Textbook Question
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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Ch.5 - Thermochemistry
Brown14th EditionChemistry: The Central ScienceISBN: 9780134414232
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Table of contents
- Ch.1 - Introduction: Matter, Energy, and Measurement140
- Ch.2 - Atoms, Molecules, and Ions192
- Ch.3 - Chemical Reactions and Reaction Stoichiometry172
- Ch.4 - Reactions in Aqueous Solution156
- Ch.5 - Thermochemistry151
- Ch.6 - Electronic Structure of Atoms137
- Ch.7 - Periodic Properties of the Elements127
- Ch.8 - Basic Concepts of Chemical Bonding143
- Ch.9 - Molecular Geometry and Bonding Theories167
- Ch.10 - Gases147
- Ch.11 - Liquids and Intermolecular Forces97
- Ch.12 - Solids and Modern Materials129
- Ch.13 - Properties of Solutions126
- Ch.14 - Chemical Kinetics175
- Ch.15 - Chemical Equilibrium107
- Ch.16 - Acid-Base Equilibria127
- Ch.17 - Additional Aspects of Aqueous Equilibria133
- Ch.18 - Chemistry of the Environment88
- Ch.19 - Chemical Thermodynamics140
- Ch.20 - Electrochemistry137
- Ch.21 - Nuclear Chemistry99
- Ch.22 - Chemistry of the Nonmetals66
- Ch.23 - Transition Metals and Coordination Chemistry69
- Ch.24 - The Chemistry of Life: Organic and Biological Chemistry11
Brown14th EditionChemistry: The Central ScienceISBN: 9780134414232Not the one you use?Change textbook
Chapter 5, Problem 62b
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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Identify the enthalpy changes for each reaction: \( \Delta H_{I} = +60 \text{kJ} \) for \( A \rightarrow B \) and \( \Delta H_{II} = -90 \text{kJ} \) for \( B \rightarrow C \).
Draw an enthalpy diagram with the y-axis representing enthalpy (H) and the x-axis representing the reaction progress.
Place substance A at the starting point on the y-axis. Since \( \Delta H_{I} = +60 \text{kJ} \), draw an upward arrow to represent the conversion of A to B, indicating an increase in enthalpy.
From B, draw a downward arrow to represent the conversion of B to C, since \( \Delta H_{II} = -90 \text{kJ} \), indicating a decrease in enthalpy.
Apply Hess's Law by adding the enthalpy changes of the two steps to find the overall enthalpy change for the reaction \( A \rightarrow C \).
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Enthalpy (ΔH)
Enthalpy is a thermodynamic quantity that represents the total heat content of a system. It is often expressed in kilojoules (kJ) and can indicate whether a reaction is exothermic (releases heat, ΔH < 0) or endothermic (absorbs heat, ΔH > 0). In the given reactions, the positive and negative values of ΔH indicate the heat changes associated with the transformations between substances A, B, and C.
Hess's Law
Hess's Law states that the total enthalpy change for a chemical reaction is the same, regardless of the number of steps taken to achieve the reaction. This principle allows us to calculate the enthalpy change for a reaction by summing the enthalpy changes of individual steps, making it useful for constructing enthalpy diagrams and understanding the overall energy changes in a series of reactions.
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Hess's Law
Enthalpy Diagram
An enthalpy diagram visually represents the enthalpy changes of reactants and products in a chemical reaction. It typically shows the relative energy levels of the substances involved, allowing for a clear understanding of how energy is absorbed or released during the reaction process. In this case, the diagram would illustrate the transitions from A to B and then from B to C, highlighting the respective enthalpy changes.
Related Practice
Textbook Question
Under constant-volume conditions, the heat of combustion of benzoic acid (C6H5O6) is 15.57 kJ/g. A 3.500-g sample of sucrose is burned in a bomb calorimeter. The temperature of the calorimeter increases from 20.94 to 24.72 °C. (b) If the size of the sucrose sample had been exactly twice as large, what would the temperature change of the calorimeter have been?
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Textbook Question
Consider the following hypothetical reactions: A → B ΔHI = +60 kJ B → C ΔHII = -90 kJ (a) Use Hess’s law to calculate the enthalpy change for the reaction A → C.
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Textbook Question
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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Textbook Question
Under constant-volume conditions, the heat of combustion of naphthalene (C10H8) is 40.18 kJ/g. A 2.50-g sample of naphthalene is burned in a bomb calorimeter. The temperature of the calorimeter increases from 21.50 to 28.83 °C. (c) Suppose that in changing samples, a portion of the water in the calorimeter were lost. In what way, if any, would this change the heat capacity of the calorimeter?
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Textbook Question
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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