A handbook lists two different values for the heat of...

A handbook lists two different values for the heat of combustion of hydrogen: $33.88 \mathrm{kcal} / \mathrm{g}$ if $\mathrm{H}_{2} \mathrm{O}(1)$ is formed, and $28.67 \mathrm{kcal} / \mathrm{g}$ if $\mathrm{H}_{2} \mathrm{O}(\mathrm{g})$ is formed. Explain why these two values are different, and indicate what property this difference represents. Devise a means of verifying your conclusions.

Key Concepts

Calorimetry

Calorimetry is an experimental technique used to measure the energy released or absorbed during a chemical reaction. By performing controlled calorimetric experiments for reactions that produce water in both its liquid and gaseous states, one can verify the difference in measured heat of combustion, thereby confirming that the discrepancy is due to the latent heat associated with the phase change.

Thermochemical Equations and Reaction Enthalpies

Thermochemical equations account for all energy changes in a reaction, including those from chemical bond formation and breakage as well as phase changes. The formation enthalpy of water depends on whether it is in the liquid or gaseous state, and the difference between these values reflects the energy involved in the phase change, highlighting the importance of reaction conditions in energy calculations.

Phase Transitions and Latent Heat of Vaporization

The difference in reported heats of combustion arises because of the latent heat of vaporization—the additional energy required or released when a substance changes its phase without changing its temperature. When water is produced as a gas rather than as a liquid, the energy associated with the condensation (or vaporization) process is not recovered under the same conditions, leading to a lower apparent heat of combustion.

Heat of Combustion

The heat of combustion is a measure of the energy released when a substance burns completely in oxygen. In thermochemistry, it provides insights into how much energy is stored in chemical bonds and released during reactions, and it is used to compare the energetic outputs of different fuels under specific conditions.

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