To ensure that my measurements are as accurate as possible, instead of using a measuring cylinder, I could use a measuring balance to give me the precise mass of water. Also, to prevent a great heat loss, draught shielding must be placed around the spirit burner to prevent energy from escaping into the surroundings as the experiment is exothermic.
Safety/ Risk Assessment
Care must be taken when carrying out this experiment. Methanol,ethanol,propan-1-ol and butan-1-ol are all highly flammable liquids. The spirit bottles should have lids on them/be stoppered and should be kept well away from naked flames. Do not let the alcohols come into contact with the skin and do not breathe the vapours. Do not open the spirit burners to fill etc in the laboratory with naked flames. Eye protection must also be worn and to minimise the risk of accidents, when the Bunsen burner is not in use it should be on the safety flame and the when the spirit burners are not in use the flame should be extinguished.
Sources
Salters Advanced Chemistry Data Sheets: Table 19: Organic compounds: physical and thermochemical data.
Salters Advanced Chemistry Activity DF 1.2: Measuring the enthalpy change of combustion of different fuels.
Salters Advanced Chemistry, Chemical Ideas 4.1, pages 57,58 and 59.
Salters’ Advanced Chemistry-Activity 1.3:
Enthalpy of combustion
Energy transferred to the water in each experiment
The formula to work out the energy transferred to the water is: Energy transferred=mass of water x temperature rise x 4.2 J.
Methanol
Energy transferred= 200g x 15ºC x 4.2 J
= 12600 J
Ethanol
Energy transferred= 200g x 15ºC x 4.2 J
= 12600 J
Propan-1-ol
Energy transferred= 200g x 15ºC x 4.2 J
= 12600 J
Propan-2-ol
Energy transferred= 200g x 15ºC x 4.2 J
= 12600 J
Butan-1-ol
Energy transferred= 200g x 15ºC x 4.2 J
= 12600 J
Enthalpy change of combustion
Methanol
Formula of alcohol= CH3OH
Mass of 1 mole of alcohol= 32g
Number of moles of alcohol used = 1.9g ÷ 32g = 0.059g/mol-1
Energy transferred by this number of moles of alcohol= 12600J
Enthalpy change of combustion= 12600J ÷ 0.059g/mol-1= 213559 Jmol-1
Conversion into KJ = -213 KJmol-1
Ethanol
Formula of alcohol= CH3CH2OH
Mass of 1 mole of alcohol= 46g
Number of moles of alcohol used= 1.35g ÷ 46g = 0.029g/mol-1
Energy transferred by this number of moles of alcohol= 12600J
Enthalpy change of combustion= 12600J ÷0.029g/mol-1= 434483Jmol-1
Conversion into KJ= -434 KJmol-1
Propan-1-ol
Formula of alcohol= CH3CH2CH2OH
Mass of 1mole of alcohol =60g
Number of moles of alcohol used= 1.35g ÷ 60g = 0.023g/mol-1
Energy transferred by this number of moles of alcohol= 12600J
Enthalpy change of combustion= 12600J ÷0.023g/mol-1= 547826 mol-1
Conversion into KJ= -548KJ mol-1
Propan-2-ol
Formula of alcohol= CH3CH(OH)CH3
Mass of 1 mole of alcohol= 60g
Number of moles of alcohol used= 0.89g ÷ 60g = 0.015g/mol-1
Energy transferred by this number of moles of alcohol= 12600J
Enthalpy change of combustion = 12600J ÷ 0.015g/mol-1= 840000J
Conversion into KJ= -840 KJ
Butan-1-ol
Formula of alcohol= CH3(CH2)2CH2OH
Mass of 1 mole of alcohol= 74g
Number of moles of alcohol used= 0.91g ÷ 74g = 0.012g/mol-1
Energy transferred by 1 mole of alcohol= 12600J ÷ 0.012g/mol-1= 1050000J
Conversion into KJ= -1050 KJ
When looking at my results, I can see that when the molecular formula mass of the alcohol is higher, the larger the enthalpy change of combustion is. This is because more energy is needed to break the bonds in the alcohol and the enthalpy change of combustion is always negative as a combustion reaction is always exothermic. When looking at the molecular formula mass of methanol, there is only one carbon atom present and the enthalpy change of combustion is -213KJ mol-1. However if we compare this to Butan-1-ol, in its molecular structure there are 4 carbon atoms which means that there are more bonds to break (the bonds that join the carbon atoms to the hydrogen atoms), therefore its enthalpy change of combustion is -1050KJ mol-1 as more energy is needed. You can see from the diagram below that as the number of carbon atoms increase, the number of hydrogen also increases.
When looking at my results, as the number of carbon atoms present in the alcohol increases, the higher the enthalpy change of combustion is. Methanol has the lowest enthalpy change as it has a two carbon structure- it doesn’t need as much energy to break down these bonds. Ethanol’s enthalpy change is higher than Methanol’s as there is an extra carbon atom in Ethanol’s molecular structure, meaning that more energy is needed to break down this extra bond. This trend continues as the alcohols molecular structure has more carbon atoms in it- there are more bonds to break, the highest enthalpy change of combustion is seen when looking at Butan-1-ol, lots of energy is needed to break down its 4 carbon structure.
Diagram of molecular structure of methanol and butan-1-ol
Methanol
Butan-1-ol
Chemical equations for the enthalpy change of combustion
Methanol
CH3OH + 11/2O2 CO2 + 2 H2O
Ethanol
CH3CH2OH + 3O2 2CO2 + 3H2O
Propan-1-ol
CH3CH2CH2OH + 41/2O2 3CO2 + 4 H2O
Propan-2-ol
CH3CH(OH)CH3 + 41/2O2 3 CO2 + 4H2O
Butan-1-ol
CH3(CH2)2CH2OH + 6O2 4CO2 + 5 H2O
Salter’s Advanced Chemistry- Activity 1.3:
Enthalpy change of combustion
Limitations of the practical procedure
Some limitations that I experienced when completing my experiment was that there was only basic equipment involved; a calorimeter, a measuring cylinder, a weighing balance and a thermometer as well as five spirit burners each containing a different alcohol. Also everybody used the same procedure which means that we all used the same equipment- sometimes I had to wait for a specific spirit burner.
When looking at the bottom of the calorimeter there was a covering of ‘soot’. The soot was produced from the carbon in the alcohol. This soot was formed when the alcohols were burning and this soot meant that more energy was needed in the experiment to heat up the water- the heat had to pass through the layer of soot before heating up the water. Therefore the enthalpy change would be higher as more energy was needed.
Another limitation that I experienced when I was completing my experiment was that we weren’t provided with great insulation to prevent major energy loss to the surroundings. We had to use heatproof mats that we placed around the sides of the calorimeter and in front and behind it, however we couldn’t place a mat on top as this would have prevented me from stirring the water and from recording the temperature. This poor insulation meant that there was a major heat loss to the surroundings. To prevent this we could have used a ‘bomb calorimeter’ instead.
Procedures important in ensuring data was reliable and precise
To produce reliable and precise results, a number of things had to be done whilst completing the experiment. Firstly, we had to use a weighing balance to measure the mass of the spirit burners before the experiment and after. The weighing balance that we used was to 2 decimal places and provided us with more accurate readings than what we would have obtained if we had of used a balance to 1 decimal place.
We also had to use a measuring cylinder to measure the amount of water that was to be needed. We used a 100cm3 measuring cylinder and had to fill this up twice to achieve the right mass of water- 200g. However it took lots of accuracy to ensure that the bottom of the meniscus rested on the calibration line. To have produced more accurate readings a burette or pipette could have been used as they have a smaller percentage error.
To make certain that we weighed the spirit burners accurately before and after the experiment we weighed them with their lids on them. This was to make sure that none of the alcohol evaporated. If some of the alcohol had of evaporated then an incorrect reading would have been taking on the weighing balance.
To measure the temperature of the water whilst it was heating we used a thermometer that had the highest reading of 110ºC. It took great care in recording the temperatures that the water had reached and we had to make sure that the readings were accurate. To make sure that I got accurate results I heated the water by the same temperature rise each time- 15ºC, and used the same mass of water- 200g. This ensured that my experiment was fair and this meant that I obtained the same result for the energy transferred to the water.
Measurement uncertainty and percentage uncertainty
When carrying out an experiment there will always be some uncertainty. It is sometimes referred to as experimental error and it does not mean that you have made a mistake. The source of the uncertainty is due to the precision of the instrument being used.
In my experiment, a measuring cylinder, weighing balance and thermometer were used. We used the weighing balance to weigh the spirit burners before and after the experiment to see how much alcohol had been used up in the experiment. The balance measured to two decimal places and therefore the percentage error of using the balance is 0.005.
Percentage errors for using the weighing balance
Methanol before the experiment
Percentage error = 0.005 x 100 = 0.0022%
225.82g
Methanol after the experiment
Percentage error = 0.005 x 100 = 0.0022%
223.92g
Ethanol before the experiment
Percentage error = 0.005 x 100 = 0.0021%
231.87g
Ethanol after the experiment
Percentage error = 0.005 x 100 = 0.0021%
230.52g
Propan-1-ol before the experiment
Percentage error = 0.005 x 100 = 0.0020%
240.25g
Propan-1-ol after the experiment
Percentage error= 0.005 x 100 = 0.0020%
238.90g
Propan-2-ol before the experiment
Percentage error= 0.005 x 100 = 0.0022%
224.77g
Propan-2-ol after the experiment
Percentage error = 0.005 x 100 = 0.0022%
223.88g
Butan-1-ol before the experiment
Percentage error = 0.005 x 100 = 0.0022%
192.88g
Butan-1-ol after the experiment
Percentage error = 0.005 x 100 = 0.0026%
191.97g
We also used a measuring cylinder and a thermometer that had their own percentage errors however these are incredibly small. Instead of using a measuring cylinder, a burette or pipette could have been used as these have smaller percentage errors than a measuring cylinder. Care had to be taken when recording measurements and readings from the thermometer and from the measuring cylinder also.
Relative significance of aspects of practical procedures and uncertainties associated with measurements.
When completing my experiment there were various limitations however I have produced results that are as accurate as possible and they follow a trend: the more carbon atoms there are, the more atoms of hydrogen are joined to them meaning that it takes more energy for these bonds to be broken- the enthalpy change of combustion is greater.
When looking at the percentage errors of the experiment all of them add up to make the total experiment error. Any experiment is not going to be perfect as there are always some uncertainty associated with measurement etc however my percentage errors are so small (around 0.002%- 0.0026%) that it doesn’t contribute that greatly to my results. The greatest error was the heat loss from the experiment to the surroundings as the experiment was exothermic and therefore produced negative enthalpy changes of combustion. This is where most of the energy was lost. To prevent this heat loss we could have used a bomb calorimeter that is specifically designed to reduce heat loss to the environment but the equipment that was available to use was an ordinary calorimeter and we insulated it with heatproof mats.
Overall, I feel that my experiment was a success as I obtained a set of results that follow the same trend and I have no anomalous results. A risk assessment was carried out before completing the experiment to ensure that people’s safety was a priority and the experiment was carried out in the most accurate way as possible.
