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Ch.9 - Thermochemistry: Chemical Energy
McMurry - Chemistry 8th Edition
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Tutti i libri di testoMcMurry 8th EditionCh.9 - Thermochemistry: Chemical EnergyProblema 142
Capitolo 9, Problema 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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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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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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Enthalpy of Formation

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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Enthalpy of Formation

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
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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):

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

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

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