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Ch.6 - Thermochemistry
Tro - Chemistry: A Molecular Approach 4th Edition
Tro4th EditionChemistry: A Molecular ApproachISBN: 9780134112831Not the one you use?Change textbook
All textbooksTro 4th EditionCh.6 - ThermochemistryProblem 84b
Chapter 6, Problem 84b

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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Identify the elements in their standard states that make up the compound MgCO3(s). Magnesium (Mg) is a solid, carbon (C) is a solid in the form of graphite, and oxygen (O2) is a gas.
Write the balanced chemical equation for the formation of magnesium carbonate (MgCO3) from its elements. The equation is: Mg(s) + C(s) + 3/2 O2(g) → MgCO3(s).
Ensure the equation is balanced with respect to the number of atoms of each element on both sides of the equation.
Consult Appendix IIB or a similar reliable source to find the standard enthalpy change of formation, ΔH °rxn, for MgCO3(s).
Use the value of ΔH °rxn obtained to discuss the thermodynamics of the reaction, considering whether it is exothermic or endothermic.

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Key Concepts

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

Standard States

Standard states refer to the physical states of substances at a defined set of conditions, typically 1 bar of pressure and a specified temperature (usually 25°C). For elements, this means their most stable form at these conditions, such as O2(g) for oxygen and C(s) for carbon. Understanding standard states is crucial for writing formation equations and calculating enthalpy changes.
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Standard Reduction Potentials

Formation Reaction

A formation reaction is a chemical reaction that forms one mole of a compound from its constituent elements in their standard states. The equation for such a reaction is essential for determining the enthalpy change (ΔH°) associated with the formation of the compound. For example, the formation of magnesium carbonate (MgCO3) from magnesium, carbon, and oxygen must be represented accurately to calculate its ΔH°.

Enthalpy Change (ΔH°)

Enthalpy change (ΔH°) is the heat content change associated with a chemical reaction at constant pressure. It can be determined using tabulated standard enthalpy values of formation for the reactants and products. For the formation of MgCO3, ΔH° can be calculated by subtracting the total enthalpy of the reactants from that of the products, providing insight into the energy dynamics of the reaction.
Related Practice
Textbook Question

Hydrazine (N2H4) is a fuel used by some spacecraft. It is normally oxidized by N2O4 according to the equation: N2H4 (l) + N2O4 (g) → 2 N2O (g) + 2 H2O (g) Calculate ΔH°rxn for this reaction using standard enthalpies of formation.

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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

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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Textbook Question

Pentane (C5H12) is a component of gasoline that burns according to the following balanced equation: C5H12(l) + 8 O2(g) → 5 CO2(g) + 6 H2O(g) Calculate ΔH°rxn for this reaction using standard enthalpies of formation. (The standard enthalpy of formation of liquid pentane is –146.8 kJ/mol.)

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Textbook Question

Calculate ΔHrxn for the reaction:

Fe2O3(s) + 3 CO(g) → 2 Fe(s) + 3 CO2(g)

Use the following reactions and given ΔH's:

2 Fe(s) + 3/2 O2(g) → Fe2O3(s) ΔH = –824.2 kJ

CO(g) + 1/2 O2(g) → CO2(g) ΔH = –282.7 kJ

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