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Ch.6 - Thermochemistry
All textbooksTro 4th EditionCh.6 - ThermochemistryProblem 87d
Chapter 6, Problem 87d

Use standard enthalpies of formation to calculate ΔH°rxn for each reaction. d. Cr2O3(s) + 3 CO(g) → 2 Cr(s) + 3 CO2(g)

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Identify the standard enthalpies of formation (ΔH°f) for each reactant and product in the reaction. These values are typically found in a table in your textbook or a reliable chemistry database. For this reaction, you need the ΔH°f for Cr2O3(s), CO(g), Cr(s), and CO2(g).
Write the balanced chemical equation and note the stoichiometric coefficients. For this reaction, it is already balanced as: Cr2O3(s) + 3 CO(g) → 2 Cr(s) + 3 CO2(g).
Apply the formula for calculating the standard enthalpy change of the reaction (ΔH°rxn): ΔH°rxn = ΣΔH°f (products) - ΣΔH°f (reactants). Remember to multiply the ΔH°f of each substance by its stoichiometric coefficient in the balanced equation.
Substitute the ΔH°f values into the equation. Multiply each ΔH°f by the respective coefficients from the balanced equation: ΔH°rxn = [2(ΔH°f for Cr) + 3(ΔH°f for CO2)] - [ΔH°f for Cr2O3 + 3(ΔH°f for CO)].
Simplify the expression to find the ΔH°rxn. This will give you the standard enthalpy change for the reaction, which indicates whether the reaction is exothermic or endothermic.

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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 value, which can be found in tables, and is essential for determining the overall enthalpy change of a reaction.
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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 achieve that reaction. This principle allows chemists to calculate the enthalpy change of a reaction by summing the enthalpy changes of individual steps, making it possible to use standard enthalpies of formation to find ΔH°rxn for complex reactions.
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Reaction Stoichiometry

Reaction stoichiometry involves the quantitative relationships between reactants and products in a chemical reaction. It is essential for balancing chemical equations and determining the amounts of substances consumed and produced. In the context of calculating ΔH°rxn, stoichiometry helps in applying the correct coefficients from the balanced equation to the standard enthalpy values to ensure accurate calculations.
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Related Practice
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Use standard enthalpies of formation to calculate ΔH°rxn for each reaction. a. C2H4(g) + H2(g) → C2H6(g)

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Use standard enthalpies of formation to calculate ΔH°rxn for each reaction. c. 3 NO2(g) + H2O(l) → 2 HNO3(aq) + NO(g)

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Use standard enthalpies of formation to calculate ΔH°rxn for each reaction. a. 2 H2S(g) + 3 O2(g) → 2 H2O(l) + 2 SO2(g)

Textbook Question

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