Apparatus required:
- 100cm³ beaker.
- Copper calorimeter and stirrer.
- Digital balance.
- Thermometer.
- Clamp Stand to hold the calorimeter.
- Scale to measure distance between tip of wick and base of calorimeter.
- Matches to ignite the wick of the spirit lamps.
- Alcohols in spirit burner:
- Propanol.
- Butanol.
- Pentanol.
- Hexanol.
- Distilled water = 50cm³
Method:
- Measure 50 cm³ of distilled water in the beaker and pour it into the calorimeter.
- Measure the mass of the spirit lamp on the digital balance.
- Record the temperature of the water using a thermometer.
- Keep the spirit lamp containing under the calorimeter in such a way that the distance between the tip of the lamp and the base of the calorimeter is 5cm apart.
- Light the wick of the spirit lamp so that the calorimeter is being heated.
- Stir the water in the calorimeter so that the heat doesn’t concentrate only at the centre of the calorimeter.
-
Since the change in temperature is supposed to be 30˚C as stated in the controlled, put off the flame once there has been an increase of 30˚C from the starting temperature.
- Measure the mass of the spirit lamp after is has been put off.
- Next, subtract the mass of the spirit lamp after the flame was put off from the mass of the spirit lamp before it was lit. This will give us the value of the mass of alcohol used for the combustion of water.
- To calculate how much heat is absorbed by the water, we must use the equation
Q = mc∆T.
In the above equation,
‘m’ represents the mass of water = 50g = 0.05kg
‘c‘ represents the specific heat capacity of water = 4.18 KJ/KgK
And, ∆T represents the change in temperature = 30˚C.
- Next, we find the number of moles combusted using the formula
No. of moles = m/Mr where m = the mass of the alcohol used and Mr = molecular mass of the compound.
- If we divide the value of the heat absorbed by the water by the number of moles (n), we get the enthalpy change of combustion of the alcohol i.e. ∆H = Q/n.
- Next we check which alcohol has the highest enthalpy change of combustion and evaluate our hypothesis.
Note: The above method should be repeated twice for each of the four alcohols.
- DATA COLLECTION AND PROCESSING.
Results table:
Calculations:
Amount of heat absorbed by water: Q = mc∆T
= 0.05 x 4.18 x 30
= 6.27 KJ.
Propanol: Mass of alcohol used = 0.95g.
Molecular Mass = 60.1 g.
Therefore, number of moles = 0.95/60.1
= 0.01580 moles.
Enthalpy change (∆H) = 6.27/0.01580
= -396.84 KJ/mol.
% error = (Theoretical value – experiment value) x 100
Theoretical value
= 2021 – 396.84 x 100
2021
= 80.00% error.
Butanol: Mass of alcohol used = 0.725g.
Molecular Mass = 74.1 g.
Number of moles = 0.00978 moles.
Enthalpy change (∆H) = 6.27/0.00978
= - 641.10 KJ/mol.
% error = 2676 – 641.10 x 100
2676
= 76.04% error.
Pentanol: Mass of alcohol used = 0.61g.
Molecular Mass = 88.15 g.
Number of moles = 0.00692 moles.
Enthalpy change (∆H) = 6.27/0.00692
= - 906.06 KJ/mol.
% error = 3329 –906.06 x 100
3329
= 72.78% error.
Hexanol: Mass of alcohol used = 0.565g.
Molecular Mass = 102.17g.
Number of moles = 0.0055 moles.
Enthalpy change (∆H) = 6.27/0.0055
= -1140.00 KJ/mol.
% error = 3984 – 1140.00 x 100
3984
= 71.39% error.
Let us display the processed data:
- CONCLUSION AND EVALUATION.
As we can see from the graph and the resultant values, the enthalpy change of combustion of an alcohol increases as the size of the alcohol increases i.e.as the number of carbon atoms increase even though there is an % error of more than 70% in all the calculated values of enthalpy change which is a vast difference between the theoretical and experiment values. This statement goes parallel to the hypothesis I made in the beginning which states that the enthalpy change increases as the length of the alcohol chain increases i.e. addition of carbon atoms.
As we know, a more suitable fuel is one which releases more energy on combustion. Therefore, from the reults we can conclude by saying that hexanol is comparatively a more suitable fuel to use than propanol.
Therefore, the hypothesis has been proved.
Since we observed that there is a % error of more than 70% in all the calculated values of enthalpy change and the method of the experiment is accurate, the limitations to this experiment is due to the faulty apparatus.
- One of the main reasons of the error is due to the heat loss as all the heat was not absorbed by the calorimeter but a significant amount of heat was lost to the surroundings, to the calorimeter and to the thermometer. Therefore, complete combustion does not take place.
- The alcohol containers had varying sizes of wicks and amount of alcohol in them. Therefore, the flame was varying in size and was one of the sources of the error.
- Some of the alcohol might be evaporating from the spirit lamp.
- The thermometer might have touched the bottom of the calorimeter giving inaccurate results.
- Another possibility could be lack of oxygen in the room which led to incomplete combustion as oxygen molecules would react with only one carbon molecule to from carbon monoxide, CO which is a poisonous gas since limited supply of oxygen causes some carbon atoms get released before they react with oxygen and hence might be present in the calorimeter in the form of soot.
Improvements:
- Use of a heat insulator such as heatproof mats to minimize heat loss and using an air tight around the spirit lamp to prevent any amount alcohol from evaporating.
- Make sure the bob of the thermometer does not touch the extreme bottom of the calorimeter and the water is stirred throughout the experiment.
- Make sure that a sufficient supply of oxygen is available; the experiment takes place in an area where there is free air circulation.
- Similar spirit lamps are used having the same width of the wick so that the flame can reach the calorimeter regardless of the alcohol used.
- Use of more advanced equipment.