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 $\mathrm{C}_{6} \mathrm{H}_{6}(I)$ 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 $\mathrm{C}_{6} \mathrm{H}_{6}(I)$ 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

Energy Density

Energy density refers to the amount of energy released per unit mass of a substance. It is a key concept in comparing fuels as it determines which fuel provides more energy for a given mass, making it crucial for efficiency assessments.

Combustion Reaction

Combustion reactions involve the reaction of a substance, often a fuel, with oxygen to produce energy along with combustion products like carbon dioxide and water. Understanding these reactions is essential when evaluating the energy output of different fuels.

Hess's Law

Hess's Law states that the total enthalpy change of a reaction is the same regardless of the pathway taken, since enthalpy is a state function. This allows us to determine reaction enthalpies by combining the standard enthalpies of formation of reactants and products.

Standard Enthalpy of Formation

This is the change in enthalpy when one mole of a compound is formed from its elements in their standard states (usually at 298 K). It is used as a reference point to calculate the enthalpy change of chemical reactions via Hess’s Law.

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