Comparing The Enthalpy Changes Of Combustion Of Different Alcohols

Aim:

The enthalpy change of combustion of a fuel is a measure of the energy transferred when one mole of fuel burns completely. A value for the enthalpy change can be obtained by using the burning fuel to heat water and using fact that 4.2j of energy are required to raise the temperature of 1g of water by 1oC. In this investigation my aim is to find the enthalpy change of combustion of a number of alcohols so that I can investigate how and why the enthalpy change is affected by the molecular structure of the alcohol.

I am going to investigate the enthalpy change of combustion for the alcohol homologous series. I will investigate how alcohols with increasing number of carbons affect the enthalpy change when an alcohol goes under combustion. The energy that alcohols release when being used is called the enthalpy change of combustion.

As I already know that alcohols are a series of organic compounds which all contain a -OH group.

My Prediction:

By using the knowledge that I already have I think that the alcohols with a larger number of carbon atoms within the molecule will have a greater alcohol change of combustion than the alcohols with less carbon atoms. So therefore I predict that as the molecular mass increases so does the enthalpy of combustion.

The alcohols that I intend to use throughout this investigation are the following 6 alcohols:

Ø Methanol – CH3OH solvent; fuel; manufacture of organic chemicals; denaturing ethanol straight chain alcohol.

Ø Ethanol – (CH3CH2OH) ethyl alcohol; grain alcohol. A colorless, flammable liquid produced by fermentation of sugars. Straight chains.

Ø Butane – CH3CH2CH2CH3, colourless, with a characteristic natural odor

Ø Pentan-1-ol -straight chains alcohol.

Ø Hexan-1-ol – straight chain alcohol.

Ø Propan-1-ol – straight chain alcohol.

Using the alcohols listed above I will measure the amount of energy produced by them when burnt in air. As I am calculating the enthalpy change of combustion, in order to calculate the enthalpy change of combustion for my chosen alcohols.

Ø I will firstly burn my chosen alcohol to heat 100cm3 of water

Ø Remembering that 4.2 j of energy is needed to increase the temperature by 1g of water by 1oC

Ø So therefore I will need to measure the quantity of alcohol that have burned in the experiment and convert it into moles.

All the combustion reactions from the alcohols will be exothermic; exothermic is a process that releases heat, in an exothermic reaction, the enthalpy of the reacting system decreases is negative. In an endothermic reaction, the enthalpy of the reacting system increases is positive. The enthalpy change for an exothermic process is negative. Examples of exothermic processes are combustion reactions and neutralisation reactions.

The graph below shows the enthalpy level diagram for an exothermic reaction, eg burning methane: CH4 + 202 CO2 + 2H20.

To carry out my investigation I will need the following equipment:

Ø Draught shield – this is to exclude draught and prevent any energy from being lost.

Ø Spirit burner – to burn the alcohols.

Ø Thermometer – ranging from 0oC -110oC, to give accurate temperature results

Ø 500ml Metal copper Calorimeter – will help to transfer all the energy into the water efficiently.

Ø The 6 different alcohols, which are: Methanol, Ethanol, Propan-1-ol, Butan-1-ol, Pentan-1-ol and Hexan-1-ol.

Ø Water – 100ml

Ø Heatproof mat – to avoid accidents from occurring, e.g. to stop the surface that I am working on from being damaged or burning.

Ø Hazard cards – so that I can follow all the safety hazards and be aware of how dangerous or harmful the chemical that I am handling can be for me and others working in the same environment as me.

Ø Goggles – to protect my eyes from being harmed by the alcohols.

Ø Electronic balance – an electronic balance of 2 decimal places to give me the right degree of accuracy to carry out my investigation precisely.

Ø Clamp – to support the calorimeter

Ø 100cm3-measuring cylinder – to keep all liquid measurements accurate.

Ø Repeat this procedure for all 6 of the alcohols that I am going to use.

Method:

The method that I am going to use to carry out my experiment is:

Ø Firstly I am going to measure out 100cm3 cold water, using the measuring cylinder (so that the water I put into the calorimeter is accurately measured out) rather than using an ordinary beaker, as an ordinary beaker is less precise compared with a measuring cylinder.

Ø Then I will record the temperature of the water in the calorimeter, using a thermometer to give a good degree of accuracy.

Ø After that I will use the clamp to support the calorimeter.

Ø Then I will place the spirit burner with one of the six alcohols that I am going to test on a heatproof mat.

Ø In order to make it a fair test and make sure that no energy is lost through surroundings, and to prevent heat from being evaporated I will use 2 draught shields and place them around either side of the calorimeter and spirit burner.

Ø I will then weigh the spirit on the electronic balance with the lid on to make sure that the test is fair because if I took the lid off the fuel would have evaporated very quickly making my investigation not precise and accurate.

Ø Then I will place the fuel that I will be using under the calorimeter on top of the heatproof mat.

Ø Then I will cover the calorimeter with foil to avoid any heat loss.

Ø I will then pierce a hole into the foil big enough for the thermometer to fit through so I can stir the water and record the temperature of the water.

Ø Then I will take the lid off the fuel and light the fuel with matches.

Ø I will constantly stir the water with the thermometer ensuring the temperature is the same throughout the whole calorimeter.

Ø Once the temperature of the water has reached around 45oC on my thermometer I will immediately put the top back onto the fuel, so that no more fuel is lost via evaporation in the air.

Ø I will then go and once again weigh the fuel on the electronic balance to see how much fuel has been used to make the temperature of the water in the calorimeter reach about 45oC.

Ø I will then work out the difference between the two measurements that I have of the fuel and work out the amount of fuel that has been used.

Ø To work out the mass used I will use the formula below:

Mass used (g) = Original Mass (g)

Final Mass (g)

I have also done a preliminary test by using my plan to guide me, and by following my plan and then making changes to my plan where it was necessary to get accurate results. At the start of the investigation I wasn’t too sure of weather to do my experiment with the lid on or the lid off. However after doing a preliminary test with the lid on and lid off on Butan-1-ol, I got the following results:

Fuel Original Mass (g) Final Mass (g) Mass used (g) Mass Of Water (g) Origin-al Temp (oC) Final Temp (oC) Temp Diff(oC) Specifi-c heat capacity of water Energy transferr-ed J Molec-ular mass of fuel Energy transferr-ed per mole J Energy transferred per mole KJ

Lid ON Butan-1-ol 194.043 193.624 0.419 100 39.1 52 12.9 4.2 5418 74 956878.2816 956.8782816

Lid OFF Butan-1-ol 193.61 192.829 0.781 100 25 41.6 16.6 4.2 6972 74 660599.2318 660.5992318

With lid = Enthalpy of combustion KJ mol

-956.8782816

Without lid = Enthalpy of combustion KJ mol

-660.5992318

I have worked out the enthalpy of combustion, using the formula below:

Energy transfer per mole = Mr Fuel* energy transferred in experiment

Mass of fuel used

After doing my preliminary test I decided to do my experiment with the lid on rather then off, as with having the lid on I was more likely to get precise results because when the lid is off fuel is evaporated into the air and is lost. So therefore by the time you can weigh the fuel again some of the fuel has been already evaporated into the air, which makes the results less precise.

Risk Assessment:

The Risk Assessment that I am doing is to ensure that I am able to identify the hazards and precautions that I will have to take while doing this experiment.

Chemical exposure occurs through ingestion, inhalation and direct contact with the skin or eyes. Accidental ingestion of chemicals in the laboratory occurs when food or drink become contaminated. It also occurs when people transfer chemicals from their hands to their mouths. Ingested chemicals may damage the digestive tract or they may be absorbed through the digestive tract and transported to organs where they can exert a toxic effect. Inhalation exposure occurs when people inhale airborne gases, vapours, particulates or finely divided liquid droplets. Inhaled chemicals can damage the nasal passages and the upper and lower regions of the respiratory tract. Contaminants – particularly fine particles – may be deposited in the lung where they can cause damage.

Or the inhaled contaminants may be absorbed through the lung. The blood stream can then transport the absorbed contaminants to other parts of the body. A small amount of inhalation of certain chemicals is not life threatening however, as the dose increases; the chemical begins to exert an effect. But, as the body is capable of repairing the damage that may have been done, the exposure in this range is without adverse consequences. At still higher doses, the intense exposure overwhelms the defense mechanisms causing irreversible damage. Finally, at even higher doses, the exposure can be fatal. The graph below shows this:

Risk Assessment.

The table below shows the risks of the 6 alcohols that I am going to use, it also shows how accidents can be avoided.

Name of Chemical. Particular hazard. Risk. Action needed for accident prevention. Action that needs to be taken incase of an accident.

Methanol Toxic Irritant to the eyes and mucous membranes. It can react vigorously with oxidising materials. Methanol possesses distinct narcotic properties. It is also a slight irritant to the mucous membranes. Its main toxic effect is exerted on the nervous system, particularly the optic nerves, which could eventually lead to blindness. Once absorbed by the body it is only very slowly eliminated and thus is regarded as an accumulative poison. It can react vigorously with other materials. Don’t stay exposed too long with methanol and do not mix methanol with any other chemicals as it may react vigorously with them. Seek medical advice and take the person out of the room to fresh air immediately.

Ethanol Highly Flammable Vapour may catch fire easily, also a risk of inhalation, even drinking the alcohol may cause narcotic effects. Keep containers closed and away from any naked flames, also warn users to use Ethanol in well-ventilated space. Cover the area to remove oxygen, use a fire extinguisher if necessary, remove everyone from the area and take them out to get fresh air. Also give the them plenty of water and seek medical attention ASAP.

Propan-1-ol Highly flammable Vapour can catch fire. Make sure the area is well ventilated and keep all containers closed and well away from any naked flames. Use a fire extinguisher to remove all oxygen.

Butan-1-ol Harmful and flammable Alcohol can catch fire. Keep bottle tops on when the alcohol is not in use, keep away from naked flames. Use a fire extinguisher if there are any BIG flames.

Pentan-1-ol Highly flammable and can also become irritant when it has contact with skin. Can cause drowsiness if it is breathed in, so do not inhale excessively, also wear gloves if available to avoid contact with skin as it can cause burning of skin. Keep bottles stoppered when not in use and well away from naked flames. If the vapours are breathed in take the person out to a fresh-aired space, if there is any contact with skin wash the area in contact with water immediately. Seek medical advice in both circumstances.

Hexan-1-ol Highly flammable. Can also be irritant if there is contact with skin. Can cause severe burning if in contact with skin, do not inhale excessive amounts of vapour as this may cause drowsiness. Keep bottles stoppered when not in use and well away from naked flames. If the vapours are breathed in take the person out to a fresh-aired space, if there is any contact with skin wash the area in contact with water immediately. Seek medical advice in both circumstances.

Throughout the investigation I must make sure the area that I am working in is well ventilated, so that there is enough windows open so that any harmful fumes in the room can escape and not be breathed in by people in the room. I must also wear my safety goggles at all times. All lose clothing’s and long hair should be tied back and kept well away from the flame.

Analysing:

The enthalpy of combustion can only be calculated by the formula, which I have already mentioned above. These calculations can simply be done using a PC, however if there is not access to a PC they can always be done manually.

To work out the MASS USED you should use the following formula;

Mass used = original mass – final mass

To work out the temperature difference you should use the formula below:

Temperature difference = final temp – original temp,

For example ethanol the original temp of ethanol is 50oC and the final temperature of ethanol is 70oC then I would do the following calculation to work out the temperature difference:

70 – 50 = 20oC

To work out the energy transfer/J you use the following formula:

Energy transfer/j = mass of water * specific heat capacity of water * temperature difference.

To work out the energy transferred per mole/j you must use the following formula:

Energy transferred per mole/j = molecular mass of fuel * Energy transferred J

Mass used

The formula for working out the energy transferred per mol/KJ you must use the following formula:

Energy transferred per mol/KJ = Energy transferred J

1000

The Final enthalpy of combustion is minus energy transferred per mol KJ.

My results (Done on Microsoft Excel)

Fuel Original mass (g) Final mass (g) Mass used (g) Mass of water (g) Original temperature

Methanol 186.622 185.935 0.687 100 79

Ethanol 230.734 229.644 1.09 100 24

Propan-1-ol 256.323 255.201 1.122 100 73

Pentan-1-ol 207.705 202.728 4.977 100 75

Butan1-ol 240.541 239.364 1.177 100 23

Hexanol 228.962 228.064 0.898 100 24

Final temperature Temperature difference Specific heat capacity of water Energy transferred J

99 20 4.2 8400

44 20 4.2 8400

93 20 4.2 8400

94 19 4.2 7980

43 20 4.2 8400

45 21 4.2 8820

Molecular mass of fuel Energy transferred per mole J Energy transferred per mole KJ

32 391266.38 391.27

32 246605.50 246.61

60 449197.86 449.20

88 141097.05 141.10

74 528122.34 528.12

102.2 1003790.65 1003.79

Enthalpy of combustion KJ mol

-391.27

-246.61

-449.20

-141.10

-528.12

-1003.79

This table shows the results of the six different alcohols and also it is showing the enthalpy of combustion. I have worked out the enthalpy of combustion using formulas in Microsoft excel.

Look at graph 1. By looking at this graph you can see that there are 2 anomalous results. One of the anomalous points has a molecular mass of 32, which means that it is methanol, because methanol has a molecular mass of 32. The second anomalous point has a molecular mass of 88, which means it is pentan-1-ol. As my results for methanol and pentan-1-ol are anomalous I have decided to repeat the experiment for both these alcohols.

Fuel Original mass (g) Final mass (g) Mass used (g) Mass of water (g) Original temperature

Methanol 183.144 182.059 1.085 100 22

Pentan-1-ol 219.058 218.054 1.004 100 75

Molecular mass of fuel Energy transferred per mole J Energy transferred per mole KJ

32 235354.84 235.35

88 699442.23 699.44

Enthalpy of combustion KJ mol

-235.35

-699.44

Now that I have repeated my experiment for both methanol and pentan-1-ol alcohols you can see on my new graph 2, that there are no anomalous points, which means that my results are correct. My errors occurred in the mass used. From my 2nd graph I can see that there is a correlation between the
molecular mass and the enthalpy of combustion, I can see that as the molecular mass increases so does the enthalpy of combustion. In my prediction I predicted that there would be a positive correlation between the enthalpy of combustion and molecular mass, now if I compare my prediction to my results I can see that my prediction was correct.

Evaluation:

There are many limitations that can be caused during the practical procedure of the experiment. Many of the obvious and most common limitations that may easily occur without greatly noticeable effects. However some of these mishaps may cause great effect in your final result of the enthalpy of combustion. For e.g. if the temperature of the water is lower then the enthalpy of combustion would be affected, because the enthalpy of combustion will also become lower than it should be. This would mean that my results would not be reliable and would be incorrect, as it will have a greater effect on my overall final result. Another problem for inaccurate results would be the soot at the bottom of the calorimeter. While the water in the calorimeter is being heated, the soot at the bottom may act as an insulator so therefore it will make the temperature of the water lower than it actually should be. This again will affect my results and make the enthalpy of combustion lower than it should be.

I had foil on top of the calorimeter, this was to ensure that no heat was lost, however, while I had the foil on the calorimeter I also had to stir the water init to ensure that the temperature of the water is the same. While I had to stir the water the foil was easily damaged and torn which resulted in minor excessive heat loss into the surroundings. This will also make the temperature of the water go down which means that the enthalpy of combustion will be affected. Another important factor is to ensure that the lid of the burner is closed properly, this is to reduce the amount of energy from being lost. If the lid is not securely put on this will result in inaccurate results when the burner is reweighed, as there has been energy lost.

To reduce the amount of heat lost it is important to cover the water with foil, however this makes it difficult to stir properly which would give inaccurate results of enthalpy of combustion. So therefore in order to reduce these limitations it is important to stir the water properly and avoid tearing the foil as this will mean that heat is lost.

Finally once the experiment is done and the burner has to be weighed it is important that the lid of the burner is put back on straight away, this is to ensure that no vapour is lost via evaporation. In order for my experiment to go well and to gain accurate results as I possibly could, it was important that I used draft shielding to reduce any heat from being lost into the surrounding areas.

Errors:

The errors that occurred in my investigation were only minor errors that didn’t have a great affect on my overall results. The errors that did occur are shown below:

Errors for hexanol:

Errors due to mass readings:

Start: 0.01/228.962=0.0000436

Finish: 0.01/228.064=0.000438

Errors due to temperature readings:

0.1/19=0.00526

Errors due to volume of water measurements:

1cm3/100cm3=0.01

Total percentage of errors due to equipments:

0.0000436+0.000438+0.00526+0.01*100=1.0%

Errors for butan-1-ol:

Errors due to mass readings:

Start: 0.01/240.541=0.0000415

Finish: 0.01/239.364=0.0000417

Errors due to temperature readings:

0.1/20=0.005

Errors due to volume of water measurements:

1cm3/100cm3=0.01

Total percentage of errors due to equipment:

0.0000415+0.0000417+0.005+0.01*100=1.0%

Errors for propan-1-ol:

Errors due to mass readings:

Start: 0.01/256.323=0.000039

Finish: 0.01/255.201=0.0000392

Errors due to temperature readings:

0.1/20=0.005

Errors due to volume of water measurements:

1cm3/100cm3=0.01

Total percentage of errors due to equipment:

0.000039+0.0000392+0.005+0.01*100=1.0%

Errors for ethanol results:

Errors for mass readings:

Start: 0.01/183.114=0.0000546

Finish: 0.01/182.059=0.0000549

Errors due to temperature readings:

0.1/20=0.0005

Errors due to the volume of water measurements:

1cm3/100cm3=0.01

Total percentage of errors due to equipment:

0.0000546+0.0000549+ 0.0005+0.01*100=1.0%

Errors for methanol results:

Errors for mass reading:

Start: 0.01/183.114=0.0000546

Finish: 0.01/182.059=0.0000549

Errors due to temperature readings:

0.1/19=0.00526

Errors due to volume of water measurements:

1cm3/100cm3=0.01

Total percentage error due to equipment:

0.0000546+0.0000549+0.00526+0.01*100=1.0%

Bibliography:

The sources that I used to gain information and knowledge for my coursework on The Enthalpy Changes Of Combustion Of Different Alcohols were:

Ø Salters Advanced Chemistry – Chemical Ideas.

– Chemical Storylines.

Ø The internet, using search engines such as: – www.google.com

– www.askjeeves.com

Ø Hazard cards: – cards with information about different alcohols and their properties etc.