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Ch.5 - Thermochemistry
All textbooksBrown 15th EditionCh.5 - ThermochemistryProblem 42c
Chapter 5, Problem 42c

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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Identify the chemical reaction involved: H_2(g) + 0.5 O_2(g) \rightarrow H_2O(g).
Understand that the enthalpy change (\Delta H) for this reaction is related to the formation of water from hydrogen and oxygen.
Recall that the enthalpy of formation for a compound is typically negative, indicating that energy is released when the compound forms from its elements.
Consider that the reactants (H_2 and O_2) are in their elemental forms, which generally have higher enthalpy compared to the compound formed (H_2O).
Conclude that the enthalpy of the reactants (H_2 and O_2) is higher than that of the product (H_2O) because energy is released during the formation of water.

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Key Concepts

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

Enthalpy

Enthalpy is a thermodynamic property that represents the total heat content of a system. It is defined as the sum of the internal energy and the product of pressure and volume. Enthalpy changes are crucial for understanding heat transfer during chemical reactions, particularly in constant pressure conditions, which is common in many chemical processes.
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Standard State Conditions

Standard state conditions refer to a set of specific conditions (usually 1 atm pressure and a specified temperature, often 25 °C) under which the properties of substances are measured. These conditions allow for consistent comparisons of enthalpy values, as substances can exist in different phases (solid, liquid, gas) under varying conditions, affecting their enthalpy.
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Phase Changes and Enthalpy of Formation

The enthalpy of formation is the change in enthalpy when one mole of a compound is formed from its elements in their standard states. Understanding the enthalpy of formation for water and the gaseous reactants is essential for predicting which has a higher enthalpy, as phase changes (from gas to liquid or solid) significantly influence the enthalpy values due to the energy associated with breaking or forming bonds.
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Related Practice
Textbook Question

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)?

Textbook Question

Without referring to tables, predict which of the following has the higher enthalpy in each case: (a) 1 mol CO2(s) or 1 mol CO2(g) at the same temperature

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

Textbook Question

Consider the following reaction: 2 CH3OH(g) → 2 CH4(g) + O2(g) ΔH = +252.8 kJ (b) Calculate the amount of heat transferred when 24.0 g of CH3OH(g) is decomposed by this reaction at constant pressure.

Textbook Question

Consider the following reaction: 2 CH3OH(g) → 2 CH4(g) + O2(g) ΔH = +252.8 kJ (d) How many kilojoules of heat are released when 38.5 g of CH4(g) reacts completely with O2(g) to form CH3OH(g) at constant pressure?