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Ch.9 - Thermochemistry: Chemical Energy
McMurry - Chemistry 8th Edition
McMurry8th EditionChemistryISBN: 9781292336145Not the one you use?Change textbook
All textbooksMcMurry 8th EditionCh.9 - Thermochemistry: Chemical EnergyProblem 106
Chapter 9, Problem 106

The standard enthalpy change for the reaciton of SO3(g) with H2O(l) to yield H2SO4(aq) is ΔH° = -227.8 kJ. Use the information in Problem 9.104 to calculate ΔH°f for H2SO4(aq) in kJ/mol. [For H2O(l), ΔH°f = -285.88 kJ/mol.]

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Identify the given reaction: \( \text{SO}_3(g) + \text{H}_2\text{O}(l) \rightarrow \text{H}_2\text{SO}_4(aq) \).
Recognize that the standard enthalpy change for the reaction, \( \Delta H^\circ \), is given as -227.8 kJ.
Recall that the standard enthalpy of formation, \( \Delta H^\circ_f \), for a compound is the enthalpy change when one mole of the compound is formed from its elements in their standard states.
Use the formula for the enthalpy change of a reaction: \( \Delta H^\circ = \sum \Delta H^\circ_f(\text{products}) - \sum \Delta H^\circ_f(\text{reactants}) \).
Substitute the known values into the equation: \( -227.8 = \Delta H^\circ_f(\text{H}_2\text{SO}_4(aq)) - [\Delta H^\circ_f(\text{SO}_3(g)) + (-285.88)] \), and solve for \( \Delta H^\circ_f(\text{H}_2\text{SO}_4(aq)) \).

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Enthalpy Change (ΔH)

Enthalpy change (ΔH) is a measure of the heat absorbed or released during a chemical reaction at constant pressure. A negative ΔH indicates that the reaction is exothermic, meaning it releases heat. Understanding ΔH is crucial for calculating the standard enthalpy of formation (ΔH°f) of compounds, as it reflects the energy changes associated with forming products from reactants.
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Standard Enthalpy of Formation (ΔH°f)

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 key value used in thermodynamic calculations to determine the energy changes in reactions. Knowing the ΔH°f values of reactants and products allows for the calculation of the overall enthalpy change for a reaction.
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Hess's Law

Hess's Law states that the total enthalpy change for a reaction is the sum of the enthalpy changes for individual steps, regardless of the pathway taken. This principle allows for the calculation of ΔH°f for compounds by using known ΔH values from related reactions. It is particularly useful when direct measurement of ΔH°f is not possible, enabling the use of other thermodynamic data to find the desired value.
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Related Practice
Textbook Question

Write balanced equations for the formation of the following compounds from their elements. (c) Dichloromethane (a liquid, CH2Cl2)

Textbook Question
Sulfuric acid (H2SO4), the most widely produced chemical in the world, is amde yb a two-step oxidaiton of sulfur to sulfur trioxide, SO3, followed by reaciton with water. Calculate ΔH°f for SO3 in kJ/mol, given the following data: S(s) + O2(g) → SO2(g) ΔH° = -296.8 kJ SO2(g) + 1/2 O2(g) → SO3(g) ΔH° = -98.9 kJ
Textbook Question
Calculate ∆H°f in kJ/mol for benzene, C6H6, from the following data: 2 C6H6(l) + 15 O2(g) → 12 CO2(g) + 6 H2O(l) ∆H°=-6534 kJ ∆H°f (CO2) = -393.5 kJ/mol ∆H°f(H2O) = -285.8 kJ/mol
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Textbook Question
Acetic acid (CH3CO2H), whose aqueous solutions are known as vinegar, is prepared by reaction of ethyl alcohol (CH3CH2OH) with oxygen: CH3CH2OH(l) + O2(g) → CH3CO2H(l) + H2O(l) Use the following data to calculate ∆H° in kilojoules for the reaction: ∆H°f [CH3CH2OH(l)] = -277.7 kJ/mol ∆H°f [CH3CO2H(l)] = -484.5 kJ/mol ∆H°f [H2O(l)] = -285.8 kJ/mol
Textbook Question
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
Textbook Question
Methyl tert-butyl ether (MTBE), C5H12O, a gasoline additive used to boost octane ratings, has ΔH°f = -313.6 kJ/mol. Write a balanced equaiton for its combustion reaciton, and calcualte its standard heat of combustion in kilojoules