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Ch.5 - Thermochemistry
Brown - Chemistry: The Central Science 15th Edition
Brown15th EditionChemistry: The Central ScienceISBN: 9780137542970Not the one you use?Change textbook
All textbooksBrown 15th EditionCh.5 - ThermochemistryProblem 41b
Chapter 5, Problem 41b

Ozone, O3(g), is a form of elemental oxygen that plays an important role in the absorption of ultraviolet radiation in the stratosphere. It decomposes to O2(g) at room temperature and pressure according to the following reaction: 2 O3(g) → 3 O2(g) ΔH= -284.6 kJ b. Which has the higher enthalpy under these conditions, 2 O3(g) or 3 O2(g)?

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Identify the reaction given: 2 O_3(g) \(\rightarrow\) 3 O_2(g) with \(\Delta\) H = -284.6 \(\text{ kJ}\).
Understand that \(\Delta\) H represents the change in enthalpy for the reaction, where a negative value indicates an exothermic reaction.
In an exothermic reaction, the products have lower enthalpy than the reactants because energy is released to the surroundings.
Since \(\Delta\) H is negative, the enthalpy of the products (3 O_2(g)) is lower than the enthalpy of the reactants (2 O_3(g)).
Therefore, 2 O_3(g) has higher enthalpy than 3 O_2(g) under the given conditions.

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Key Concepts

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

Enthalpy Change

Enthalpy change (ΔH) is a measure of the heat content of a system at constant pressure. In the given reaction, the negative value of ΔH (-284.6 kJ) indicates that the reaction is exothermic, meaning it releases heat. This concept is crucial for understanding how the energy of reactants and products compares, influencing their stability and favorability.

Stability of Molecules

The stability of a molecule is often related to its enthalpy; lower enthalpy indicates greater stability. In the reaction provided, 2 O3(g) decomposes into 3 O2(g), and since the formation of O2 releases energy, O2 is generally more stable than O3 under standard conditions. Understanding this stability helps in determining which species has a higher enthalpy.
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Thermodynamic Favorability

Thermodynamic favorability refers to the tendency of a reaction to proceed based on the energy changes involved. A reaction is favored if it leads to products with lower enthalpy. In this case, since the decomposition of O3 to O2 releases energy, the products (O2) are thermodynamically favored, indicating that 3 O2(g) has a lower enthalpy than 2 O3(g).
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Related Practice
Textbook Question

The decomposition of Ca(OH)2 into CaO(s) and H2O at constant pressure requires the addition of 109 kJ of heat per mole of Ca(OH)2 . b. Draw an enthalpy diagram for the reaction.

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Textbook Question

Without referring to tables, predict which of the following has the higher enthalpy in each case: (c) 1 mol H2(g) and 0.5 mol O2(g) at 25 °C or 1 mol H2O(g) at 25 °C

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Textbook Question

Without referring to tables, predict which of the following has the higher enthalpy in each case: (b) 2 mol of hydrogen atoms or 1 mol of H2

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Textbook Question

The complete combustion of ethanol, C2H5OH(l), to form H2O(g) and CO2(g) at constant pressure releases 1235 kJ of heat per mole of C2H5OH. b. Draw an enthalpy diagram for the reaction.

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Textbook Question

A gas is confined to a cylinder under constant atmospheric pressure, as illustrated in Figure 5.4. When 0.49 kJ of heat is added to the gas, it expands and does 214 J of work on the surroundings. What are the values of H and E for this process?

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