Unit 5 Worksheets 4 Energy cycles
1. In Section 19 of the data booklet the enthalpy of aqueous solution of potassium chloride is given
as +17.22 kJ mol-1. Draw an energy cycle showing how the enthalpy of solution is related to the
lattice enthalpy and the relevant hydration enthalpies for potassium chloride. Use the values given
in Sections 18 and 20 to calculate the enthalpy of solution and compare the value you obtain with
the value given in Section 19.
2. (a) Draw a Born-Haber cycle for the formation of rubidium chloride from its elements in their
standard states.
(b) Use the information given to calculate a value for the lattice enthalpy of rubidium chloride.
Enthalpy of formation of rubidium chloride = – 431 kJ mol-1
Enthalpy of atomization of rubidium = + 85.8 kJ mol-1
Enthalpy of atomization of chlorine = + 121 kJ mol-1
First ionization energy of rubidium = + 403 kJ mol-1
Electron affinity of chlorine = – 349 kJ mol-1
(c) Compare your value with the theoretical value of − 680 kJ mol-1.
What can be inferred from this comparison about the bonding in rubidium chloride?
3. Explain the following:
(a) The lattice enthalpy of magnesium oxide is larger than the lattice enthalpy of calcium oxide.
(b) The lattice enthalpy of magnesium chloride is larger than the lattice enthalpy of sodium chloride.
(c) The lattice enthalpy of sodium fluoride is larger than the lattice enthalpy of sodium chloride.
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4. The following shows an enthalpy level diagram for the formation of strontium oxide.
(a) Draw the Born-Haber cycle for the formation of strontium oxide.
(b) Identify the correct names for X and Y.
(c) Calculate the value for the enthalpy of formation of strontium oxide.
Answers
1.
ΔH⦵solution(KCl) = ΔH⦵lattice enthalpy(KCl) + ΔH⦵hydration(K+) + ΔH⦵hydration(Cl–)
ΔH⦵solution(KCl) = +720 + (–340) + (–359) = +21 kJ mol-1
This differs by 3.78 kJ mol-1 (22%) from the value given in Section 19.
2. (a)
(b) ∆H f(RbCl) = ∆H
at(Rb)
+ ∆H
at(Cl)
+ ∆H
– 431 = + 85.8 + 121 + 403 + (– 349) + ∆H
∆H
latt(RbCl)
= − 692 kJ mol-1
IE(Rb)
latt(RbCl)
+ ∆H
EA(Cl)
+ ∆H
latt(RbCl)
(c) The difference between the experimental value and the theoretical value is 12 kJ mol-1 or1.7%.
Since there is very little difference, the bonding in rubidium chloride is close to being 100% ionic.
3. (a) The ionic radius of the Mg2+ ion is smaller than the ionic radius of the Ca2+ ion so the charge
density on the magnesium ion is greater so there is greater attraction to the negative oxide ion.
(b) The ionic radius of the Mg2+ ion is smaller than the ionic radius of the Na+ ion and the charge is
twice as large so the charge density on the magnesium ion is much greater so there is greater
attraction to the negative chloride ions.
(c) The ionic radius of the F– ion is smaller than the ionic radius of the Cl– ion so the charge
density on the fluoride ion is greater so there is greater attraction to the positive sodium ion.
4. (a)
(b) X represents the sum of the 1st and 2nd electron affinities of oxygen.
Y represents the sum of the 1st and 2nd ionization energies of strontium.
(c) ∆H f(SrO) = + 164 + 249 + 1608 + 657 + (-3223) = −545 kJ mol-1