Atomic/molecular collisions and pressure

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley

Atomic/molecular collisions and pressure

• The kinetic-molecular theory relates pressure to elastic collisions between individual particles and the walls of the container.

• Even though atoms/molecules are light, they move at hundreds or even thousands of m/s, and each contributes significant kinetic energy.


Consider two specimens of ideal gas at the same temperature. The molecules in specimen #1 have greater molar mass than the molecules in specimen #2. How do the rms speed of molecules (vrms) and the average translational kinetic energy per molecule (KE) compare in the two specimens?

A. vrms and KE are both greater in specimen #2.

B. vrms is greater in specimen #2; KE is the same in both specimens.

C. vrms is greater in specimen #2; KE is greater in specimen #1.

D. Both vrms and KE are the same in both specimens.

E. None of the above is correct.

Q18.3


Consider two specimens of ideal gas at the same temperature. The molecules in specimen #1 have greater molar mass than the molecules in specimen #2. How do the rms speed of molecules (vrms) and the average translational kinetic energy per molecule (KE) compare in the two specimens?

A. vrms and KE are both greater in specimen #2.

B. vrms is greater in specimen #2; KE is the same in both specimens.

C. vrms is greater in specimen #2; KE is greater in specimen #1.

D. Both vrms and KE are the same in both specimens.

E. None of the above is correct.

A18.3


Consider two specimens of ideal gas at the same temperature. Specimen #1 has the same total mass as specimen #2, but the molecules in specimen #1 have greater molar mass than the molecules in specimen #2. In which specimen is the total translational kinetic energy of the entire gas greater?

A. specimen #1

B. specimen #2

C. The answer depends on the particular mass of gas.

D. The answer depends on the particular molar masses.

E. Both C. and D. are correct.

Q18.4


Consider two specimens of ideal gas at the same temperature. Specimen #1 has the same total mass as specimen #2, but the molecules in specimen #1 have greater molar mass than the molecules in specimen #2. In which specimen is the total translational kinetic energy of the entire gas greater?

A. specimen #1

B. specimen #2

C. The answer depends on the particular mass of gas.

D. The answer depends on the particular molar masses.

E. Both C. and D. are correct.

A18.4


Expanding Helium

Helium gas starts at a volume Vi = 1L and Pi = 1 atm. It expands linearly to Vf = 3L and Pf = 3 atm.

1. Draw the pV diagram for this process

2. What is the work done by the gas when it expands?

3. What is its increase in thermal energy?

4. How much heat is gained or lost by the gas?


Speed of air molecules

• What is the average translational kinetic energy of one molecule an ideal gas of oxygen (molar mass M = 32 g/mol) at 27°C? What about nitrogen (molar mass M = 28 g/mol)?

• What is the rms speed of nitrogen and oxygen?


How much heat energy can ensembles contain?

• An atom can absorb energy as the kinetic energy of its motion.

• A molecule can absorb energy in its translation, and also in its rotation and in the vibrations of one atom in its structure with respect to the others.

• Atomic/molecular energy absorbed is termed its “heat capacity.”


Illustration of heat absorption into degrees of freedom


Energy of monatomic and diatomic gases

• How much heat is required to raise 4g of He gas from 300 K to 310 K?

• How about hydrogen gas H2?

• What is the rms speed of He gas and H2 at 300K?

Documents

Atomic/molecular collisions and pressure