Textbook Question
The following reaction is exothermic: (c) Is the reaction likely to be spontaneous at lower temperatures only, at higher temperatures only, or at all temperatures?
Ch.9 - Thermochemistry: Chemical Energy
McMurry8th EditionChemistryISBN: 9781292336145
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Table of contents
- Ch.1 - Chemical Tools: Experimentation & Measurement170
- Ch.2 - Atoms, Molecules & Ions160
- Ch.3 - Mass Relationships in Chemical Reactions169
- Ch.4 - Reactions in Aqueous Solution199
- Ch.5 - Periodicity & Electronic Structure of Atoms102
- Ch.6 - Ionic Compounds: Periodic Trends and Bonding Theory92
- Ch.7 - Covalent Bonding and Electron-Dot Structures61
- Ch.8 - Covalent Compounds: Bonding Theories and Molecular Structure59
- Ch.9 - Thermochemistry: Chemical Energy122
- Ch.10 - Gases: Their Properties & Behavior155
- Ch.11 - Liquids & Phase Changes54
- Ch.12 - Solids and Solid-State Materials73
- Ch.13 - Solutions & Their Properties120
- Ch.14 - Chemical Kinetics98
- Ch.15 - Chemical Equilibrium125
- Ch.16 - Aqueous Equilibria: Acids & Bases110
- Ch.17 - Applications of Aqueous Equilibria147
- Ch.18 - Thermodynamics: Entropy, Free Energy & Equilibrium86
- Ch.19 - Electrochemistry154
- Ch.20 - Nuclear Chemistry96
- Ch.21 - Transition Elements and Coordination Chemistry156
- Ch.22 - The Main Group Elements187
- Ch.23 - Organic and Biological Chemistry51
Chapter 9, Problem 38b
The reaction of A with B to give D proceeds in two steps: (1) A + B → C ΔH° = -20 kJ (2) C + B → D ΔH° = +50 kJ (3) A + 2B → D ΔH° = ? (b) What is the value of ΔH° for the overall reaction A + 2 B → D ΔH° = ?
Verified step by step guidance1
Identify the given reactions and their enthalpy changes. The first reaction is A + B → C with an enthalpy change ΔH° = -20 kJ. The second reaction is C + B → D with an enthalpy change ΔH° = +50 kJ.
Understand that the overall reaction A + 2B → D can be obtained by adding the two given reactions. This means you sum up the left-hand sides and the right-hand sides of the reactions respectively.
Add the reactions: (A + B) + (C + B) = C + (D). Simplify this to get A + 2B = D, which matches the overall reaction provided.
Add the enthalpy changes for the individual steps to find the overall enthalpy change ΔH° for the reaction A + 2B → D. This is done by adding ΔH° = -20 kJ and ΔH° = +50 kJ.
The sum of the enthalpy changes will give you the ΔH° for the overall reaction A + 2B → D. This is the value you need to find.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Enthalpy Change (ΔH°)
Enthalpy change (ΔH°) is a measure of the heat absorbed or released during a chemical reaction at constant pressure. A negative ΔH° indicates an exothermic reaction, where heat is released, while a positive ΔH° indicates an endothermic reaction, where heat is absorbed. Understanding ΔH° is crucial for predicting the energy changes associated with chemical reactions.
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Enthalpy of Formation
Hess's Law
Hess's Law states that the total enthalpy change for a reaction is the sum of the enthalpy changes for the individual steps of the reaction, regardless of the pathway taken. This principle allows us to calculate the overall ΔH° for a reaction by adding the ΔH° values of the steps involved, making it essential for solving multi-step reactions.
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Reaction Stoichiometry
Reaction stoichiometry involves the quantitative relationships between reactants and products in a chemical reaction. It is important for determining how much of each substance is consumed or produced. In this context, understanding stoichiometry helps in balancing the overall reaction and applying Hess's Law correctly to find the overall ΔH°.
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Related Practice
Textbook Question
Redraw the following diagram to represent the situation (b) when work has been lost by the system.
Textbook Question
The following reaction is exothermic:
(a) Write a balanced equation for the reaction (red spheres represent A atoms and ivory spheres represent B atoms)
Textbook Question
The reaction of A with B to give D proceeds in two steps: (1) A + B → C ΔH° = -20 kJ (2) C + B → D ΔH° = +50 kJ (3) A + 2B → D ΔH° = ? (a) Which Hess's law diagram represents the reaction steps and the overall reaction? Diagram 1 Diagram 2
Textbook Question
A reaction is carried out in a cylinder fitted with a movable piston. The starting volume is V = 5.00 L, and the appa- ratus is held at constant temperature and pressure. Assum- ing that ∆H = -35.0 kJ and ∆E = -34.8 kJ, redraw the piston to show its position after reaction. Does V increase,decrease, or remain the same?
Textbook Question
Acetylene, C2H2, reacts with H2 in two steps to yield ethane, CH3CH3:(1) HC?CH + H2 → H2C?CH2 ΔH° = -175.1 kJ(2) H2C?CH2 + H2 → CH3CH3 ΔH° = -136.3 kJWhich arrow (a–c) in the Hess's law diagram corresponds to which step, an2d which a2rrow cor2responds to the net reaction? Where are the reactants located on the diagram, and where are the products located?