Test on bonding and energetics
1. (i) Define the terms lattice enthalpy and electron affinity.
(ii) Use the data in the following table and from the data booklet to construct the Born-Haber cycle for sodium chloride, NaCl, and determine the lattice enthalpy of NaCl(s).
Na(s) + Cl2(g) → NaCl(g) ∆H
ο = –411 kJ mol–1
Na(s) → Na(g) ∆H
ο = +108 kJ mol–1
(iii) Describe the structure of sodium chloride.
(Total 8 marks)
2. In some countries, ethanol is mixed with gasoline (petrol) to produce a fuel for cars called gasohol.
(i) Define the term average bond enthalpy.
(ii) Use the information from Table 10 of the Data Booklet to determine the standard enthalpy change for the complete combustion of ethanol.
CH3CH2OH(g) + 3O2(g) → 2CO2(g) + 3H2O(g)
(iii) The standard enthalpy change for the complete combustion of octane, C8H18, is
–5471 kJ mol–1. Calculate the amount of energy produced in kJ when 1 g of ethanol and 1 g of octane is burned completely in air.
(Total 7 marks)
3. Transition elements form complexes such as [Fe(CN)6]4– and [FeCl4]–. Deduce the oxidation number of iron in each of these complex ions.
(Total 2 marks)
4. Describe the delocalization of pi (π) electrons and explain how this can account for the structure and stability of the carbonate ion, CO32–.
(Total 3 marks)
5. The standard enthalpy change of three combustion reactions is given below in kJ.
2C2H6(g) + 7O2(g) → 4CO2(g) + 6H2O(l) ∆H
O = –3120
2H2(g) + O2(g) → 2H2O(l) ∆H
O = –572
C2H4(g) + 3O2(g) → 2CO2(g) + 2H2O(l) ΔH
O = –1411
Based on the above information, calculate the standard change in enthalpy, ∆H
O, for the following reaction.
C2H6(g) → C2H4(g) + H2(g)
(Total 4 marks)
6. In an experiment to measure the enthalpy change of combustion of ethanol, a student heated a copper calorimeter containing 100 cm3 of water with a spirit lamp and collected the following data.
Initial temperature of water: 20.0 °C
Final temperature of water: 55.0 °C
Mass of ethanol burned: 1.78 g
Density of water: 1.00 g cm–3
(i) Use the data to calculate the heat evolved when the ethanol was combusted.
(ii) Calculate the enthalpy change of combustion per mole of ethanol.
(iii) Suggest two reasons why the result is not the same as the value in the Data Booklet.
(Total 6 marks)