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Ch.5 - Thermochemistry
Brown - Chemistry: The Central Science 14th Edition
Brown14th EditionChemistry: The Central ScienceISBN: 9780134414232Not the one you use?Change textbook
All textbooksBrown 14th EditionCh.5 - ThermochemistryProblem 66
Chapter 5, Problem 66

Given the data N2(g) + O2(g) → 2 NO(g) ΔH = +180.7 kJ 2 NO(g) + O2(g) → 2 NO2(g) ΔH = -113.1 kJ 2 N2O(g) → 2 N2(g) + O2(g) ΔH = -163.2 kJ use Hess's law to calculate ΔH for the reaction N2O(g) + NO2(g) → 3 NO(g)

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Identify the target reaction: \( \text{N}_2\text{O}(g) + \text{NO}_2(g) \rightarrow 3 \text{NO}(g) \).
Write down the given reactions and their enthalpy changes: \( \text{N}_2(g) + \text{O}_2(g) \rightarrow 2 \text{NO}(g) \), \( \Delta H = +180.7 \text{ kJ} \); \( 2 \text{NO}(g) + \text{O}_2(g) \rightarrow 2 \text{NO}_2(g) \), \( \Delta H = -113.1 \text{ kJ} \); \( 2 \text{N}_2\text{O}(g) \rightarrow 2 \text{N}_2(g) + \text{O}_2(g) \), \( \Delta H = -163.2 \text{ kJ} \).
Reverse the third reaction to match the target reaction's reactants: \( 2 \text{N}_2(g) + \text{O}_2(g) \rightarrow 2 \text{N}_2\text{O}(g) \), changing \( \Delta H \) to \(+163.2 \text{ kJ} \).
Adjust the stoichiometry of the reactions to match the target reaction: Divide the first reaction by 2 to get \( \text{N}_2(g) + \frac{1}{2} \text{O}_2(g) \rightarrow \text{NO}(g) \), and divide the second reaction by 2 to get \( \text{NO}(g) + \frac{1}{2} \text{O}_2(g) \rightarrow \text{NO}_2(g) \).
Combine the adjusted reactions to form the target reaction, ensuring that the intermediates cancel out, and sum the enthalpy changes to find \( \Delta H \) for the target reaction.

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Key Concepts

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

Hess's Law

Hess's Law states that the total enthalpy change for a chemical reaction is the sum of the enthalpy changes for the individual steps of the reaction, regardless of the pathway taken. This principle allows chemists to calculate the enthalpy change for a reaction that may be difficult to measure directly by using known enthalpy changes of related reactions.
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Hess's Law

Enthalpy Change (ΔH)

Enthalpy change (ΔH) is a measure of the heat content of a system at constant pressure. It indicates whether a reaction is exothermic (releases heat, ΔH < 0) or endothermic (absorbs heat, ΔH > 0). Understanding ΔH is crucial for predicting the energy changes associated with chemical reactions and for applying Hess's Law effectively.

Stoichiometry

Stoichiometry involves the calculation of reactants and products in chemical reactions based on the balanced chemical equation. It is essential for determining the proportions of substances involved in reactions, which is necessary when applying Hess's Law to combine multiple reactions and find the overall enthalpy change for a target reaction.
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For each of the following compounds, write a balanced thermochemical equation depicting the formation of one mole of the compound from its elements in their standard states and then look up ΔH°f for each substance in Appendix C. (a) NO2(g) (b) SO3(g) (c) NaBr(s) (d) Pb(NO3)2(s).

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Write balanced equations that describe the formation of the following compounds from elements in their standard states, and then look up the standard enthalpy of formation for each substance in Appendix C: (a) CH3OH(l)

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From the enthalpies of reaction H2(g) + F2(g) → 2 HF(g) ΔH = -537 kJ C(s) + 2 F2(g) → CF4(g) ΔH = -680 kJ 2 C(s) + 2 H2(g) → C2H4(g) ΔH = +52.3 kJ Calculate H for the reaction of ethylene with F2: C2H4(g) + 6 F2(g) → 2 CF4(g) + 4 HF(g)

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Consider the following hypothetical reactions: A → B ΔHI = +60 kJ B → C ΔHII = -90 kJ (b) Construct an enthalpy diagram for substances A, B, and C, and show how Hess's law applies.

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Calculate the enthalpy change for the reaction P4O6(s) + 2 O2(g) → P4O10(s) given the following enthalpies of reaction: P4(s) + 3 O2(g) → P4O6(s) ΔH = -1640.1 kJ P4(s) + 5 O2(g) → P4O10(s) ΔH = -2940.1 kJ

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