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Ch.5 - Thermochemistry
Brown - Chemistry: The Central Science 15th Edition
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Tutti i libri di testoBrown 15th EditionCh.5 - ThermochemistryProblema 117
Capitolo 5, Problema 117

The hydrocarbons cyclohexane (C6H12), ΔHf° = -156 kJ/mol, and 1-hexene (C6H12), ΔHf° = -74 kJ/mol, have the same empirical formula. (a) Calculate the standard enthalpy change for the transformation of cyclohexane to 1-hexene. (b) Which has greater enthalpy, cyclohexane or 1-hexene?

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Step 1: Understand the problem by identifying the given data: Cyclohexane (C6H12) has a standard enthalpy of formation (ΔHf°) of -156 kJ/mol, and 1-hexene (C6H12) has a ΔHf° of -74 kJ/mol.
Step 2: To find the standard enthalpy change (ΔH°) for the transformation of cyclohexane to 1-hexene, use the formula: ΔH° = ΔHf°(products) - ΔHf°(reactants).
Step 3: Substitute the given values into the formula: ΔH° = (-74 kJ/mol) - (-156 kJ/mol).
Step 4: Simplify the expression by subtracting the enthalpy of formation of cyclohexane from that of 1-hexene.
Step 5: To determine which compound has greater enthalpy, compare the ΔHf° values: the compound with the less negative (or more positive) ΔHf° has greater enthalpy.

Concetti chiave

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Standard Enthalpy of Formation (ΔHf°)

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 in chemical reactions. In this question, the ΔHf° values for cyclohexane and 1-hexene are essential for determining the enthalpy change during the transformation between these two hydrocarbons.
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Enthalpy of Formation

Enthalpy Change (ΔH)

Enthalpy change (ΔH) is the heat content change associated with a chemical reaction at constant pressure. It can be calculated using the formula ΔH = ΣΔHf°(products) - ΣΔHf°(reactants). In this case, the enthalpy change for the transformation from cyclohexane to 1-hexene can be determined by subtracting the ΔHf° of cyclohexane from that of 1-hexene.
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Enthalpy of Formation

Comparison of Enthalpy Values

Comparing the enthalpy values of different compounds helps determine their stability and energy content. A compound with a lower ΔHf° is generally more stable and has lower energy than one with a higher ΔHf°. In this question, analyzing the ΔHf° values of cyclohexane and 1-hexene allows us to conclude which compound has greater enthalpy and thus is less stable.
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Percorso guidato
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Enthalpy of Formation
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