Ch.5 - Thermochemistry

Problem 67

Chapter 5, Problem 67

(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).

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.

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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Related Practice

Textbook Question

From the enthalpies of reaction 2 C(s) + O₂(g) → 2 CO(g) ΔH = -221.0 kJ 2 C(s) + O₂(g) + 4 H₂(g) → 2 CH₃OH(g) ΔH = -402.4 kJ Calculate ΔH for the reaction CO(g) + 2 H₂(g) → CH₃OH(g)

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Textbook Question

From the enthalpies of reaction H₂(g) + F₂(g) → 2 HF(g) ΔH = -537 kJ C(s) + 2 F₂(g) → CF₄(g) ΔH = -680 kJ 2 C(s) + 2 H₂(g) → C₂H₄(g) ΔH = +52.3 kJ Calculate H for the reaction of ethylene with F₂: C₂H₄(g) + 6 F₂(g) → 2 CF₄(g) + 4 HF(g)

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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

(a) Why does the standard enthalpy of formation of both the very reactive fluorine (F₂) and the almost inert gas nitrogen (N₂) both read zero?

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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) SO₃(g)

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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) CH₃OH(l)

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