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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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Enthalpy of Formation

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)

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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
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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
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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
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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
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