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Ch.7 - Thermochemistry
Tro - Chemistry: A Molecular Approach 5th Edition
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Tutti i libri di testoTro 5th EditionCh.7 - ThermochemistryProblema 82
Capitolo 7, Problema 82

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

Guida verificata passo dopo passo
1
Identify the target reaction: CH4(g) + 4 Cl2(g) \(\rightarrow\) CCl4(g) + 4 HCl(g).
List the given reactions and their enthalpy changes: 1) C(s) + 2 H2(g) \(\rightarrow\) CH4(g), \(\Delta\) H = -74.6 \(\text{ kJ}\); 2) C(s) + 2 Cl2(g) \(\rightarrow\) CCl4(g), \(\Delta\) H = -95.7 \(\text{ kJ}\); 3) H2(g) + Cl2(g) \(\rightarrow\) 2 HCl(g), \(\Delta\) H = -92.3 \(\text{ kJ}\).
Use Hess's Law to manipulate the given reactions to derive the target reaction. This involves reversing and/or multiplying the given reactions to match the stoichiometry of the target reaction.
Reverse the first reaction to get CH4(g) \(\rightarrow\) C(s) + 2 H2(g), and change the sign of \(\Delta\) H to +74.6 \(\text{ kJ}\).
Combine the manipulated reactions to form the target reaction, ensuring that all intermediate species cancel out, and sum the \(\Delta\) H values to find \(\Delta\) H_{\(\text{rxn}\)} for the target reaction.

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Enthalpy Change (ΔH)

Enthalpy change (ΔH) is a measure of the heat content of a system 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 chemical reactions, which is essential for calculating the overall enthalpy change for a reaction using Hess's law.
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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 the individual steps of the reaction, 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 from related reactions.
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Hess's Law

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. It provides a reference point for calculating the enthalpy changes of reactions. In the given problem, the ΔH values for the reactions provided can be used to derive the ΔHrxn for the overall reaction by applying Hess's Law.
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Enthalpy of Formation
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