5.3.1 Molecular Speeds Flashcards Preview

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Flashcards in 5.3.1 Molecular Speeds Deck (12)
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1
Q

Molecular Speeds

A
  • The molecules in a gas have a distribution of speeds, with the majority near the average speed.
  • The root-mean-square speed (u rms ) is related to the temperature and molecular mass of the gas.
2
Q

note

A
  • The molecules in a gas have a distribution of speeds, with the majority near the average speed.
  • As the molecular mass of a gas increases, the distribution of molecular speeds shifts toward lower averages.
  • As the temperature of a gas rises, the distribution of molecular speeds shifts toward higher averages.
  • The speeds of gases are compared using the root-mean-square speed (u rms ). The root-mean-square speed is the square root of the mean square speed of the gas particles.
  • The mean square speed of the gas particles is the sum of their individual squared speeds, divided by the number of particles.
  • By the ideal gas law, the mean square speed is related to the temperature (T) and molecular mass (M) of the gas by the universal gas constant (R).
  • This allows the root-mean-square speed to be calculated directly, given the temperature and molecular mass of the gas.
3
Q

Study the graph. Which of the following statements is not true about O2?

A

O2 has more molecules at higher speeds than helium.

4
Q

Based on the graph for O2, find the statement below that is not correct.

A

There is a greater distribution (or amount) of O2 particles at 1000°C than O2 particles at 25°C for any molecular speed.

5
Q

What is the root mean square speed of methane, CH4, at 25°C?

A

681 m / sec

6
Q

Which of the following statements referring to the graph is true?

A

O2 particles at 25°C have a smaller range of distribution than O2 particles at 1000°C.

7
Q

Maxwell-Boltzman’s speed distribution theory predicts the probably distributions for the speeds of molecules in a gas at thermal equilibrium. The graphs you saw in the lecture were based on this theory. The kinetic molecular theory says that the mean square speed of a gas particle, u^2 = 3(R)(T)/M where R=8.13 J/mol*K, T is the absolute temperature (K), and M is the molar mass (kg/mol). Which statement best predicts and explains what will happen to the mean square speed of a gas particle if you increase the temperature, T?

A

The mean square speed will increase at a higher temperature.

8
Q

The formula for average kinetic energy is (1/2) m u 2, where m is the mass of a particle and u is the average molecular speed. It is assumed, for this equation, that the temperature is constant. In addition, at any given temperature, the average kinetic energy for any two gas particles is the same. Suppose that in an experiment Gas A has a lower mass than Gas B. Based on this information and the graph from Question 1, which statement is true if you are comparing the two gases at the same temperature?

A

The mean square speed, u 2, of Gas B is less than the mean square speed of Gas A.

9
Q

Which of the following best describes the information for nitrogen (N2 )?

A

Nitrogen has a greater maximum number of molecules than the maximum distribution values for H2O and He. The plot for nitrogen is roughly bell-shaped.

10
Q

Which of the following statements about the graph is not correct?

A

The mass of a nitrogen particle must be larger than the mass of an oxygen particle at a given temperature.

11
Q

Study the graph. Which of the following statements is true about the H2O information?

A

H2O has a distribution range that is similar to N2 and O2 but very different from He.

12
Q

Which of the following statements about the graph is not correct?

A

The O2 molecules show the greatest speed of all the gas molecules.

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