

= 4.9%

Anomalous Results

Any anomalous results which I may have could be due to errors in
measurement and reading. This includes, measuring the initial and
final mass of the alcohol, measuring out the volume of water and
taking the start and final temperature of the water.

Errors

I had varied starting temperatures of the water at the start of each
test. This will probably be because I didn't allow the calorimeter
enough time to cool following its use, and so when new water was put
in, it instantly heated up to a higher temperature. This would mean
that it would be easier for the water to increase in temperature
further and so it makes it an unfair test. Some of the water may have
evaporated because of this as well.

The heat of the room, which the experiment was carried out in, may
have varied, resulting in different speeds of evaporation so sometimes
the water may have heated up quicker than others. This again is an
unfair test as there would be different amounts of water.

When measuring the water, I ensured that it was measured accurately.
However, there may still have been some excess water left in the
calorimeter from the previous test and so there may have been more
water in the calorimeter than expected. This would have taken it
longer to heat up, so it would have needed more alcohol to complete
the change in temperature.

Also, the water I used came from a tap. This water may therefore have
had impurities which could have affected the specific heat capacity of
the water.

The heat which wasn't absorbed by the calorimeter or the water could
have been lost to the surroundings. Also, the fuel may have lost some
of its mass between when it was weighed and when it was used, although
this is unlikely as it was a very short time period.

The water may not have been heated evenly. This would have given
different temperature readings. To ensure this didn't happen, I
stirred the water often.

Also, there may have been incomplete combustion. Some carbon soot
formed on the calorimeter after each combustion.

The Effects of Errors

If there was too much water, then it would have required more alcohol
to increase its temperature. This would have had two different
affects.

1. The mass of the water was higher than expected so, if read
correctly, the enthalpy would have been higher because of the
calculation cmrT.

2. The mass of alcohol was more than needed so it would make rH
smaller when the amount of moles used was found than it actually was.

How Could I Change My Experiment?

I could have extended my experiment by testing other alcohols as well
as the 6 I used. These could include pentanol, heptanol, hexanol,
octanol and decanol, as well as various isomers.

A bomb calorimeter could have been used to retain heat within the
calorimeter and obtian more accurate results.

If I were to do this experiment again, I would ensure that the area
where the combustion was being carried out was well insulated. This
would mean less heat would escape and so the water would get to absorb
more of it and I would be able to produce more accurate results.

Conclusion

Overall, I am happy with the way I have carried out this experiment.
The results I have obtained reflect how well the investigation was
carried out and show he correct trends between both the alcohols and
the isomers.

The enthalpy change of combustion increases with its mass number. For
alcohols of the same mass number, the less branches it has, the higher
its enthalpy of combustion.

Bibliography

Salters Advanced Chemistry 2000

Data Sheets

Salters Advanced Chemistry - Chemical Ideas (2nd Edition)

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