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Ch.5 - Thermochemistry
All textbooksBrown 14th EditionCh.5 - ThermochemistryProblem 79b
Chapter 5, Problem 79b

Ethanol (C2H5OH) is blended with gasoline as an automobile fuel. (b) Calculate the standard enthalpy change for the reaction, assuming H2O(g) as a product.

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Identify the balanced chemical equation for the combustion of ethanol: \( \text{C}_2\text{H}_5\text{OH}(l) + 3\text{O}_2(g) \rightarrow 2\text{CO}_2(g) + 3\text{H}_2\text{O}(g) \).
Look up the standard enthalpy of formation (\( \Delta H_f^\circ \)) for each substance involved in the reaction: \( \text{C}_2\text{H}_5\text{OH}(l) \), \( \text{O}_2(g) \), \( \text{CO}_2(g) \), and \( \text{H}_2\text{O}(g) \).
Apply Hess's Law to calculate the standard enthalpy change of the reaction (\( \Delta H_{rxn}^\circ \)) using the formula: \( \Delta H_{rxn}^\circ = \sum \Delta H_f^\circ (\text{products}) - \sum \Delta H_f^\circ (\text{reactants}) \).
Substitute the values of \( \Delta H_f^\circ \) for each product and reactant into the formula. Remember to multiply the \( \Delta H_f^\circ \) of each substance by its stoichiometric coefficient from the balanced equation.
Calculate the sum of the enthalpies of the products and subtract the sum of the enthalpies of the reactants to find the standard enthalpy change for the reaction.

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Key Concepts

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

Standard Enthalpy Change

Standard enthalpy change (ΔH°) refers to the heat change that occurs at constant pressure when reactants are converted to products under standard conditions (1 atm pressure and a specified temperature, usually 25°C). It is a crucial concept in thermodynamics, allowing chemists to predict the energy changes associated with chemical reactions.
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Combustion Reactions

Combustion reactions involve the reaction of a substance with oxygen, producing heat and light. In the case of ethanol, the combustion reaction typically produces carbon dioxide and water. Understanding the stoichiometry of these reactions is essential for calculating the enthalpy change associated with the combustion of ethanol.
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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 chemists to calculate the enthalpy change for complex reactions by using known enthalpy values of simpler reactions, facilitating the determination of the standard enthalpy change for the combustion of ethanol.
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Related Practice
Textbook Question

Calcium carbide (CaC2) reacts with water to form acetylene (C2H2) and Ca(OH)2. From the following enthalpy of reaction data and data in Appendix C, calculate H°f for CaC2(s): CaC2(s) + 2 H2O(l) → Ca(OH2)(s) + C2H2(g) ΔH° = -127.2 kJ

Textbook Question

Gasoline is composed primarily of hydrocarbons, including many with eight carbon atoms, called octanes. One of the cleanest–burning octanes is a compound called 2,3,4- trimethylpentane, which has the following structural formula: The complete combustion of one mole of this compound to CO2(g) and H2O(g) leads to ΔH° = -5064.9 kJ. (b) By using the information in this problem and data in Table 5.3, calculate H°f for 2,3,4-trimethylpentane.

Textbook Question

Diethyl ether, C4H10O(l), a flammable compound that was once used as a surgical anesthetic, has the structure The complete combustion of 1 mol of C4H10O(l) to CO2(g) and H2O(l) yields ΔH° = -2723.7 kJ. (a) Write a balanced equation for the combustion of 1 mol of C4H10O(l).

Textbook Question

Ethanol (C2H5OH) is blended with gasoline as an automobile fuel. (c) Calculate the heat produced per liter of ethanol by combustion of ethanol under constant pressure. Ethanol has a density of 0.789 g/mL.

Textbook Question

Ethanol (C2H5OH) is blended with gasoline as an automobile fuel. (d) Calculate the mass of CO2 produced per kJ of heat emitted.

Textbook Question

Methanol (CH3OH) is used as a fuel in race cars. (b) Calculate the standard enthalpy change for the reaction, assuming H2O(g) as a product.