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Ch.5 - Thermochemistry
Brown - Chemistry: The Central Science 15th Edition
Brown15th EditionChemistry: The Central ScienceISBN: 9780137542970Not the one you use?Change textbook
All textbooksBrown 15th EditionCh.5 - ThermochemistryProblem 71b
Chapter 5, Problem 71b

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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Identify the elements involved in the formation of FeCl_3(s): iron (Fe) and chlorine (Cl).
Determine the standard states of these elements: Fe is a solid (s) and Cl is a diatomic gas (Cl_2(g)).
Write the unbalanced chemical equation for the formation of FeCl_3(s) from its elements: Fe(s) + Cl_2(g) -> FeCl_3(s).
Balance the chemical equation: Since FeCl_3 contains three chlorine atoms, you need 3/2 Cl_2 molecules to provide three Cl atoms. The balanced equation is: Fe(s) + \(\frac{3}{2}\)Cl_2(g) -> FeCl_3(s).
Look up the standard enthalpy of formation (\(\Delta\) H^\(\circ\)_f) for FeCl_3(s) in Appendix C to complete the thermochemical equation.

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Key Concepts

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

Thermochemical Equations

Thermochemical equations represent the heat changes associated with chemical reactions. They show the relationship between the enthalpy change (ΔH) and the stoichiometry of the reaction. In the context of formation reactions, these equations depict how one mole of a compound is formed from its elements in their standard states, allowing for the calculation of enthalpy changes during the formation process.
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Standard State

The standard state of a substance refers to its physical state (solid, liquid, gas) at a defined set of conditions, typically 1 bar of pressure and a specified temperature, usually 25°C. For elements, the standard state is the most stable form at these conditions. Understanding standard states is crucial for accurately writing formation equations and determining standard enthalpy changes.
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Enthalpy of Formation (ΔH°f)

The 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 key thermodynamic quantity used to assess the stability of compounds and predict the heat released or absorbed during chemical reactions. Values of ΔH°f can be found in thermodynamic tables, which are essential for calculating reaction enthalpies.
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Related Practice
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)

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.

Textbook Question

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

Textbook Question

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

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