IB Chemistry HL 1.2.4 Hl Hesss Law Applications Questions

Use real HL exam questions to apply products-minus-reactants formation data, reactants-minus-products combustion data and sign checks.

Exam points

Calculate standard reaction enthalpy from formation data using ΣΔHf°(products) − ΣΔHf°(reactants), applying all equation coefficients and state-specific values.
Calculate standard reaction enthalpy from combustion data using ΣΔHc°(reactants) − ΣΔHc°(products), applying stoichiometric coefficients and the correct subtraction order.
Rearrange a formation or combustion enthalpy expression to determine an unknown value and check the sign and magnitude against the reaction direction.

[Maximum number: 1]

Phosphoryl chloride, $\mathrm{POCl}_{3}$ , is a dehydrating agent.

$\quad \mathrm{POCl}_{3}(\mathrm{~g})$ decomposes according to the following equation.

2 \mathrm{POCl}_{3}(\mathrm{~g}) \rightarrow 2 \mathrm{PCl}_{3}(\mathrm{~g})+\mathrm{O}_{2}(\mathrm{~g})

Calculate the standard enthalpy change for the reaction, $\Delta H^{\ominus}$ , in $\mathrm{kJ} \mathrm{mol}^{-1}$ , using the data below.

$\left.\left(\Delta H^{\ominus}=[(2)(-288.1)]-[(2)(-542.2)]\right)=\right)(+) 508.2\left(\mathrm{~kJ} \mathrm{~mol}^{-1}\right)$ ;

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