IB Chemistry SL 1.2.2 Hesss Law Questions

Use real exam questions to reverse and scale equations, cancel species and calculate pathway-independent enthalpy changes.

Exam points

Construct or manipulate a Hess cycle by reversing and scaling component reactions so unwanted species cancel and the target reaction remains.
Calculate the target ΔH from the signed sum of component enthalpy changes in a Hess cycle or reaction set, reporting the correct sign and units.
Interpret an enthalpy cycle and use formation, combustion or solution data consistently to justify the pathway-independent enthalpy change.

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

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The students repeated the experiment using 6.16 g of solid hydrated magnesium sulfate, $\mathrm{MgSO}_{4} \cdot 7 \mathrm{H}_{2} \mathrm{O}(\mathrm{s})$ , and $50.0 \mathrm{~cm}^{3}$ of water. They found the enthalpy change, $\Delta H_{2}$ , to be $+18 \mathrm{~kJ} \mathrm{~mol}^{-1}$ .

The enthalpy of hydration of solid anhydrous magnesium sulfate is difficult to determine experimentally, but can be determined using the diagram below.

\mathrm{MgSO}_{4} \cdot 7 \mathrm{H}_{2} \mathrm{O}(\mathrm{~s}) \xrightarrow[\text { water }]{\Delta H_{2}} \mathrm{Mg}^{2+}(\mathrm{aq})+\mathrm{SO}_{4}^{2-}(\mathrm{aq})

Determine the enthalpy change, $\Delta H$ , in $\mathrm{kJ} \mathrm{mol}^{-1}$ , for the hydration of solid anhydrous magnesium sulfate, $\mathrm{MgSO}_{4}$ .

$\quad \Delta H\left(=\Delta H_{1}-\Delta H_{2}\right)=-99\left(\mathrm{~kJ} \mathrm{~mol}^{-1}\right)$ ;

Marking guidance:

Award [1] if-86 is used giving an answer of $-104\left(\mathrm{~kJ} \mathrm{~mol}^{-1}\right)$ .

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IB Chemistry SL 1.2.2 Hesss Law Questions question 1