Web Resource: Ideal Gas Simulation. Kinetic Theory of Gases. Ideal Gas. Ideal Gas Assumptions

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1 Web Resource: Ideal Gas Simulation Kinetic Theory of Gases Physics Enhancement Programme Dr. M.H. CHAN, HKBU Link: Ideal Gas Ideal Gas Assumptions The interatomic forces within the gas are ery weak. There is no equilibrium separation for the atoms. Thus, no standard olume at a gien temperature. For a gas, the olume is entirely determined by the container holding the gas. Equations inoling gases will contain the olume, V, as a ariable This is instead of focusing on V The number of molecules in the gas is large, and the aerage separation between the molecules is large compared with their dimensions. The molecules occupy a negligible olume within the container. This is consistent with the macroscopic model where we assumed the molecules were point like. The molecules obey Newton s laws of motion, but as a whole they moe randomly. Any molecule can moe in any direction with any speed. 3 4

2 Ideal Gas Assumptions Notes on Ideal Gas The molecules interact only by short range forces during elastic collisions. This is consistent with the macroscopic model, in which the molecules exert no long range forces on each other. The molecules make elastic collisions with the walls. These collisions lead to the macroscopic pressure on the walls of the container. The gas under consideration is a pure substance. All molecules are identical. An ideal gas is often pictured as consisting of single atoms. Howeer, the behaior of molecular gases approximate that of ideal gases quite well. At low pressures. Molecular rotations and ibrations hae no effect, on aerage, on the motions considered. 5 6 Gas: Equation of State Ideal Gas Model It is useful to know how the olume, pressure and temperature of the gas of mass m are related. The equation that interrelates these quantities is called the equation of state. These are generally quite complicated. If the gas is maintained at a low pressure, the equation of state becomes much easier. This type of a low density gas is commonly referred to as an ideal gas. The ideal gas model can be used to make predictions about the behaior of gases. If the gases are at low pressures, this model adequately describes the behaior of real gases. 7 8

3 The Mole The Mole The amount of gas in a gien olume is coneniently expressed in terms of the number of moles. One mole of any substance is that amount of the substance that contains Aogadro s number of constituent particles. Aogadro s number N A = 6.0 x 0 3. The constituent particles can be atoms or molecules. The number of moles can be determined from the mass of the substance: n = m /M. M is the molar mass of the substance. Example: Helium, M = 4 g/mol. m is the mass of the sample. n is the number of moles. 9 0 Gas Laws Ideal Gas Law Boyle s law: When a gas is kept at a constant temperature, its pressure (P) is inersely proportional to its olume (V). PV = Constant. Charles law: When a gas is kept at a constant pressure, its olume (V) is directly proportional to its temperature (T). V / T = V / T. Guy Lussac s law: When the olume of the gas is kept constant, the pressure (P) is directly proportional to the temperature (T). P / T = P / T. The ideal gas law is the equation of state of a hypothetical ideal gas. The equation of state for an ideal gas combines and summarizes the Ideal Gas Law: PV = nrt R = Uniersal Gas Constant = 8.34 J/mol K = L atm/mol K Example: Determine the olume of any gas ( mole, atmospheric pressure, 0 o C) Answer:.4 L

4 Ideal Gas Law Application: Eco Cooler The ideal gas law is often expressed in terms of the total number of molecules, N, present in the sample. PV = nrt = (N/N A ) RT = Nk B T k B = Boltzmann s constant =.38 x 0 3 J/K It is common to call P, V, and T the thermodynamic ariables of an ideal gas. If the equation of state is known, one of the ariables can always be expressed as some function of the other two. The Eco Cooler is a zero electricity air cooler, which made of plastic bottles. Helps to reduce temperatures in tin huts to make them bearable to lie in. Because of the simplicity, eeryone is expected to be able to adopt this idea and make their own. Reference: green.com/air conditioner less 5/ 3 4 Application: Eco Cooler Application: Eco Cooler The DIY air conditioner has no copyright. Instruction manual is aailable at: cooler/eco Cooler.HowToMake.pdf Blow With you mouth wide open. Blow With you mouth with your lips pursed. Can you relate this effect to thermodynamics? 5 6

5 Molecular Model of an Ideal Gas Pressure and Kinetic Energy Macroscopic properties of a gas were pressure, olume and temperature. Can be related to microscopic description Matter is treated as a collection of molecules. Newton s Laws of Motion can be applied statistically. Assume a container is a cube. Edges are length d. Motion of the molecule: in terms of its elocity components. Momentum and the aerage force. 7 8 Pressure and Kinetic Energy Pressure and Kinetic Energy Assume perfectly elastic collisions with the walls of the container. The relationship between the pressure and the molecular kinetic energy comes from momentum and Newton s Laws. Pressure is proportional to the number of molecules per unit olume (N/V) and to the aerage translational kinetic energy of the molecules: P 3 N V mo This equation relates the macroscopic quantity of pressure with a microscopic quantity of the aerage alue of the square of the molecular speed. Ways to increase pressure: increase the number of molecules per unit olume; and increase the speed (kinetic energy) of the molecules. 9 0

6 Molecular Interpretation of Temperature Total Kinetic Energy Temperature is a direct measure of the aerage molecular kinetic energy: N P mo NkBT 3 V Theorem of Equipartition of Energy: Each translational degree of freedom contributes an equal amount to the energy of the gas mo mo mo mo x y z 3 kbt kbt kbt kbt Total Kinetic Energy (translational): K Tot N m 3 3 NkBT nrt If a gas has only translational energy, this is the internal energy of the gas. Internal energy of an ideal gas depends only on the temperature. RMS Speed Root Mean Square Speed of molecules: 3kT m M = molar mass (in kg per mole) = m o N A RMS o 3RT M Quick Quiz Two containers hold an ideal gas at the same temperature and pressure. Both containers hold the same type of gas, but container B has twice the olume of container A. (i) What is the aerage translational kinetic energy per molecule in container B? [ Answer: (b), translational KE is a function of temperature only ] (ii) Describe the internal energy of the gas in container B. [ Answer: (a), there are twice as many molecules ] Multiple Choices: (a) twice that of container A. (b) the same as that of container A. (c) half that of container A. (d) impossible to determine. 3 4

7 Molar Specific Heat Molar Specific Heat Consider: An ideal gas undergoes seeral processes such that change in temperature is T. Temperature change can be achieed by arious paths (from one isotherm to another). T is the same for each process E int is also the same. Work done on gas (negatie area under the cure) is different for each path. From the first law of thermodynamics (E int = Q + W), the heat associated with a particular change in temperature is not unique. Specific heats for two processes that frequently occur: Changes with constant pressure Changes with constant olume 5 6 Molar Specific Heat Ideal Monatomic Gas Constant olume processes: Q n C Constant pressure processes: Q n C p T T () () Monatomic gas: one atom per molecule. When energy is added to a monatomic gas in a container with a fixed olume, all of the energy goes into increasing the translational kinetic energy of the gas. There is no other way to store energy in such a gas. C = molar specific heat at constant olume C p = molar specific heat at constant pressure Q in Eqn () is greater than Q in Eqn () for a gien n and T. Why? 7 8

8 Exercise on Molar Specific Heat Find: (a) C (molar specific heat at constant olume, State i f ); (b) C p (molar specific heat at constant pressure,, State i f ) of an ideal monatomic gas. Express your answers in terms of R (Uniersal Gas Constant). Answers: C = 3R/ C p = 5R/ Remark: C C p 5 3 Molar Specific Heat of Monatomic Gases Constant Volume: C Constant Pressure: C p Applicable to any ideal gas: 3 R.5 J/ molk 5 R 0.8 J/ molk C p C R 9 30 Molar Specific Heat of Various Gases Quick Quiz ) How does the internal energy (E int ) of an ideal gas change as it follows path i f? ) How does the internal energy of an ideal gas change as it follows path f f along the isotherm labeled T + T? (A) E int increases. (B) E int decreases. (C) E int stays the same. Answers: ) A ) C 3 3

9 Exercise: Heating a cylinder of helium gas Adiabatic Processes A cylinder contains 3 moles helium gas at 300 K. i. The gas is heated at constant olume, calculate the heat required to transfer to the gas in order to increase the temperature to 500 K. ii. The gas is heated at constant pressure, calculate the heat required to transfer to the gas in order to increase the temperature to 500 K. Adiabatic process: No energy is transferred by heat between a system and its surroundings. Assume an ideal gas is in an equilibrium state and PV = nrt is alid. The pressure and olume of an ideal gas at any time during an adiabatic process are related by PV = constant. = C P / C V is assumed to be constant during the process. All three ariables in the ideal gas law (P, V, T ) can change during an adiabatic process Exercise: Adiabatic Process Exercise: Diesel Engine Cylinder Considering an ideal gas undergoes an adiabatic process, proe: i. ii. PV where = C P / C V. i i TV constant T V f f Air (0C) in a diesel engine cylinder is compressed from an initial pressure of atm and a olume of 800 cm 3 to 60 cm 3. Assume air behaes as an ideal gas with =.4 and the compression is adiabatic. Find the final pressure and temperature of the air

10 Equipartition of Energy Equipartition of Energy In addition to internal energy, one possible energy is the translational motion of the center of mass. Rotational motion about the arious axes also contributes. For this molecule, we can neglect the rotation around the y axis since it is negligible compared to the x and z axes. The molecule can also ibrate. There is kinetic energy and potential energy associated with the ibrations Molecular Vibrations Equipartition of Energy Translational motion: 3 degrees of freedom. Rotational motion: degrees of freedom. Vibrational motion: degrees of freedom. Symmetrical Stretching Antisymmetrical Stretching Scissoring Rocking Wagging Twisting Animation Source: Wikipedia Suppose: E C int 3N kbt N kbt N kbt de n dt Translational int 7R Rotational Vibrational 39 40

11 Molar specific heat of hydrogen as a function of temperature. Hydrogen liquefies at 0 K. Complex Molecules For molecules with more than two atoms, the ibrations are more complex. The number of degrees of freedom is larger. The more degrees of freedom aailable to a molecule, the more ibration modes there are to store energy. This results in a higher molar specific heat. 4 4 Quantization of Energy Vibrational states are separated by larger energy gaps than are rotational states. At low temperatures, the energy gained during collisions is generally not enough to raise it to the first excited state of either rotation or ibration. Quantization of Energy As the temperature increases, the speed (or energy) of the molecules increases. In some collisions, the molecules hae enough energy to excite to the first excited state. As the temperature continues to increase, more molecules are in excited states

12 Quantization of Energy ~ Room temperature, rotational energy contributes fully to the internal energy. ~ 000 K, ibrational energy leels are reached. ~0,000 K, ibration contributes fully to the internal energy. Monatomic Gas C = 3R/ C p = 5R/ Diatomic Gas Room Temperature C = 5R/ C p = 7R/ High Temperature C = 7R/ = 5/3 =.67 = 7/5 =.4 Translational only Translational and Rotational Translational, Rotational, and Vibrational Quick Quiz Quick Quiz The molar specific heat of a diatomic gas is measured at constant olume and found to be 9. J/(molK). What are the types of energy that are contributing to the molar specific heat? A. Translation only. B. Translation and rotation only. C. Translation and ibration only. D. Translation, rotation, and ibration. Answer: D [ 9. J/(molK) 7R/ ] The molar specific heat of a gas is measured at constant olume and found to be R/. Is the gas most likely to be A. monatomic, B. diatomic, or C. polyatomic? Answer: C [ C = R/, which is larger than the highest possible alue of C (7R/) for a diatomic gas ] 47 48

13 Web Resource: Kinetic Theory Boltzmann Distribution Law Probability of finding a molecule in a particular energy state: n E n o exp E k T B Web Resource where n V (E ) de is the number of molecules per unit olume with energy between E and E + de Exercise on Number Density Consider a gas at a temperature of 500 K whose atoms can occupy only two energy leels separated by.5 ev. Determine the ratio of the number of atoms in the higher energy leel to the number in the lower energy leel. Ludwig Boltzmann Austrian physicist Contributed to: Kinetic Theory of Gases. Electromagnetism. Thermodynamics. Pioneer in statistical mechanics. 5 5

14 Maxwell Boltzmann Speed Distribution Distribution of speeds in N gas molecules: N P m 4N k o B T M 4 RT 3 3 M = Molar Mass of the Gas [kg], R = Gas Constant = 8.3 J/(molK), T = Temperature [K], and = Speed of Gas Molecules. m o exp kbt M exp RT 0 P d 53 P() (s/m).5 x 0-3 Maxwell's Speed Distribution, M= kg, T= K Speed Distribution Mean Speed RMS Speed Most Probable Speed Escape,Earth =. km/s Speed (km) P() E E E E E-93.04E E Speed (m/s) 54.4 x 0-3 Maxwell's Speed Distribution, M= kg, T= K. Speed Distribution Mean Speed RMS Speed Most Probable Speed Escape,Mercury = 4.5 km/s.6 x 0-3 Maxwell's Speed Distribution, M= kg, T= K.4. Speed Distribution Mean Speed RMS Speed Most Probable Speed Escape,Venus = 0.4 km/s P() (s/m) P( > 4.5 km/s).9e 0 P() (s/m) P( > 0.4 km/s) Speed (m/s) Speed (m/s) 55 56

15 Distribution of Molecular Speeds Distribution of Molecular Speeds Fraction of molecules with speeds in an interal [, ]: Mean Speed: RMS Speed: Fraction[, ] 0 P P( ) d RMS 0 3RT M d 8RT M 3RT P( ) d M 0 57 Most Probable Speed ( mp ): The speed at which P() is maximum. Note: mp dp( ) Sole 0 d RMS RT mp M 58 Maxwell Boltzmann Speed Distribution Speed Distribution As T increases, the peak shifts to the right. This shows that the aerage speed increases with increasing temperature

16 Exercise Summary 0.5 mole hydrogen gas at 300 K. (a) Find the mean speed, the rms speed, and the most probable speed. (b) Find the number of hydrogen molecules with speeds between 400 ms and 40 ms. Answers: (a).78 km/s,.93 km/s,.57 km/s (b).60 9 molecules. 6 6 Summary 63