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Ch.9 - Thermochemistry: Chemical Energy
All textbooksMcMurry 8th EditionCh.9 - Thermochemistry: Chemical EnergyProblem 98
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Chapter 9, Problem 98

What is a compound's standard heat of formation?

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Key Concepts

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

Standard Heat of Formation

The standard heat 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 measured under standard conditions, typically at 1 atm pressure and a specified temperature, usually 25°C. This value is crucial for understanding the energy changes associated with chemical reactions.
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Standard State

The standard state of a substance refers to its physical state (solid, liquid, gas) at a specified temperature and pressure, usually 1 atm and 25°C. For elements, the standard state is the most stable form at these conditions. Understanding standard states is essential for accurately determining the standard heat of formation, as it provides a reference point for the enthalpy changes.
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Enthalpy Change

Enthalpy change (ΔH) is a measure of the total heat content of a system and reflects the energy absorbed or released during a chemical reaction. It is a key concept in thermodynamics, allowing chemists to predict whether a reaction is exothermic (releases heat) or endothermic (absorbs heat). The standard heat of formation specifically quantifies the enthalpy change associated with the formation of compounds from their elements.
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Related Practice
Textbook Question
Hess's law can be used to calculate reaction enthalpies for hypothetical processes that can't be carried out in the labo- ratory. Set up a Hess's law cycle that will let you calculate ∆H° for the conversion of methane to ethylene: 2 CH4(g) → C2H4(g) + 2 H2(g) You can use the following information: 2 C2H6(g) + 7 O2(g) → 4 CO2(g) + 6 H2O(l) ∆H° = -3120.8 kJ CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(l) ∆H° = -890.3 kJ C2H4(g) + H2(g) → C2H6(g) ∆H° = -136.3 kJ H2O(l) ∆H°f = -285.8 kJ/mol
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Textbook Question
Find ∆H° in kilojoules for the reaciton of nitric oxide with oxygen, 2 NO(g) + O2(g) → N2O4(g), given the following data: N2O4(g) → 2 NO2(g) ∆H° = 55.3 kJ NO(g) + 1/2 O2(g) → NO2(g) ∆H° = -58.1 kJ
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Textbook Question
Set up a Hess's law cycle, and use the following information to calculate ΔH°f for aqueous nitiric acid, HNO3(aq). You will need to use fractional coefficients for some equations. 3 NO2(g) + H2O(l) → 2 HNO3(aq) + NO(g) ΔH° = -137.3 kJ 2 NO(g) + O2(g) → 2 NO2(g) ΔH° = -116.2 kJ 4 NH3(g) + 5 O2(g) → 4 NO (g) + 6 H2O(l) ΔH° = -1165.2 kJ NH3(g) ΔH°f = -46.1 kJ/mol H2O(l) ΔH°f = -285.8 kJ/mol
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Textbook Question
How is the standard state of an element defined? Why do elements always have ∆H°f = 0?
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Textbook Question
What phase of matter is associated with the standard states of the following elements and compounds? (a) Cl2 (b) Hg (c) CO2 (d) Ga
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Textbook Question
What is the phase of the standard states of the following ele- ments and compounds? (a) NH3 (b) Fe (c) N2 (d) Br2
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