Use standard enthalpies of formation in Appendix L to...

Use standard enthalpies of formation in Appendix L to calculate enthalpy changes for the following: (a) $0.054 \mathrm{g}$ of sulfur burns, forming $\mathrm{SO}_{2}(\mathrm{g})$ (b) 0.20 mol of $\mathrm{HgO}(\mathrm{s})$ decomposes to $\mathrm{Hg}(\ell)$ and $\mathbf{O}_{2}(\mathbf{g})$ (c) $2.40 \mathrm{g}$ of $\mathrm{NH}_{3}(\mathrm{g})$ is formed from $\mathrm{N}_{2}(\mathrm{g})$ and excess $\mathrm{H}_{2}(\mathrm{g})$ (d) $1.05 \times 10^{-2}$ mol of carbon is oxidized to $\mathrm{CO}_{2}(\mathrm{g})$

Use standard enthalpies of formation in Appendix L to calculate enthalpy changes for the following:
(a) $0.054 \mathrm{g}$ of sulfur burns, forming $\mathrm{SO}_{2}(\mathrm{g})$
(b) 0.20 mol of $\mathrm{HgO}(\mathrm{s})$ decomposes to $\mathrm{Hg}(\ell)$ and $\mathbf{O}_{2}(\mathbf{g})$
(c) $2.40 \mathrm{g}$ of $\mathrm{NH}_{3}(\mathrm{g})$ is formed from $\mathrm{N}_{2}(\mathrm{g})$ and excess $\mathrm{H}_{2}(\mathrm{g})$
(d) $1.05 \times 10^{-2}$ mol of carbon is oxidized to $\mathrm{CO}_{2}(\mathrm{g})$

Key Concepts

Stoichiometry in Thermochemical Calculations

Stoichiometry involves the quantitative relationship between reactants and products in a chemical reaction. In thermochemical calculations, stoichiometry is used to convert between the amount of substance (in moles or grams) and the energy change per mole, ensuring that computed enthalpy changes correctly reflect the actual quantities involved in the reaction.

Molar Mass and Conversion Calculations

Molar mass is the mass of one mole of a substance, which is crucial for converting between mass and moles in chemical calculations. In problems involving non-molar amounts, such as grams or millimoles, knowing the molar mass permits the correct conversion to moles, thereby allowing the use of standard enthalpies of formation on a per-mole basis.

Standard Enthalpy of Formation

This concept refers to the enthalpy change when one mole of a compound is formed from its constituent elements in their standard states under standard conditions (25°C and 1 atm). It is a fundamental parameter tabulated in reference materials and is used as the building block for calculating reaction enthalpies.

Hess's Law

Hess's Law states that the total enthalpy change for a reaction is the same, regardless of the number of steps in the reaction. This means that by using standard enthalpies of formation for reactants and products, the overall reaction enthalpy can be determined by summing the enthalpies of formation of products, weighted by their stoichiometric coefficients, and subtracting those of the reactants.

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