Question
For the reactions, where experimental determination of cnthalpy is not possible the enthalpy can be calculated by(1) Kirchoff's law(2) Hess's law(3) Henry's law(4) Clapeyron equation
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Kirchhoff's law states that the total current entering a junction or node in a network equals the total current leaving the same junction or node. This law is not applicable to the calculation of enthalpy. Hess's law states that the total enthalpy change during Show more…
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Explain how Hess's law can be used to determine the enthalpy of a reaction by using enthalpy values of other reactions.
Hess's law is used in the determination of (1) Heat of formation (2) Heat of reaction (3) Heat of transition (4) All of these
Hess's law can be used to calculate reaction enthalpies for hypothetical processes that can't be carried out in the laboratory. Set up a Hess's law cycle that will let you calculate $\Delta H^{\circ}$ for the conversion of methane to ethylene: $$ 2 \mathrm{CH}_{4}(g) \longrightarrow \mathrm{C}_{2} \mathrm{H}_{4}(g)+2 \mathrm{H}_{2}(g) $$ You can use the following information: $$ 2 \mathrm{C}_{2} \mathrm{H}_{6}(g)+7 \mathrm{O}_{2}(g) \longrightarrow 4 \mathrm{CO}_{2}(g)+6 \mathrm{H}_{2} \mathrm{O}(l) $$ $$ \Delta H^{\circ}=-3120.8 \mathrm{kJ} $$ $$ \mathrm{CH}_{4}(g)+2 \mathrm{O}_{2}(g) \longrightarrow \mathrm{CO}_{2}(g)+2 \mathrm{H}_{2} \mathrm{O}(l) $$ $$ \begin{array}{l}{\Delta H^{\circ}=-890.3 \mathrm{kJ}} \\ {\Delta H^{\circ}=-136.3 \mathrm{kJ}}\end{array} $$ $$ \begin{array}{l}{\mathrm{C}_{2} \mathrm{H}_{4}(g)+\mathrm{H}_{2}(g) \longrightarrow \mathrm{C}_{2} \mathrm{H}_{6}(g)} \\ {\mathrm{H}_{2} \mathrm{O}(l) \quad \Delta H_{\mathrm{f}}^{\circ}=-285.8 \mathrm{kJ} / \mathrm{mol}}\end{array} $$
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