Theoretically, isomers of the same alcohols do not affect the enthalpy change because they contain the same number of bonds. Only the physical structure is different, e.g. propan-1-ol and propan-2-ol.
Bonds of different atoms have different bond strengths. This is because as you go across and down the Periodic Table, the electro negativity increases, so some bonds between atoms will have stronger forces of attraction than others. This means that atoms, which are nearer to right side or lower down the Periodic Table, have higher nuclear density (more protons) and the size of the nuclei are smaller.
All combustion reactions are exothermic. This means that the reaction releases energy, so the products will have lower enthalpy change than the reactants. The reaction is also ΔH negative.
Plan
These are the actions I will take to ensure I will achieve accurate and reliable results:
- Record temperature, in °Celsius, before and after each test.
- Keep temperature range the same for all of the alcohols.
- Keep water level the same throughout the experiment.
- Weigh each alcohol, in grams, before and after each test.
- Record any other results.
- Do the experiment twice.
- Since this experiment cannot be conducted under standard conditions, the following formulas must be used to calculate the percentage error:
These calculations will tell me have I conducted the experiment accurately or not.
Procedure
- Set up equipment and acquire all materials needed.
-
Fill measuring cylinder with water up to 200cm³.
- Pour water into the copper calorimeter.
- Attach the copper calorimeter to the clamp.
- Put two mats around the copper calorimeter.
- Weigh methanol before the test and record the mass.
- Record temperature of water before the test.
- Put the alcohol under the copper calorimeter.
- Use the splinter to light it by using the Bunsen burner.
- Light the wick, which is attached to the alcohol.
- While the alcohol is burning, stir the water by using the thermometer from time to time.
- Keep on heating the water inside the copper calorimeter until the temperature rises 20°C above the recorded temperature before the test.
- Extinguish the burning alcohol.
- Record the temperature after the test, before it decreases.
- Weigh the methanol after the test and record the mass.
- Pour the water into the sink.
- Repeat procedures 2 to 15 for the rest of the alcohols, as stated above under ‘Materials’. Remember to keep the conditions for the other alcohols the same as the first alcohol, in order to make the test fair.
- Repeat the experiment again to cancel out any anomalies.
Implementing
Results
Combustion of different alcohols by mass
Table 1 *Anomaly
Water level: 200cm³
ΔT: 20°
Analyse
I have used the following formulas to calculate the values of enthalpies of combustion for all the alcohols I used during the experiment:
Average enthalpy change of combustion
Methanol
- Use the Periodic Table to calculate the molecular mass of methanol. Write the mass number of each element stated in the molecular formula. If you see a number or the symbol of an element appears more than once, write the appropriate number and the mass number in brackets. Then multiply the sum in brackets first then add up the whole sum to obtain the molecular mass number, e.g.:
= 12+4+16
= 32
- To calculate the average number of moles, use the average mass and divide it by the molecular mass number, e.g.:
32
= 0.039 mol (3 dp)
- Calculate the delta temperature by subtracting the initial temperature from the final temperature, e.g.:
- Calculate the enthalpy change of combustion per mole in kilo joules by multiplying the mass of water by 4.2 Jg‾¹ then by ΔT. Then multiply the number of moles by 1000. Then divide the mass, 4.2 Jg‾¹ and ΔT by the number of moles × 1000, e.g.:
-
ΔHc = 200 g × 4.2 Jg‾¹ × 20°C
0.039mol × 1000
= 16800/39
= -430.77 kJ mol‾¹
Ethanol
-
Mr(CH3CH2OH) = (2×12)+(6×1)+16
= 24+6+16
= 46
46
= 0.021 mol (3 dp)
-
ΔHc = 200 g × 4.2 Jg‾¹ × 20°C
0.021mol × 1000
= 16800/21
= -680 kJ mol‾¹
Propan-1-ol
-
Mr(CH3CH2CH2OH) = (3×12)+(8×1)+16
= 36+8+16
= 60
-
Mol(CH3CH2CH2OH) = 0.931 g
60
= 0.016 mol (3 dp)
- ΔT = 42-22=20
-
ΔHc = 200 g × 4.2 Jg‾¹ × 20°C
0.016mol × 1000
= 16800/16
= -1050 kJ mol‾¹
Propan-2-ol
-
Mr(CH3CH(OH)CH3) = (3×12)+(8×1)+16
= 36+8+16
= 60
-
Mol((CH3CH(OH)CH3) = 0.664 g
60
= 0.011 mol (3 dp)
- ΔT = 42-22=20
-
ΔHc = 200 g × 4.2 Jg‾¹ × 20°C
0.011mol × 1000
= 16800/11
= -1527.28 kJ mol‾¹
Butan-1-ol
-
Mr(CH3(CH2)2CH2OH) = (4×12)+(10×1)+16
= 48+10+16
= 74
-
Mol(CH3(CH2)2CH2OH) = 0.942 g
74
= 0.013 mol (3 dp)
- ΔT = 42-22=20
-
ΔHc = 200 g × 4.2 Jg‾¹ × 20°C
0.013mol × 1000
= 16800/13
= -1292.31 kJ mol‾¹
Average Enthalpy Change of Combustion for each of the alcohols
Background
Alcohols are a series of organic compounds which contain the OH group, which is a functional group. The length of the molecular structure and the position of the OH group determine the difference of the alcohols. All alcohols which I have used during the experiment are monohydric, which contains only one OH group. The number in some names of the alcohols indicates the OH group position, e.g. Propan-1-ol and propan-2-ol. Some isomers have a lower enthalpy change of combustion due to the position of the OH group, e.g. propan-1-ol: -2021 kJ mol‾¹, propan-2-ol: -2006 kJ mol‾¹.
The general formula of alcohols is Cn+H2n-1.
Conclusion
My results reinforce the fact that the longer the alcohol chain (or longer the alkyl chain with an OH functional group), the higher the enthalpy changes of combustion per mole. This is because that longer alcohol chains contain more carbon and hydrogen atoms, which more energy is required to break more C-H, C-C (from ethanol onwards) and O=O bonds and more energy is required to form more C=O and O-H bonds, resulting higher enthalpy change of combustion.
Methanol has the lowest enthalpy change of combustion per mole at -430.77 kJ mol‾¹ because it has the fewest bonds to break and form. The number of carbon atoms increase by 1 and hydrogen atoms increase by 2. Ethanol has the second lowest enthalpy change of combustion per mole at -630 kJ mol‾¹ because it has 11 bonds to break and 10 bonds to form. Propan-1-ol has an enthalpy change of combustion per mole at -1050 kJ mol‾¹ and propan-2-ol releases -1527.28 kJ mol‾¹, which has the highest enthalpy change of combustion per mole in this experiment. I did not prove, in this experiment, that isomers of the same compound make no difference to the enthalpy change of combustion because I did not stir the water to obtain the true temperature reading. Butan-1-ol has the second highest enthalpy change of combustion per mole at -1292.31 kJ mol‾¹.
The graph on page 11A shows a clear pattern that as the number of carbon and hydrogen atom increase, the enthalpy change of combustion per mole increases. I have found an anomaly in my results, concerning propan-2-ol, which is well away from my line of best fit. This is because I did not stir the water to obtain the true temperature reading when the temperature has risen to 42°C, so the temperature was certainly below 42°C. My experiment was not conducted under standard conditions so I will need to use uncertainty formulas in order obtain accurate results and to check if I have conducted the experiment fairly and accurately.
Evaluating
I have an anomalous result, which is propan-2-ol has an enthalpy of combustion per mol at -1527.28 kJ mol‾¹. This is because, during the second experiment, I did not obtain the true temperature reading by stirring the water, as this lowers the temperature to the true reading. This means that the temperature did not increase 20°C above the initial temperature.
I will use the uncertainty formula to prove if I have conducted the experiment accurately. Here are the uncertainty values for each measurement:
The formulas for uncertainty are under ‘Plan’. I have calculated the percentage error values for each measurement and alcohols:
-
Divide the volume error value by the volume of water, which is 200cm³. Then multiply it by 100.
- Divide the mass error value by the average mass of each alcohol. Then multiply it by 100.
- Divide the temperature error by the Δ temperature. Then multiply it by 100.
According to the percentage error values, I have measured the volume of water, temperature and mass of each alcohol accurately as possible.
I have calculated the percentage error for each enthalpy change of combustion of the alcohols:
- Divide the average enthalpy change of combustion by the theoretical enthalpy of combustion. Then multiply it by 100.
-
Methanol: -430.77 kJ mol‾¹
-
Propan-1-ol: -1050 kJ mol‾¹
-
Propan-2-ol: -1527.28 kJ mol‾¹
-
Butan-1-ol: -1292.31 kJ mol‾¹
The percentage error values suggest that my results, concerning the average enthalpy change of combustion, were unreliable. This is because when I put each alcohol on the electronic scale, the values tend to move up and down, making measurement of the mass of each alcohol difficult. There are also limitations when using a thermometer and a measuring cylinder because I had to record the mass or temperature to the nearest gram or degree. I conducted the experiment in different conditions at 20°C (293K) and 1 atm. Then the temperature has decreased to 18°C (291K) during the experiment. I also manipulated the results by rounding it up or down, so this accounts about 5% of the error percentages.
The main causes of anomalous results are heat loss by heating the environment, due to poor insulation of the copper calorimeter, soot on the bottom of the copper calorimeter, incomplete combustion, tiny amounts of water has been lost when pouring from the measuring cylinder into the copper calorimeter, impurities in the water and the experiment was not conducted under standard conditions; at 25°C (298K), 1 atm and using 1 mol dm‾¹ of each alcohol.
Improvements can be made to the procedure by:
- Using the same burner
- Keep the copper calorimeter at the same height
- Pump enriched oxygen to the burner
- Insulate the copper calorimeter.
These procedures will ensure that the results obtained are accurate and reliable because if the same burner is used, the same amount of fuel would be burned since the length of the wick is the same. Keeping the copper calorimeter at the same height, means that the fire can reach the copper calorimeter from the burner at the same time. Pumping enriched oxygen would make combustion of the alcohols complete, so soot is not deposited on the bottom of the copper calorimeter. Finally, insulating the copper calorimeter prevents heat loss by heating the environment.
