Textbook Question
b. If energy can flow in and out of the system to maintain a constant temperature during the process, what can you say about the entropy change of the surroundings as a result of this process?
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Ch.19 - Chemical Thermodynamics
Chapter 19, Problem 6a
Isomers are molecules that have the same chemical formula but different arrangements of atoms, as shown here for two isomers of pentane, C5H12.
(a) Do you expect a significant difference in the enthalpy of combustion of the two isomers? Explain.
Verified step by step guidance1
Understand the concept of isomers: Isomers are compounds with the same molecular formula but different structural arrangements. In this case, we are dealing with two isomers of pentane, C_5H_12.
Consider the enthalpy of combustion: The enthalpy of combustion is the heat released when one mole of a substance is burned in excess oxygen. It is typically measured in kJ/mol.
Recognize that the enthalpy of combustion depends on the stability of the isomers: More stable isomers generally have lower energy and thus release less energy upon combustion.
Analyze the structural differences: The two isomers of pentane may have different branching, which affects their stability. Generally, more branched isomers are more stable due to lower surface area and less van der Waals forces.
Conclude the expectation: Since the isomers have different stabilities, you can expect a difference in their enthalpy of combustion, with the more branched isomer likely having a slightly lower enthalpy of combustion due to its higher stability.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Isomerism
Isomerism refers to the phenomenon where two or more compounds have the same molecular formula but differ in the arrangement of atoms. This can lead to variations in physical and chemical properties, including boiling points, melting points, and reactivity. In the case of pentane, the two isomers, n-pentane and isopentane, illustrate how structural differences can influence the behavior of molecules.
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Isomerism in Coordination Complexes Example
Enthalpy of Combustion
The enthalpy of combustion is the heat released when one mole of a substance is completely burned in oxygen. It is a crucial measure in thermodynamics, reflecting the energy content of fuels. Different isomers can have varying enthalpies of combustion due to differences in their molecular structure, which affects the stability of the reactants and products formed during combustion.
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02:24Combustion Apparatus
Thermodynamic Stability
Thermodynamic stability refers to the relative energy levels of different molecular structures. More stable isomers typically have lower enthalpy values, meaning they release less energy upon combustion. The stability of isomers can influence their enthalpy of combustion, as less stable isomers may release more energy when they undergo combustion due to the greater energy difference between reactants and products.
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01:18First Law of Thermodynamics
Related Practice
Textbook Question
Predict the signs of ΔH and ΔS for this reaction. Explain your choice.
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Textbook Question
The accompanying diagram shows how entropy varies with temperature for a substance that is a gas at the highest temperature shown. (c) If this substance is a perfect crystal at T = 0 K, what is the value of S at this temperature?
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Textbook Question
Isomers are molecules that have the same chemical formula but different arrangements of atoms, as shown here for two isomers of pentane, C5H12.
(b) Which isomer do you expect to have the higher standard molar entropy? Explain.
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Textbook Question
The accompanying diagram shows how ΔH (red line) and TΔS (blue line) change with temperature for a hypothetical reaction.
(b) In what temperature range is this reaction spontaneous?
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Open Question
Using data from Appendix C, write the equilibrium-constant expression and calculate the value of the equilibrium constant and the free-energy change for these reactions at 298 K: (b) 2 HBr(g) + Cl2(g) ⇌ 2 HCl(g) + Br2(g) (c) 2 SO2(g) + O2(g) ⇌ 2 SO3(g)