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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
Step 1
Step 1: Hess's law, also known as Hess's law of constant heat summation, states that the total enthalpy change during the complete course of a chemical reaction is the same whether the reaction is made in one step or in several steps. Show more…
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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: $$ 2 \mathrm{C}_{2} \mathrm{H}_{6}(g)+7 \mathrm{O}_{2}(g) \longrightarrow 4 \mathrm{CO}_{2}(g)+6 \mathrm{H}_{2} \mathrm{O}(l) $$ $$ \Delta H^{\circ}=-3120.8 \mathrm{kJ} $$ $$ \mathrm{CH}_{4}(g)+2 \mathrm{O}_{2}(g) \longrightarrow \mathrm{CO}_{2}(g)+2 \mathrm{H}_{2} \mathrm{O}(l) $$ $$ \begin{array}{l}{\Delta H^{\circ}=-890.3 \mathrm{kJ}} \\ {\Delta H^{\circ}=-136.3 \mathrm{kJ}}\end{array} $$ $$ \begin{array}{l}{\mathrm{C}_{2} \mathrm{H}_{4}(g)+\mathrm{H}_{2}(g) \longrightarrow \mathrm{C}_{2} \mathrm{H}_{6}(g)} \\ {\mathrm{H}_{2} \mathrm{O}(l) \quad \Delta H_{\mathrm{f}}^{\circ}=-285.8 \mathrm{kJ} / \mathrm{mol}}\end{array} $$
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
Hess's Law The enthalpy changes for the following reactions can be measured: $\mathrm{CH}_{4}(\mathrm{g})+2 \mathrm{O}_{2}(\mathrm{g}) \rightarrow \mathrm{CO}_{2}(\mathrm{g})+2 \mathrm{H}_{2} \mathrm{O}(\mathrm{g})$ $$ \Delta_{r} H^{\circ}=-802.4 \mathrm{kJ} / \mathrm{mol}-\mathrm{rxn} $$ $\mathrm{CH}_{3} \mathrm{OH}(\mathrm{g})+3 / 2 \mathrm{O}_{2}(\mathrm{g}) \rightarrow \mathrm{CO}_{2}(\mathrm{g})+2 \mathrm{H}_{2} \mathrm{O}(\mathrm{g})$ $\Delta_{1} H^{\circ}=-676 \mathrm{kJ} / \mathrm{mol}-\mathrm{rxn}$ (a) Use these values and Hess's law to determine the enthalpy change for the reaction $\mathrm{CH}_{4}(\mathrm{g})+1 / 2 \mathrm{O}_{2}(\mathrm{g}) \rightarrow \mathrm{CH}_{3} \mathrm{OH}(\mathrm{g})$ (b) Draw an energy level diagram that shows the relationship between the energy quantities involved in this problem.
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