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Ch.5 - Thermochemistry
All textbooksBrown 15th EditionCh.5 - ThermochemistryProblem 69
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Chapter 5, Problem 69

(c) What is meant by the term standard enthalpy of formation?

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The standard enthalpy of formation, denoted as \( \Delta H_f^\circ \), is the change in enthalpy when one mole of a compound is formed from its elements in their standard states under standard conditions (1 atm pressure and 298 K temperature).
Standard states refer to the most stable form of an element at 1 atm and 298 K. For example, the standard state of oxygen is \( O_2(g) \), and for carbon, it is graphite.
The standard enthalpy of formation for any element in its standard state is defined as zero. This provides a reference point for measuring the enthalpy changes of reactions.
The standard enthalpy of formation is used to calculate the enthalpy changes of chemical reactions using Hess's Law, which states that the total enthalpy change for a reaction is the sum of the enthalpy changes for each step of the reaction.
In practice, \( \Delta H_f^\circ \) values are used in thermochemical equations to determine the heat absorbed or released during the formation of compounds from their constituent elements.

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Key Concepts

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

Standard Enthalpy of Formation

The standard enthalpy of formation (ΔH°f) is defined as the change in enthalpy when one mole of a compound is formed from its elements in their standard states at a specified temperature, usually 25°C. This value is crucial for understanding the energy changes associated with chemical reactions and is typically expressed in kilojoules per mole (kJ/mol).
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Standard State

The standard state of a substance refers to its physical state (solid, liquid, or gas) at a pressure of 1 atmosphere and a specified temperature, usually 25°C. For elements, the standard state is the most stable form of the element under these conditions. Understanding standard states is essential for accurately calculating enthalpy changes.
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Enthalpy Change

Enthalpy change (ΔH) is a measure of the heat content of a system at constant pressure. It reflects the energy absorbed or released during a chemical reaction. The standard enthalpy of formation specifically quantifies the energy change when forming a compound from its elements, providing insight into the stability and reactivity of substances.
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Textbook Question

The concentration of alcohol 1CH3CH2OH2 in blood, called the 'blood alcohol concentration' or BAC, is given in units of grams of alcohol per 100 mL of blood. The legal definition of intoxication, in many states of the United States, is that the BAC is 0.08 or higher. What is the concentration of alcohol, in terms of molarity, in blood if the BAC is 0.08?

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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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Textbook Question

We can use Hess's law to calculate enthalpy changes that cannot be measured. One such reaction is the conversion of methane to ethane: 2 CH4(g) → C2H6(g) + H2(g) Calculate the ΔH° for this reaction using the following thermochemical data: CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(l) ΔH° = -890.3 kJ 2 H2(g) + O2(g) → 2 H2O(l) H° = -571.6 kJ 2 C2H6(g) + 7 O2(g) → 4 CO2(g) + 6 H2O(l) ΔH° = -3120.8 kJ

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Textbook Question

What is the value of the standard enthalpy of formation of an element in its most stable form?

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Textbook Question

For each of the following compounds, write a balanced thermochemical equation depicting the formation of one mole of the compound from its elements in their standard states and then look up H °f for each substance in Appendix C. (b) FeCl3(s)

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Textbook Question

Write balanced equations that describe the formation of the following compounds from elements in their standard states, and then look up the standard enthalpy of formation for each substance in Appendix C: (a) NH4NO3(s)

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