Measuring the Enthalpy of Vapourisation of Water
Measuring the enthalpy change of vaporisation of water
You should attempt this part of the activity even if you have not seen the demonstration of the practical.
1. Calculate a value for ∆Hvap for water in KJ mol-1 using the data from the demonstration. Also, calculate ∆Hvap from the following data gathered by a student; 8100 J were needed to distil 2.95g of water.
a) Explain why the boiling tube was surrounded with expanded polystyrene (or other similar insulation), but the condenser was left uncovered.
b) Before any water was collected, the water was allowed to boil for several minutes until drops steadily fell from the condenser. Explain why this helps achieve greater accuracy when estimating ∆Hvap.
The table below lists values for ∆Hvap and boiling point (Tb) for water and four organic compounds.
Liquid
Formula
∆Hvap/KJ mol-1
Tb/K
Trichloromethane
CHCl3
+29.3
335
Hexane
C6H14
+28.8
342
Cyclohexane
C6H12
+30.1
354
Methylbenzene
C7H8
+33.4
384
water
H2O
+40.6
373
2. Water has a high ∆Hvap value compared with the other substances in the above table. Use your knowledge of intermolecular forces to explain this observation.
a) Ethanol also has an unusually high ∆Hvap value of +38.5 kJ mol-1. Explain how the changes which occur during boiling are similar for ethanol and water.
3. From your studies you should be aware that water plays a vital role in terms of the ‘global climate’. Write a brief report (max. 150 words) explaining why the fact that water carries much more energy with it than other substances is important for the climate of the Earth.
You should attempt this part of the activity even if you have not seen the demonstration of the practical.
1. Calculate a value for ∆Hvap for water in KJ mol-1 using the data from the demonstration. Also, calculate ∆Hvap from the following data gathered by a student; 8100 J were needed to distil 2.95g of water.
a) Explain why the boiling tube was surrounded with expanded polystyrene (or other similar insulation), but the condenser was left uncovered.
b) Before any water was collected, the water was allowed to boil for several minutes until drops steadily fell from the condenser. Explain why this helps achieve greater accuracy when estimating ∆Hvap.
The table below lists values for ∆Hvap and boiling point (Tb) for water and four organic compounds.
Liquid
Formula
∆Hvap/KJ mol-1
Tb/K
Trichloromethane
CHCl3
+29.3
335
Hexane
C6H14
+28.8
342
Cyclohexane
C6H12
+30.1
354
Methylbenzene
C7H8
+33.4
384
water
H2O
+40.6
373
2. Water has a high ∆Hvap value compared with the other substances in the above table. Use your knowledge of intermolecular forces to explain this observation.
a) Ethanol also has an unusually high ∆Hvap value of +38.5 kJ mol-1. Explain how the changes which occur during boiling are similar for ethanol and water.
3. From your studies you should be aware that water plays a vital role in terms of the ‘global climate’. Write a brief report (max. 150 words) explaining why the fact that water carries much more energy with it than other substances is important for the climate of the Earth.