Ch.5 - Thermochemistry

Brown - Chemistry: The Central Science 15th Edition

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Brown 15th Edition

Ch.5 - Thermochemistry

Problem 103

Chapter 5, Problem 103

The air bags that provide protection in automobiles in the event of an accident expand because of a rapid chemical reaction. From the viewpoint of the chemical reactants as the system, what do you expect for the signs of q and w in this process?

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Identify the system and surroundings: The system is the chemical reactants inside the airbag, and the surroundings are everything outside the system.

Understand the process: The airbag inflates due to a rapid chemical reaction, which typically involves the production of gas.

Consider the sign of q (heat): If the reaction is exothermic, it releases heat to the surroundings, so q would be negative.

Consider the sign of w (work): As the gas expands, it does work on the surroundings by pushing against the airbag, so w would be negative.

Summarize the signs: For an exothermic reaction with gas expansion, q is negative and w is negative.

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Key Concepts

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

Enthalpy (q)

Enthalpy, represented as 'q', refers to the heat content of a system at constant pressure. In the context of the airbag deployment, the rapid chemical reaction generates heat, indicating that the system is releasing energy to the surroundings. Therefore, for the reactants, the sign of q is expected to be negative, as the system loses heat during the exothermic reaction.

Work (w)

Work, denoted as 'w', is the energy transferred when a force is applied over a distance. In the case of an airbag, the rapid expansion of gas against the atmospheric pressure represents work done by the system. Since the system is expanding, the work done by the system is considered positive, leading to a positive sign for w.

First Law of Thermodynamics

The First Law of Thermodynamics states that energy cannot be created or destroyed, only transformed from one form to another. In the airbag scenario, the chemical energy from the reactants is converted into thermal energy (heat) and mechanical energy (work) as the gas expands. This principle helps in understanding the relationship between q, w, and the internal energy change of the system.

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First Law of Thermodynamics

Related Practice

Textbook Question

The standard enthalpies of formation of gaseous propyne (C₃H₄), propylene (C₃H₆), and propane (C₃H₈) are +185.4, +20.4, and -103.8 kJ/mol, respectively. (c) Which is the most efficient fuel in terms of heat evolved per unit mass?

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Textbook Question

Consider a system consisting of the following apparatus, in which gas is confined in one flask and there is a vacuum in the other flask. The flasks are separated by a valve. Assume that the flasks are perfectly insulated and will not allow the flow of heat into or out of the flasks to the surroundings. When the valve is opened, gas flows from the filled flask to the evacuated one. (a) Is work performed during the expansion of the gas? (b) Why or why not?

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Textbook Question

A system consists of a sample of gas contained in a cylinder-and-piston arrangement. It undergoes the change in state shown in the drawing under two different situations: In Case 1, the cylinder and piston are perfect thermal insulators that do not allow heat to be transferred. In Case 2, the cylinder and piston are made up of a thermal conductor such as a metal, and during the state change, the cylinder gets warmer to the touch. Let 𝑞1,𝑤1, and Δ𝐸1 be the values of q, w, and Δ𝐸 for Case 1, and let 𝑞2,𝑤2, and Δ𝐸2 be the values for Case 2. b. What is the sign of 𝑤1?

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Textbook Question

The heat of combustion of ethanol, C₂H₅OH(l), is -1367 kJ/mol. A batch of Sauvignon Blanc wine contains 10.6% ethanol by mass. Assuming the density of the wine to be 1.0 g/mL, what is the caloric content due to the alcohol (ethanol) in a 6-oz glass of wine (177 mL)?

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Textbook Question

Two positively charged spheres, each with a charge of 2.0⨉10^-5 C, a mass of 1.0 kg, and separated by a distance of 1.0 cm, are held in place on a frictionless track. (a) What is the electrostatic potential energy of this system?

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