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Ch.5 - Thermochemistry
Brown - Chemistry: The Central Science 14th Edition
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Tutti i libri di testoBrown 14th EditionCh.5 - ThermochemistryProblema 64
Capitolo 5, Problema 64

From the enthalpies of reaction 2 C(s) + O2(g) → 2 CO(g) ΔH = -221.0 kJ 2 C(s) + O2(g) + 4 H2(g) → 2 CH3OH(g) ΔH = -402.4 kJ Calculate ΔH for the reaction CO(g) + 2 H2(g) → CH3OH(g)

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Identify the target reaction: CO(g) + 2 H_2(g) → CH_3OH(g).
Use Hess's Law, which states that the total enthalpy change for a reaction is the same, no matter how many steps the reaction is carried out in.
Write the given reactions and their enthalpy changes: 1) 2 C(s) + O_2(g) → 2 CO(g), ΔH = -221.0 kJ; 2) 2 C(s) + O_2(g) + 4 H_2(g) → 2 CH_3OH(g), ΔH = -402.4 kJ.
Manipulate the given reactions to derive the target reaction. Divide both given reactions by 2 to match the stoichiometry of the target reaction.
Subtract the enthalpy of the first reaction from the second to find the enthalpy change for the target reaction.

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Enthalpy of Reaction

Enthalpy of reaction (ΔH) is the heat change associated with a chemical reaction at constant pressure. It indicates whether a reaction is exothermic (releases heat, ΔH < 0) or endothermic (absorbs heat, ΔH > 0). Understanding ΔH is crucial for predicting the energy changes during reactions and for applying Hess's law to calculate unknown enthalpies.
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Enthalpy of Formation

Hess's Law

Hess's Law states that the total enthalpy change for a reaction is the sum of the enthalpy changes for individual steps, regardless of the pathway taken. This principle allows chemists to calculate the enthalpy change for a reaction that may be difficult to measure directly by using known enthalpy changes of related reactions.
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Hess's Law

Stoichiometry in Reactions

Stoichiometry involves the quantitative relationships between the reactants and products in a chemical reaction. It is essential for balancing chemical equations and determining the amounts of substances consumed or produced. In this context, stoichiometry helps in relating the enthalpy changes of the given reactions to find the ΔH for the target reaction.
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Stoichiometry Concept
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