Energy cycles questions
Assignment: Energy cycles short answer questions
This page of questions on energy cycles can be used in two different ways.
1. It can be set by the teacher as a formal written assignment. This can either be over a set period of time (i.e. as a test or exam) or for students to complete as and when they can by the deadline set by the teacher. Once they have answered all the questions it is automatically sent to the teacher to mark (with comments to return to the student if required) and the marks are automatically recorded in their mark book. After it has been sent and marked the student will then be able to see the mark they achieved, any comments the teacher has made and the fully worked model answers.
2. Students can be given direct access to the page by their teacher so it they can work through it in their own time for revision or self-testing etc. and students will then be able to also see the fully worked answers.
The standard enthalpy of aqueous solution, ΔH⦵solution, for potassium chloride, is given as +17.22 kJ mol−1 in Section 19 of the data booklet .
(a) Draw an energy cycle showing how the enthalpy of solution is related to the lattice enthalpy and the relevant hydration enthalpies for potassium chloride.
(b) Use the values given in Sections 18 and 20 of the data booklet to calculate the enthalpy of solution and compare the value you obtain with the value given in Section 19.
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.
(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
– 431 = + 85.8 + 121 + 403 + (– 349) + ∆Hlatt(RbCl)
∆Hlatt(RbCl) = − 692 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?
The difference between the experimental value and the theoretical value is 12 kJ mol−1 (or 1.7%).
Since there is very little difference, the bonding in rubidium chloride is close to being 100% ionic.
Explain the following:
(a) The lattice enthalpy of magnesium oxide is larger than the lattice enthalpy of calcium oxide.
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 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.
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.