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Ch.7 - Thermochemistry
Tro - Chemistry: A Molecular Approach 6th Edition
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Capitolo 7, Problema 91a

Use standard enthalpies of formation to calculate ΔH°rxn for each reaction. a. C2H4(g) + H2(g) → C2H6(g)

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Identify the standard enthalpy of formation (ΔH_f°) for each compound involved in the reaction. These values are typically found in tables in chemistry textbooks or reliable online resources.
Write the balanced chemical equation for the reaction: \[ \text{C}_2\text{H}_4(g) + \text{H}_2(g) \rightarrow \text{C}_2\text{H}_6(g) \]
Apply Hess's Law, which states that the change in enthalpy for a reaction is the sum of the enthalpy changes for each step of the reaction. Use the formula: \[ \Delta H_{\text{rxn}}^\circ = \sum \Delta H_f^\circ(\text{products}) - \sum \Delta H_f^\circ(\text{reactants}) \]
Substitute the standard enthalpies of formation into the formula: \[ \Delta H_{\text{rxn}}^\circ = [\Delta H_f^\circ(\text{C}_2\text{H}_6(g))] - [\Delta H_f^\circ(\text{C}_2\text{H}_4(g)) + \Delta H_f^\circ(\text{H}_2(g))] \]
Calculate the value of \( \Delta H_{\text{rxn}}^\circ \) using the substituted values to find the enthalpy change for the reaction.

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Enthalpy of Formation

The standard enthalpy of formation (ΔH°f) is the change in enthalpy when one mole of a compound is formed from its elements in their standard states. It is a crucial value used in thermodynamics to calculate the heat changes in chemical reactions. Each substance has a specific ΔH°f, which can be found in tables, and is typically expressed in kilojoules per mole (kJ/mol).
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Enthalpy of Formation

Hess's Law

Hess's Law states that the total enthalpy change for a reaction is the same, regardless of the number of steps taken to achieve the reaction. This principle allows for the calculation of the enthalpy change of a reaction by summing the enthalpy changes of individual steps, making it possible to use standard enthalpies of formation to find ΔH°rxn for complex reactions.
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Hess's Law

Reaction Enthalpy (ΔH°rxn)

The reaction enthalpy (ΔH°rxn) is the overall change in enthalpy when reactants are converted to products at standard conditions. It can be calculated using the formula ΔH°rxn = ΣΔH°f(products) - ΣΔH°f(reactants). This value indicates whether a reaction is exothermic (releases heat, ΔH°rxn < 0) or endothermic (absorbs heat, ΔH°rxn > 0), which is essential for understanding the energy dynamics of chemical processes.
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Enthalpy of Formation
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