IB Chemistry HL 1.2.2 Hesss Law Questions

Apply Hess-cycle reasoning to annotate solution-energy cycles, combine lattice and hydration terms and calculate signed enthalpy changes.

Exam points

Construct or annotate a Hess or solution-energy cycle by identifying lattice, hydration, formation or combustion steps and their state symbols.
Combine cycle contributions with correct signs and stoichiometric factors to calculate the target enthalpy change, including enthalpy of solution.
Use Hess-cycle or formation/combustion data consistently to explain pathway independence, compare routes and report the correct sign and units.

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Study 1.2.2—Hess's law conceptsOpen the corresponding Concept lesson for explanations, examples and exam connections.

[Maximum number: 3]

Solid ionic compounds form crystal lattices.

(a)

Entalpy of solution, enthalpy of hydration and lattice enthalpy are related in an energy cycle.

[ 3 ]

(i)

Annotate the energy cycle for the enthalpy of solution of solid magnesium chloride, $\mathrm{MgCl}_{2}$ (s), by naming the processes A, B and C and completing the boxes. Include state symbols.

Figure for Question (i) — IB Chemistry HL

A:
B:
C:

[ 2 ]

Figure for Answer to Question (i) — IB Chemistry HL

A: enthalpy of solution / $\Delta H$ solution / $\Delta H_{\text{sol}}$ .
B: lattice enthalpy / $\Delta H_{\text{lattice}}$ .
C: enthalpy of hydration / $\Delta H_{\text{hydration}}$ .

(ii)

Calculate the enthalpy of solution for magnesium chloride, $\mathrm{MgCl}_{2}$ . Use sections 18 and 20 of the data booklet.

[ 1 ]

$\Delta H_{\text{solution}}=\Delta H_{\text{lattice}}+\Delta H_{\text{hydration}}$ $\Delta H_{\text{solution}}=2540+(-1963)+2(-359) =-141\ \mathrm{kJ\ mol^{-1}}$

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IB Chemistry HL 1.2.2 Hesss Law Questions question 1

Question (a)

Question (i)

Question (ii)