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Ch.6 - Thermochemistry
All textbooksTro 4th EditionCh.6 - ThermochemistryProblem 84a
Chapter 6, Problem 84a

Write an equation for the formation of each compound from its elements in their standard states, and find ΔH°rxn for each in Appendix IIB. a. NO2(g)

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1
Identify the elements involved in the formation of \( \text{NO}_2(g) \). These are nitrogen (N) and oxygen (O).
Write the balanced chemical equation for the formation of \( \text{NO}_2(g) \) from its elements in their standard states: \( \frac{1}{2} \text{N}_2(g) + \text{O}_2(g) \rightarrow \text{NO}_2(g) \).
Consult Appendix IIB to find the standard enthalpy of formation, \( \Delta H^\circ_f \), for \( \text{NO}_2(g) \).
Use the standard enthalpy of formation value from Appendix IIB to determine \( \Delta H^\circ_{rxn} \) for the reaction, which is the same as \( \Delta H^\circ_f \) for \( \text{NO}_2(g) \) since it is formed from its elements.
Ensure the units are consistent and the value is reported in kilojoules per mole (kJ/mol).

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Key Concepts

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

Standard State

The standard state of a substance is its most stable form at 1 bar of pressure and a specified temperature, usually 25°C. For elements, this means their natural state, such as O2(g) for oxygen and N2(g) for nitrogen. Understanding standard states is crucial for writing formation equations, as it ensures that the reactants are in their correct physical states.
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Formation Reaction

A formation reaction is a chemical reaction in which one mole of a compound is formed from its constituent elements in their standard states. The equation for the formation of NO2(g) would involve nitrogen gas and oxygen gas combining. This concept is essential for determining the enthalpy change associated with the formation of a compound.
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Enthalpy Change (ΔH°rxn)

The enthalpy change (ΔH°rxn) for a reaction at standard conditions represents the heat absorbed or released during the reaction. It can be found in thermodynamic tables, such as Appendix IIB, and is crucial for understanding the energy dynamics of the formation reaction. Knowing ΔH°rxn helps predict whether a reaction is exothermic or endothermic.
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Related Practice
Textbook Question

Calculate ΔHrxn for the reaction:

CH4(g) + 4 Cl2(g) → CCl4(g) + 4 HCl(g)

Use the following reactions and given ΔH's:

C(s) + 2 H2(g) → CH4(g) ΔH = –74.6 kJ

C(s) + 2 Cl2(g) → CCl4( g) ΔH = –95.7 kJ

H2(g) + Cl2(g) → 2 HCl( g) ΔH = –92.3 kJ

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Textbook Question

Write an equation for the formation of each compound from its elements in their standard states, and find ΔH °f for each in Appendix IIB. a. NH3(g)

Textbook Question

Write an equation for the formation of each compound from its elements in their standard states, and find ΔH°rxn for each in Appendix IIB. b. MgCO3(s)

Textbook Question

Write an equation for the formation of each compound from its elements in their standard states, and find ΔH°rxn for each in Appendix IIB. d. CH3OH(l)

Textbook Question

Hydrazine (N2H4) is a fuel used by some spacecraft. It is normally oxidized by N2O4 according to the equation: N2H4 (l) + N2O4 (g) → 2 N2O (g) + 2 H2O (g) Calculate ΔH°rxn for this reaction using standard enthalpies of formation.