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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Ch.5 - Thermochemistry
Chapter 5, Problem 70
What is the value of the standard enthalpy of formation of an element in its most stable form?
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Identify the concept of standard enthalpy of formation, which is 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 atm pressure and a specified temperature, usually 25°C (298 K).
Understand that for any element in its most stable form, the standard enthalpy of formation is defined as zero.
This definition is based on the convention that no energy is required to form an element from itself.
Therefore, the value of the standard enthalpy of formation for an element in its most stable form is always zero.
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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 the change in enthalpy when one mole of a compound is formed from its elements in their standard states. It is a crucial concept in thermodynamics and helps in calculating the energy changes during chemical reactions.
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02:34
Enthalpy of Formation
Most Stable Form of an Element
The most stable form of an element refers to its physical state and allotrope under standard conditions (1 atm pressure and 25°C). For example, the most stable form of carbon is graphite, while oxygen exists as O2 gas. This stability is essential for determining the standard enthalpy of formation.
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02:35Elemental Forms of Elements
Zero Enthalpy of Formation for Elements
The standard enthalpy of formation for any element in its most stable form is defined as zero. This serves as a reference point for calculating the enthalpy changes of compounds formed from these elements, simplifying thermodynamic calculations in chemistry.
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Related Practice
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
(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 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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Textbook Question
Many portable gas heaters and grills use propane, C3H8(g), as a fuel. Using standard enthalpies of formation, calculate the quantity of heat produced when 10.0 g of propane is completely combusted in air under standard conditions.
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