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Ch 19: The First Law of Thermodynamics
Young & Freedman Calc - University Physics 14th Edition
Young & Freedman Calc14th EditionUniversity PhysicsISBN: 9780321973610Not the one you use?Change textbook
Chapter 19, Problem 24

Propane gas (C3H8) behaves like an ideal gas with g=1.127g = 1.127. Determine the molar heat capacity at constant volume and the molar heat capacity at constant pressure.

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First, understand that the molar heat capacities at constant volume \( C_v \) and at constant pressure \( C_p \) for an ideal gas are related to the specific heat ratio \( \gamma \) (also denoted as \( g \) in this problem) by the formula \( \gamma = \frac{C_p}{C_v} \).
Recall the relationship between \( C_p \) and \( C_v \) for an ideal gas: \( C_p = C_v + R \), where \( R \) is the universal gas constant, approximately \( 8.314 \text{ J/mol·K} \).
Using the given \( \gamma = 1.127 \), set up the equation \( 1.127 = \frac{C_p}{C_v} \).
Substitute \( C_p = C_v + R \) into the equation \( 1.127 = \frac{C_v + R}{C_v} \) and solve for \( C_v \).
Once \( C_v \) is determined, use the equation \( C_p = C_v + R \) to find \( C_p \).

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Key Concepts

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

Ideal Gas Law

The Ideal Gas Law is a fundamental equation in thermodynamics that describes the behavior of ideal gases. It is expressed as PV = nRT, where P is pressure, V is volume, n is the number of moles, R is the universal gas constant, and T is temperature. This law helps in understanding the relationship between these variables and is crucial for calculating properties like heat capacities.
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Heat Capacity

Heat capacity is a measure of the amount of heat energy required to change the temperature of a substance by a certain amount. Molar heat capacity, specifically, refers to the heat capacity per mole of a substance. It is important to distinguish between heat capacity at constant volume (Cv) and constant pressure (Cp), as they differ due to the work done by or on the system during expansion or compression.
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Ratio of Heat Capacities (Gamma, γ)

The ratio of heat capacities, denoted as γ (gamma), is defined as Cp/Cv, where Cp is the molar heat capacity at constant pressure and Cv is the molar heat capacity at constant volume. For an ideal gas, this ratio is crucial for determining the individual heat capacities using the relation Cp = γCv. Understanding γ allows us to calculate these values, which are essential for thermodynamic processes.
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Related Practice
Textbook Question

A cylinder contains 0.01000.0100 mol of helium at T=27.0T = 27.0°C. If the gas is ideal, what is the change in its internal energy in part (a)? In part (b)? How do the two answers compare? Why?

(a) How much heat is needed to raise the temperature to 67.067.0°C while keeping the volume constant? Draw a pVpV-diagram for this process.

(b) If instead the pressure of the helium is kept constant, how much heat is needed to raise the temperature from 27.027.0°C to 67.067.0°C? Draw a pVpV-diagram for this process.

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Textbook Question

A monatomic ideal gas that is initially at 1.50×1051.50\(\times\)10^5 Pa and has a volume of 0.08000.0800 m3 is compressed adiabatically to a volume of 0.04000.0400 m3. What is the final pressure?

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Textbook Question

An experimenter adds 970970 J of heat to 1.751.75 mol of an ideal gas to heat it from 10.010.0°C to 25.025.0°C at constant pressure. The gas does +223+223 J of work during the expansion. Calculate γ\(\gamma\) for the gas.

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Textbook Question

Five moles of monatomic ideal gas have initial pressure 2.50×1032.50\(\times\)10^3 Pa and initial volume 2.102.10 m3. While undergoing an adiabatic expansion, the gas does 14801480 J of work. What is the final pressure of the gas after the expansion?

Textbook Question

The temperature of 0.1500.150 mol of an ideal gas is held constant at 77.077.0°C while its volume is reduced to 25.0%25.0\% of its initial volume. The initial pressure of the gas is 1.251.25 atm. Does the gas exchange heat with its surroundings? If so, how much? Does the gas absorb or liberate heat?

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Textbook Question

Heat QQ flows into a monatomic ideal gas, and the volume increases while the pressure is kept constant. What fraction of the heat energy is used to do the expansion work of the gas?

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