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Ch.6 - Thermochemistry
All textbooksTro 4th EditionCh.6 - ThermochemistryProblem 88b
Chapter 6, Problem 88b

Use standard enthalpies of formation to calculate ΔH°rxn for each reaction. b. SO2(g) + 1/2 O2(g) → SO3(g)

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Identify the standard enthalpy of formation (ΔH°<sub>f</sub>) for each compound involved in the reaction: SO<sub>2</sub>(g), O<sub>2</sub>(g), and SO<sub>3</sub>(g). Note that the standard enthalpy of formation for any element in its standard state, such as O<sub>2</sub>(g), is zero.
Write the balanced chemical equation for the reaction: SO<sub>2</sub>(g) + \frac{1}{2} O<sub>2</sub>(g) → SO<sub>3</sub>(g).
Use the formula for the standard enthalpy change of the reaction: ΔH°<sub>rxn</sub> = Σ ΔH°<sub>f</sub>(products) - Σ ΔH°<sub>f</sub>(reactants).
Substitute the standard enthalpies of formation into the formula: ΔH°<sub>rxn</sub> = [ΔH°<sub>f</sub>(SO<sub>3</sub>(g))] - [ΔH°<sub>f</sub>(SO<sub>2</sub>(g)) + \frac{1}{2} ΔH°<sub>f</sub>(O<sub>2</sub>(g))].
Calculate the value of ΔH°<sub>rxn</sub> using the substituted values to find the enthalpy change for the reaction.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Enthalpy of Formation

The standard enthalpy of formation (ΔH°f) is the change in enthalpy when one mole of a compound is formed from its elements in their standard states. It is a crucial value used in thermodynamics to calculate the heat changes in chemical reactions. Each substance has a specific ΔH°f, which can be found in tables, and is typically expressed in kilojoules per mole (kJ/mol).
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Hess's Law

Hess's Law states that the total enthalpy change for a reaction is the same, regardless of the number of steps taken to complete the reaction. This principle allows us to calculate the enthalpy change of a reaction by using the enthalpy values of formation of the reactants and products. By applying Hess's Law, we can sum the ΔH°f values to find the overall ΔH°rxn for a given reaction.
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Standard Conditions

Standard conditions refer to a set of specific conditions used as a reference point in thermodynamic calculations, typically defined as 1 atm pressure and a temperature of 25°C (298 K). Under these conditions, the standard enthalpy of formation values are measured, ensuring consistency in thermodynamic data. Understanding standard conditions is essential for accurately calculating reaction enthalpies and comparing thermodynamic properties.
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