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Ch.5 - Thermochemistry
All textbooksBrown 14th EditionCh.5 - ThermochemistryProblem 68
Chapter 5, Problem 68

(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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1
Understand that the standard enthalpy of formation is defined as the change in enthalpy when one mole of a substance is formed from its elements in their standard states.
Recognize that the standard state of an element is its most stable form at 1 atmosphere of pressure and a specified temperature, usually 25°C (298 K).
Note that for any element in its standard state, the standard enthalpy of formation is defined to be zero because no formation reaction is needed to produce the element from itself.
Identify that both fluorine (F_2) and nitrogen (N_2) are in their most stable forms as diatomic molecules under standard conditions.
Conclude that since F_2 and N_2 are in their standard states, their standard enthalpies of formation are zero by definition.

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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 is defined as the change in enthalpy when one mole of a compound is formed from its elements in their standard states. For elements in their standard state, such as F<sub>2</sub> and N<sub>2</sub>, the standard enthalpy of formation is assigned a value of zero. This serves as a reference point for calculating the enthalpy changes of reactions involving these elements.
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Standard State

The standard state of a substance is its physical state at a defined set of conditions, typically 1 bar of pressure and a specified temperature, usually 25°C. For diatomic gases like F<sub>2</sub> and N<sub>2</sub>, their standard state is the gaseous form at these conditions. Understanding standard states is crucial for interpreting thermodynamic data and calculating reaction enthalpies.
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Reactivity and Stability of Elements

Reactivity refers to how readily a substance undergoes chemical reactions, while stability indicates how likely it is to remain unchanged. Fluorine (F<sub>2</sub>) is highly reactive due to its strong electronegativity and tendency to form bonds, whereas nitrogen (N<sub>2</sub>) is relatively inert due to the strong triple bond between nitrogen atoms. Despite their differences in reactivity, both elements have a standard enthalpy of formation of zero because they are in their elemental forms.
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Related Practice
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

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

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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 ΔH°f for each substance in Appendix C. (a) NO2(g) (b) SO3(g) (c) NaBr(s) (d) Pb(NO3)2(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) CH3OH(l)

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Open Question
The following is known as the thermite reaction: 2 Al(s) + Fe2O3(s) → Al2O3(s) + 2 Fe(s). This highly exothermic reaction is used for welding massive units, such as propellers for large ships. Using standard enthalpies of formation in Appendix C, calculate _x001F_H ° for this reaction.