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Ch.9 - Thermochemistry: Chemical Energy
McMurry - Chemistry 8th Edition
McMurry8th EditionChemistryISBN: 9781292336145Not the one you use?Change textbook
All textbooksMcMurry 8th EditionCh.9 - Thermochemistry: Chemical EnergyProblem 142
Chapter 9, Problem 142

Metallic mercury is obtained by heating the mineral cinnabar (HgS) in air:HgS1s2 + O21g2 S Hg1l2 + SO21g2(a) Use the data in Appendix B to calculate ΔH° in kilojoules for the reaction.

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1
Identify the chemical reaction: HgS(s) + O2(g) -> Hg(l) + SO2(g).
Use the standard enthalpy of formation (ΔH_f°) values from Appendix B for each compound involved in the reaction.
Calculate the total ΔH° for the products by summing the ΔH_f° values of Hg(l) and SO2(g).
Calculate the total ΔH° for the reactants by summing the ΔH_f° values of HgS(s) and O2(g).
Determine ΔH° for the reaction by subtracting the total ΔH° of the reactants from the total ΔH° of the products: ΔH° = ΣΔH_f°(products) - ΣΔH_f°(reactants).

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Key Concepts

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

Enthalpy Change (ΔH°)

Enthalpy change (ΔH°) is the heat content change of a system at constant pressure during a chemical reaction. It indicates whether a reaction is exothermic (releases heat, ΔH° < 0) or endothermic (absorbs heat, ΔH° > 0). Calculating ΔH° involves using standard enthalpy of formation values for reactants and products, which are typically found in thermodynamic tables.
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Standard Enthalpy of Formation

The standard enthalpy of formation (ΔHf°) is the change in enthalpy when one mole of a compound is formed from its elements in their standard states. Each substance has a specific ΔHf° value, which is essential for calculating the overall ΔH° of a reaction. For elements in their standard state, ΔHf° is defined as zero.
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Stoichiometry in Chemical Reactions

Stoichiometry involves the quantitative relationships between the amounts of reactants and products in a chemical reaction. It is crucial for balancing chemical equations and determining the moles of substances involved. In the context of calculating ΔH°, stoichiometry allows for the correct application of ΔHf° values based on the coefficients in the balanced equation.
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Stoichiometry Concept
Related Practice
Textbook Question
(d) What is the kinetic energy of an electron traveling at velocity (c)?
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Textbook Question

Ethyl alcohol has ΔHfusion = 5.02 kJ/mol and melts at - 114.1 °C. What is the value of ΔSfusion for ethyl alcohol?

Textbook Question

Methanol (CH3OH) is made industrially in two steps from CO and H2. It is so cheap to make that it is being considered for use as a precursor to hydrocarbon fuels, such as methane (CH4):

Step 1. CO(g) + 2 H2(g) → CH3OH(l) ΔS° = –332 J/K

Step 2. CH3OH(l) → CH4(g) + 1/2 O2(g) ΔS° = 162 J/K

(a) Calculate ΔH° in kilojoules for step 1.

Textbook Question

What is the melting point of benzene in kelvin if ΔHfusion = 9.95 kJ/mol and ΔSfusion = 35.7 J/(K mol)?

Textbook Question

Methanol (CH3OH) is made industrially in two steps from CO and H2. It is so cheap to make that it is being considered for use as a precursor to hydrocarbon fuels, such as methane (CH4):

Step 1. CO(g) + 2 H2(g) S CH3OH(l) ΔS° = - 332 J/K

Step 2. CH3OH(l) → CH4(g) + 1/2 O2(g) ΔS° = 162 J/K

(e) In what temperature range is step 1 spontaneous?