Skip to main content
Ch.7 - Thermochemistry
Tro - Chemistry: A Molecular Approach 6th Edition
Tro6th EditionChemistry: A Molecular ApproachISBN: 9780137832217Not the one you use?Change textbook
All textbooksTro 6th EditionCh.7 - ThermochemistryProblem 91c
Chapter 7, Problem 91c

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

Verified step by step guidance
1
Identify the standard enthalpy of formation (ΔH_f°) for each compound involved in the reaction. These values are typically found in tables in chemistry textbooks or reliable online resources.
Write the balanced chemical equation: 3 \(\text{NO}\)_2(g) + \(\text{H}\)_2\(\text{O}\)(l) \(\rightarrow\) 2 \(\text{HNO}\)_3(aq) + \(\text{NO}\)(g).
Apply the formula for the standard enthalpy change of the reaction: ΔH_\(\text{rxn}\)° = \(\sum\) \(\Delta\) H_f°(\(\text{products}\)) - \(\sum\) \(\Delta\) H_f°(\(\text{reactants}\)).
Calculate the sum of the standard enthalpies of formation for the products: 2 \(\times\) \(\Delta\) H_f°(\(\text{HNO}\)_3) + 1 \(\times\) \(\Delta\) H_f°(\(\text{NO}\)).
Calculate the sum of the standard enthalpies of formation for the reactants: 3 \(\times\) \(\Delta\) H_f°(\(\text{NO}\)_2) + 1 \(\times\) \(\Delta\) H_f°(\(\text{H}\)_2\(\text{O}\)).

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 essential for determining the overall enthalpy change of a reaction.

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 the reaction. This principle allows for the calculation of ΔH°rxn by summing the enthalpy changes of individual steps, using the standard enthalpies of formation of reactants and products. It emphasizes the state function nature of enthalpy, making it independent of the pathway taken.
Recommended video:
Guided course
02:03
Hess's Law

Reaction Stoichiometry

Reaction stoichiometry involves the quantitative relationships between reactants and products in a chemical reaction, as represented by the balanced chemical equation. Understanding stoichiometry is essential for calculating the enthalpy change (ΔH°rxn) because it dictates the molar ratios of substances involved. Accurate stoichiometric coefficients ensure that the correct amounts of ΔH°f values are used in the calculations.
Recommended video:
Guided course
01:16
Stoichiometry Concept
Related Practice
Textbook Question

Use standard enthalpies of formation to calculate ΔH°rxn for each reaction. a. C2H4(g) + H2(g) → C2H6(g)

1003
views
Textbook Question

Pentane (C5H12) is a component of gasoline that burns according to the following balanced equation: C5H12(l) + 8 O2(g) → 5 CO2(g) + 6 H2O(g) Calculate ΔH°rxn for this reaction using standard enthalpies of formation. (The standard enthalpy of formation of liquid pentane is –146.8 kJ/mol.)

8543
views
3
rank
Textbook Question

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

1060
views
Textbook Question

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)

396
views