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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Ch.9 - Thermochemistry: Chemical Energy
Chapter 9, Problem 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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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
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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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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Related Practice
Textbook Question
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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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
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
Textbook Question
Methyl tert-butyl ether (MTBE) is prepared by reaciton of methanol (l) (ΔH°f = -239.2 kJ/mol) with 2-methyl-propene (g), according to the requation Calculate ΔH°f in kJ/mol for 2-methylpropene
Textbook Question
Answer the following questions.(e) A gallon of ice cream contains exactly 32 servings, and each serving has 165 Calories, of which 30.0% are derived from fat. How many Calories derived from fat would you consume if you ate one half of a gallon of ice cream?
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