3.3

Question 1

3.3 Determination of a enthalpy change of combustion• To determine the enthalpy change of combustion of an alcohol.

Answer 1

You will determine the enthalpy change of combustion of an alcohol. You will heat some water by burning a small amount of an alcohol. You will need to record
• The mass of the alcohol that burns
• The mass of the water that is heated
• The temperature change of the water

Question 2

Methanol

Answer 2

Flammable

Question 3

Ethanol

Answer 3

Flammable

Question 4

Propan-1-ol

Answer 4

Flammable and irritant

Question 5

Butan-1-ol

Answer 5

Irritant

Question 6

Pentan-1-ol

Answer 6

Hazardous if inhaled

Question 7

Equipment

Answer 7

• 100 cm3 measuring cylinder
• Beaker (250 cm3)
• Spirit burner with wick
• Access to a balance that records to two decimal places
• Thermometer (10º –110 ºC) graduated in divisions of 0.5 °C or less
• Heatproof mat

Question 8

Procedure

Answer 8

1. Measure 100 cm3 of water into a beaker.
2. Accurately weigh a ‘burner’ containing one of the alcohols. Record all of your measurements in an appropriate table.
3. Arrange the apparatus in such a way that the burner can be used to heat the water in the beaker. You should try to protect your apparatus from draughts.
4. Take the temperature of the water before you start heating.
5. Use your burner to heat the water. Stop heating when you have heated the water enough to give a reasonable temperature rise. Take the final temperature of the water and reweigh the burner.

Question 9

Analysis of results

Answer 9

6. From your results, calculate the energy given out by the alcohol in heating the water.
7. Calculate the amount, in mol, of the alcohol burnt and determine the enthalpy change for the combustion of the alcohol.
8. Write an equation for the enthalpy change of combustion of the alcohol.
9. Evaluate your results by
   a. Comparing your experimental value to the accepted enthalpy change for the combustion of the alcohol and suggest three reasons why there is a difference.
10. Suggest how the experiment could be improved to give more accurate results.

Question 10

1. Calculate the enthalpy change of combustion for methanol, ethanol, propan-1-ol, butan-1-ol and pentan-1-ol using bond enthalpies. (You will need to look up values for the relevant bonds).

Answer 10

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Question 11

2. Work out the difference between the enthalpy values for each successive member of the alcohol homologous series. Why is the difference a constant value?

Answer 11

As the number of carbon atoms in an alcohol chain increases, its respective standard enthalpy change of combustion also increases. As it can be seen, the standard enthalpy combustion values increase as the number of carbon atoms increase. This can be explained by each successive member of the series contains one more methylene group (-CH2-) than the previous one. When it is burnt, there will be one extra C-C bond and two extra C-H bonds to be broken.

Question 12

3. Predict a value for the enthalpy change of combustion of dodecan-1-ol, CH3(CH2)11OH.

Answer 12

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Question 13

4. Why are these calculations not standard enthalpy changes?

Answer 13

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Question 14

5. Find the other information needed to convert the bond enthalpy values into standard enthalpy changes.

Answer 14

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Question 15

note

Answer 15

Greater accuracy is obtained in this activity when using a balance to 2 decimal places, an accurate thermometer marked with 1intervals, a large volume of water, a small temperature rise and protection from draughts