At 298 K, the standard enthalpies of formation for C2 H2(g) and C6 H6(l) are 227 kJ / mol and

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At 298 K, the standard enthalpies of formation for C2 H2(g)...

At $298 \mathrm{~K}$, the standard enthalpies of formation for $\mathrm{C}_{2} \mathrm{H}_{2}(g)$ and $\mathrm{C}_{6} \mathrm{H}_{6}(l)$ are $227 \mathrm{~kJ} / \mathrm{mol}$ and $49 \mathrm{~kJ} / \mathrm{mol}$, respectively. a. Calculate $\Delta H^{\circ}$ for $$ \mathrm{C}_{6} \mathrm{H}_{6}(l) \longrightarrow 3 \mathrm{C}_{2} \mathrm{H}_{2}(g) $$ b. Both acetylene $\left(\mathrm{C}_{2} \mathrm{H}_{2}\right)$ and benzene $\left(\mathrm{C}_{6} \mathrm{H}_{6}\right)$ can be used as fuels. Which compound would liberate more energy per gram when combusted in air?

At $298 \mathrm{~K}$, the standard enthalpies of formation for $\mathrm{C}_{2} \mathrm{H}_{2}(g)$ and $\mathrm{C}_{6} \mathrm{H}_{6}(l)$ are $227 \mathrm{~kJ} / \mathrm{mol}$ and $49 \mathrm{~kJ} / \mathrm{mol}$, respectively.
a. Calculate $\Delta H^{\circ}$ for
$$
\mathrm{C}_{6} \mathrm{H}_{6}(l) \longrightarrow 3 \mathrm{C}_{2} \mathrm{H}_{2}(g)
$$
b. Both acetylene $\left(\mathrm{C}_{2} \mathrm{H}_{2}\right)$ and benzene $\left(\mathrm{C}_{6} \mathrm{H}_{6}\right)$ can be used as fuels. Which compound would liberate more energy per gram when combusted in air?

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Key Concepts

Standard Enthalpy of Formation

This is the change in enthalpy when one mole of a compound is formed from its constituent elements in their standard states under standard conditions (usually 298 K and 1 atm). It serves as a fundamental reference for calculating the enthalpy changes in chemical reactions via tabulated values.

Hess's Law

Hess's Law states that the total enthalpy change for a chemical reaction is the same regardless of the number of steps or the specific pathway taken. This principle allows for the use of known enthalpies of formation to determine unknown reaction enthalpies by summing the individual contributions.

Reaction Enthalpy Calculation

Reaction enthalpy involves computing the net energy change for a chemical reaction using the difference between the sum of the standard enthalpies of formation for products and that of reactants. This calculation is crucial for understanding the heat flow in chemical transformations.

Energy per Unit Mass in Fuels

Energy per unit mass is a measure of how much energy is liberated per gram of a substance during combustion. This concept is important in evaluating the efficiency and practicality of fuels, as it relates to how effectively a fuel can release energy relative to its weight.

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