IB Chemistry SL Reactivity 1.1.4 Standard Enthalpy Change

Practise calculating standard enthalpy changes from calorimetry measurements. Determine ΔT, use q = mcΔT with consistent units, identify the reacting amount and report ΔH = −q/n…

Exam points

calculate heat gained by water or solution from mass, heat capacity and temperature change
divide the opposite heat transfer by reacting moles to obtain ΔH in kJ mol⁻¹
rearrange calorimetry data to find temperature change or a sample's specific heat capacity

[Maximum number: 2]

Magnesium sulfate can exist in either the hydrated form or in the anhydrous form. Two students wished to determine the enthalpy of hydration of anhydrous magnesium sulfate. They measured the initial and the highest temperature reached when anhydrous magnesium sulfate, $\mathrm{MgSO}_{4}(\mathrm{~s})$ , was dissolved in water. They presented their results in the following table.

Table for Question 1.1.4—Standard enthalpy change (ΔH⦵) question 1 — IB Chemistry SL

Calculate the enthalpy change, $\Delta H_{1}$ , for anhydrous magnesium sulfate dissolving in water, in $\mathrm{kJ} \mathrm{mol}^{-1}$ . State your answer to the correct number of significant figures.

energy released $=50.0 \times 4.18 \times 9.7=2027(\mathrm{~J}) / 2.027(\mathrm{~kJ})$ ;
$\Delta H_{1}=-81\left(\mathrm{~kJ} \mathrm{~mol}^{-1}\right) ;$

Marking guidance:

Award [2] for correct answer.
Award [2] if 53.01 is used giving an answer of $-86\left(\mathrm{~kJ} \mathrm{~mol}^{-1}\right)$ .
Award [1 max] for $+81 / 81 /+86 / 86\left(\mathrm{~kJ} \mathrm{~mol}^{-1}\right)$ .
Award [1 max] for -81000/-86000 if units are stated as $\mathrm{J} \mathrm{mol}^{-1}$ .
Allow answers to 3 significant figures.

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1.1.4—Standard enthalpy change (ΔH⦵) question 1