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Ch.9 - Thermochemistry: Chemical Energy
McMurry - Chemistry 8th Edition
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Tutti i libri di testoMcMurry 8th EditionCh.9 - Thermochemistry: Chemical EnergyProblema 93
Capitolo 9, Problema 93

The following steps occur in the reaction of ethyl alcohol (CH3CH2OH) wiht oxygen to yield acetic acid (CH3CO2H). Show that equations 1 and 2 sum to give the net equation and calculate ΔH° for the net equation. (1) CH3CH2OH(l) + 1/2 O2(g) → CH3CHO (g) + H2O(l) ΔH° = -174.2 kJ (2) CH3CHO(g) + 1/2 O2(g) → CH3CO2H(l) ΔH° = -318.4 kJ (Net) CH3CH2OH(l) + O2(g) → CH3CO2H(l) + H2O(l) ΔH° = ?

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Write down the given equations and their respective enthalpy changes (ΔH°). Equation 1: CH3CH2OH(l) + 1/2 O2(g) → CH3CHO(g) + H2O(l), ΔH° = -174.2 kJ. Equation 2: CH3CHO(g) + 1/2 O2(g) → CH3CO2H(l), ΔH° = -318.4 kJ.
Add the two equations together. When you do this, ensure that the reactants and products that appear on both sides of the equations cancel each other out. In this case, CH3CHO(g) appears as a product in Equation 1 and as a reactant in Equation 2.
Verify that the resulting equation matches the net equation provided: CH3CH2OH(l) + O2(g) → CH3CO2H(l) + H2O(l).
Sum the ΔH° values from Equation 1 and Equation 2 to find the ΔH° for the net equation. This is done by adding -174.2 kJ and -318.4 kJ.
The resulting sum of ΔH° values will give you the ΔH° for the net reaction, which represents the total enthalpy change for the conversion of ethyl alcohol to acetic acid in the presence of oxygen.

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Hess's Law

Hess's Law states that the total enthalpy change for a chemical reaction is the sum of the enthalpy changes for the individual steps of the reaction, regardless of the pathway taken. This principle allows us to calculate the enthalpy change of a net reaction by adding the enthalpy changes of the intermediate reactions, as long as the initial and final states are the same.
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Hess's Law

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). In this context, calculating ΔH° for the net reaction involves summing the ΔH° values of the individual steps to find the overall energy change.
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Enthalpy of Formation

Balancing Chemical Equations

Balancing chemical equations is essential to ensure that the number of atoms of each element is conserved in a reaction. In this case, the net equation must reflect the stoichiometry of the reactants and products accurately. This involves adjusting coefficients to balance the equation, which is crucial for correctly calculating the enthalpy change and understanding the reaction's dynamics.
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Balancing Chemical Equations
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