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Ch.6 - Thermochemistry
All textbooksTro 4th EditionCh.6 - ThermochemistryProblem 84
Chapter 6, Problem 84

Write an equation for the formation of C2H4(g) from its elements in their standard states, and find ΔH°rxn for it in Appendix IIB.

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Identify the elements that make up \( \text{C}_2\text{H}_4(g) \), which are carbon (C) and hydrogen (H).
Determine the standard states of these elements: carbon is solid graphite \( \text{C(s)} \) and hydrogen is diatomic gas \( \text{H}_2(g) \).
Write the balanced chemical equation for the formation of \( \text{C}_2\text{H}_4(g) \) from its elements: \( 2\text{C(s)} + 2\text{H}_2(g) \rightarrow \text{C}_2\text{H}_4(g) \).
Look up the standard enthalpy of formation \( \Delta H_f^\circ \) for \( \text{C}_2\text{H}_4(g) \) in Appendix IIB.
Use the \( \Delta H_f^\circ \) value from the appendix to find \( \Delta H^\circ_{rxn} \) for the reaction, which is the same as \( \Delta H_f^\circ \) for \( \text{C}_2\text{H}_4(g) \) since it is formed from its elements in their standard states.

Key Concepts

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

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 writing thermochemical equations and calculating enthalpy changes.
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Formation Reaction

A formation reaction is a specific type of chemical reaction where one mole of a compound is formed from its constituent elements in their standard states. This reaction is essential for determining the standard enthalpy of formation (ΔH°f), which is a key value used in thermodynamic calculations to find the enthalpy change of other reactions.
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Enthalpy Change (ΔH°rxn)

The enthalpy change (ΔH°rxn) for a reaction at standard conditions represents the heat absorbed or released during the reaction. It can be calculated using standard enthalpies of formation of the reactants and products. This concept is fundamental in thermodynamics, allowing chemists to predict the energy changes associated with chemical reactions.
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Related Practice
Textbook Question

Calculate ΔHrxn for the reaction:

5 C(s) + 6 H2(g) → C5H12(l)

Use the following reactions and given ΔH's:

C5H12(l) + 8 O2(g) → 5 CO2(g) + 6 H2O(g) ΔH = –3244.8 kJ

C(s) + O2(g) → CO2(g) ΔH = –393.5 kJ

2 H2(g) + O2(g) → 2 H2O(g) ΔH = –483.5 kJ

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Textbook Question

Calculate ΔHrxn for the reaction:

CH4(g) + 4 Cl2(g) → CCl4(g) + 4 HCl(g)

Use the following reactions and given ΔH's:

C(s) + 2 H2(g) → CH4(g) ΔH = –74.6 kJ

C(s) + 2 Cl2(g) → CCl4( g) ΔH = –95.7 kJ

H2(g) + Cl2(g) → 2 HCl( g) ΔH = –92.3 kJ

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Textbook Question

Write an equation for the formation of each compound from its elements in their standard states, and find ΔH °f for each in Appendix IIB. a. NH3(g)

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Textbook Question

Write an equation for the formation of each compound from its elements in their standard states, and find ΔH°rxn for each in Appendix IIB. a. NO2(g)

Textbook Question

Write an equation for the formation of each compound from its elements in their standard states, and find ΔH°rxn for each in Appendix IIB. b. MgCO3(s)

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Textbook Question

Write an equation for the formation of each compound from its elements in their standard states, and find ΔH°rxn for each in Appendix IIB. d. CH3OH(l)

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