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Ch.9 - Thermochemistry: Chemical Energy
McMurry - Chemistry 8th Edition
McMurry8th EditionChemistryISBN: 9781292336145Non è quello che usi tu?Cambia libro di testo
Capitolo 9, Problema 108

Styrene (C8H8), the precursor of polystyrene polymers, has a standard heat of combustion of -4395 kJ/mol. Write a balanced equation for the combustion reaction, and calculate ΔH°f for styrene in kJ/mol. ΔH°f [CO2(g)] = -393.5 kJ/mol; ΔH°f [H2O(l)] = -285.8 kJ/mol

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Write the balanced chemical equation for the combustion of styrene: C_8H_8 + 12O_2 -> 8CO_2 + 4H_2O.
Use the formula for the standard heat of reaction: ΔH°_reaction = ΣΔH°_f(products) - ΣΔH°_f(reactants).
Substitute the given values into the equation: ΔH°_combustion = [8(-393.5) + 4(-285.8)] - [ΔH°_f(C_8H_8) + 0].
Rearrange the equation to solve for ΔH°_f(C_8H_8): ΔH°_f(C_8H_8) = [8(-393.5) + 4(-285.8)] - (-4395).
Calculate the sum of the products' ΔH°_f and solve for ΔH°_f(C_8H_8).

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Combustion Reactions

Combustion reactions involve the reaction of a substance with oxygen, producing heat and light. In organic chemistry, these reactions typically yield carbon dioxide and water as products. Understanding the stoichiometry of combustion is essential for balancing the chemical equation and calculating enthalpy changes.
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Combustion Apparatus

Enthalpy of Formation (ΔH°f)

The 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 concept for calculating the overall enthalpy change of a reaction using Hess's law, which states that the total enthalpy change is the sum of the enthalpy changes for individual steps.
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Enthalpy of Formation

Hess's Law

Hess's Law states that the total enthalpy change for a chemical reaction is the same, regardless of the number of steps or the pathway taken. This principle allows for the calculation of ΔH°f for compounds by using known enthalpy values of products and reactants, facilitating the determination of enthalpy changes in complex reactions.
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