According to Hess's law, the standard enthalpy of a reaction can be calculated as the sum of the standard enthalpies of the intermediate reactions into which the overall reaction can be divided.
Added by Yvonne B.
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What is the enthalpy.
Isaac H.
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: $$ 2 \mathrm{C}_{2} \mathrm{H}_{6}(g)+7 \mathrm{O}_{2}(g) \longrightarrow 4 \mathrm{CO}_{2}(g)+6 \mathrm{H}_{2} \mathrm{O}(l) $$ $$ \Delta H^{\circ}=-3120.8 \mathrm{kJ} $$ $$ \mathrm{CH}_{4}(g)+2 \mathrm{O}_{2}(g) \longrightarrow \mathrm{CO}_{2}(g)+2 \mathrm{H}_{2} \mathrm{O}(l) $$ $$ \begin{array}{l}{\Delta H^{\circ}=-890.3 \mathrm{kJ}} \\ {\Delta H^{\circ}=-136.3 \mathrm{kJ}}\end{array} $$ $$ \begin{array}{l}{\mathrm{C}_{2} \mathrm{H}_{4}(g)+\mathrm{H}_{2}(g) \longrightarrow \mathrm{C}_{2} \mathrm{H}_{6}(g)} \\ {\mathrm{H}_{2} \mathrm{O}(l) \quad \Delta H_{\mathrm{f}}^{\circ}=-285.8 \mathrm{kJ} / \mathrm{mol}}\end{array} $$
Using standard formation enthalpies, calculate the standard reaction enthalpy for this reaction. 4HCl(g) + O2(g) → 2H2O(g) + 2Cl2(g) _____ kJ/mol
David C.
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