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Heat Capacity Ratio of Gases

Kaveendi Chandrasiri

kchandrasiri@chm.uri.edu

Office Hrs: Thursday. 11 a.m.,

Beaupre 305

mailto:kchandrasiri@chm.uri.edu

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Purpose

• To determine the heat capacity ratio for a

monatomic and a diatomic gas.

• To understand and mathematically model

reversible & irreversible adiabatic

processes for ideal gases.

• To practice error propagation for complex

functions.

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Key Physical Concepts

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Theory: Heat Capacity

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Theory: Determination of

Heat Capacity Ratio

• We will subject a gas to an adiabatic expansion and then allow the gas to return to its original temperature via an isovolumetric process, during which time it will cool.

• This expansion and warming can be modeled in two different ways.

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Reversible Expansion(Textbook)

• Assume that pressure in carboy (P1) and exterior pressure (P2) are always close enough that entire process is always in equilibrium

• Since system is in equilibrium, each step must be reversible

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Irreversible Expansion(Lab Manual)

• Assume that pressure in carboy (P1) and exterior pressure (P2) are not close enough; there is sudden deviation in pressure; the system is not in equilibrium

• Since system is not in equilibrium, the process becomes irreversible.

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Part I: Experimental Procedure

Set Up of Apparatus

Use

line A to input Ar, which reaches the bottom of

the carboy.

Line B will be the output line.

Use

line B to input N2, which does not reach the

bottom of the carboy.

Line A will be the output line.

ma

no

me

ter

Nit

rog

en

A

BC

ma

no

me

ter

Arg

on

A

BC

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1) Set up apparatus, insert rubber stopper.

2) Open tubes A & B, attach C to manometer.– Attach one gas to the appropriate input line.

– Loosely place clamp on output line.

3) Allow gas to flow into carboy (flush system) at 15 mbar for 15 minutes. Turn off gas.

4) Close clamp. Slowly turn on the gas a very small amount while holding down stopper.

5) When manometer reaches 60 mbar turn off gas.

6) Wait until manometer reading is constant. Record manometer reading (Man1).

Part I: Experimental Procedure

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7) Remove stopper 2-3” vertically -- Partner 1: Replace stopper tightly as quickly as

possible & hold down stopper.

-- Partner 2: Record lowest manometer reading (Man2).

8) Record manometer reading again when manometer reading is constant (Man3).

9) Flush system for 3 minutes.

10) Repeat steps 4 through 9 for a total of 3 measurements.

11) Switch gases, repeat experiment.-- Between runs of the same gas flush (step 3) for only 3 minutes.

Between different gasses flush for 15 minutes. Why?

12) Record lab temperature & barometric pressure.

Part I: Experimental Procedure

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Part II: Data Analysis

• For an adiabatic expansion, three states of gas will be expressed as:– Before expansion: P1, V1, T1, n1

– Immediately after expansion: P2, V2, T2, n1

– After returning to room temperature: P3, V2, T1, n1

Pressures:

• P1 = Man1 + Barometer

• P2 = Man2 + Barometer

• P3 = Man3 + Barometer

Expressions:

• Reversible: γ = [ln(P1 / P2)] / [ln(P1 / P3)]

• Irreversible: γ = [(P1 / P2) – 1] / [(P1 / P3) – 1]

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Part III: Laboratory Report

✓ Title Page: Title, name, partner(s), date of experiment.

✓ Abstract: One (1) paragraph of what, why, how, and results.

✓ Introduction: Discussion of purpose and general nature of experiment, derive expansion equations.

✓ Theory: State all assumptions, define all variables, give variations on formulas.– Knowledge: Compare and contrast reversible & irreversible

adiabatic expansion expressions.

– Hypothesis: Explain which you expected to closest resemble this experiment & why.

✓ Procedure & Original Data: Both signed.

✓ Results Table: All data & calculated values.

MAT/2014

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Part III: Laboratory Report✓Calculations: (SHOW ALL WORK)

• Calculate theoretical heat capacity for each gas.

• For each trial, determine γ for both expressions.

• Include at least one sample calculation of each type of calculation used in numerical analysis.

✓Error Analysis: (SHOW ALL WORK)• Calculate γ as the average of all trials for each gas

and each expression.

• Propagate errors in each pressure.

• Propagate statistical error in γ for each expression.

• Choose one trial to calculate error in γ for each expression; identify which trial is used.

• Assume error of ±2 in the last recorded figure of manometer and barometer.

MAT/2014

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Part III: Lab ReportError Analysis

Propagate error in each of the following:

• ξ (P1 )2 = ξ( Man1 ) 2 + ξ( Barometer) 2

• ξ (P2 ) 2 = ξ (Man2 ) 2 + ξ (Barometer) 2

• ξ (P3 ) 2 = ξ (Man3 ) 2 + ξ (Barometer) 2

•Reversible: ξ γ = [ln(P1 / P2)] / [ln(P1 / P3)]

•Irreversible: ξ γ = [(P1 / P2) – 1] / [(P1 / P3) – 1]

MAT/2014

Use General

Rule

Need to

derive

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Part III: Laboratory Report• Summary of Data:

For each gas, report both expressions with associated error [γ±ε(γ)]

and correct significant figures, and indicate which is the best

expression and why.

• Conclusions:

– Discuss significance of results. Do they match your hypothesis?

– Compare γ±ε(γ) for both reversible and irreversible expansions.

Do the two values lie within their respective statistical errors?

– Compare γ±ε(γ) with the expected values for a monatomic and

diatomic gas. Do the theoretical results lie outside the

experimental errors? If so, explain plausible reasons for the lack

of agreement.

– Would you be able to tell the monatomic and diatomic gases

apart based solely on your data? Explain.

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• Use line A for Argon input.

• Use line B for N2 input.

• Make sure one partner is holding stopper

tightly in carboy from time = 0s.

• When calculating pressures, add

manometer readings with barometric

pressure of room.

• TA will only sign data AFTER you have

closed both gases.

Important Reminders