Engineering Questions with Answers - Multiple Choice Questions

MCQs on Microscopic Description of Gases

1 - Question

At higher temperatures, the assumption of a calorically perfect gas is valid.
a) True
b) False

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Explanation: For studying modern compressible flow, the assumption of calorically perfect has is not valid at high temperature because the gases become highly reactive. The molecules of the gas vibrate as the temperature rises causing deviation from the calorically perfect gas behavior.

2 - Question

What is specific heat a function of at high temperatures?
a) Time
b) Temperature
c) Entropy
d) Density

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Explanation: The vibrational motion of the molecules becomes significant by absorbing some of the energy that would otherwise go into the translational and rotational molecular motion as the temperature of a diatomic or polyatomic gas is raised above normal conditions. The excitation of vibrational energy, allows the specific heat to become a function of temperature function making it thermally perfect rather than calorically perfect.

3 - Question

At high temperatures, what does dissociation of molecule mean?
a) Atoms break away from molecular structure
b) Electrons break away from the atom
c) Protons and neutrons break away
d) Electrons jump to the higher energy state

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Explanation: At higher temperature molecules dissociate or ionize depending on the temperature range. Dissociation is a process where the atoms which are a part of the molecule break away from the molecular structure. This leads to the gas becoming highly chemically reactive.

4 - Question

At 4000 K what happens to the gas?
a) Oxygen is dissociated completely
b) Nitrogen is dissociated completely
c) Oxygen is dissociated completely, nitrogen begins to dissociate
d) Ionization of both oxygen and nitrogen

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Explanation: Up to 2500 K, there is no reaction taking place and oxygen starts to dissociated. At 4000 K, oxygen gets completely dissociated and nitrogen begins the process of dissociation. The reaction is as follows:
O2 → 2O

5 - Question

How many thermal degrees of freedom is there for a diatomic molecule having translation energy?
a) 2
b) 3
c) 4
d) 6

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Explanation: For a diatomic molecule having translation energy, there are three thermal degrees of freedom. This energy is at the center of mass present due to the translation kinetic energy. The velocity components are in x, y, z direction of the Cartesian space.

6 - Question

How many degrees of freedom is there for a CO2 molecule with rotational kinetic energy?
a) 1
b) 2
c) 3
d) 6

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Explanation: CO2 is a linear polyatomic molecule that has similar properties as the diatomic molecule having rotational kinetic energy. The moment of inertia for the molecule in z – axis is negligible small compared to x and y – axis. Due to this reason, diatomic and even linear polyatomic molecules have only two thermal degrees of freedom.

7 - Question

The source of electronic energy is kinetic energy due to translation motion.
a) True
b) False

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Explanation: Electrons revolve around the nucleus of an atom which makes up the molecule. This molecule possesses electrical energy. This is a result of two factors – kinetic energy due to translation motion and potential energy.

8 - Question

For a single atom what is the total energy a sum of?
a) Translation and rotational energy
b) Electrical and vibrational energy
c) Translation and electronic energy
d) Rotational, electronic and translation energy

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Explanation: For a molecule, the total energy is a summation of rotational, translation, vibrational and electronic energy. But for a single atom, the total energy is a sum of only translation and electronic energy.

9 - Question

What is a ground state?
a) Energy when gas is at absolute zero
b) Energy when gas is at 273.15 K
c) State where energies are non – existent
d) State where only translation energy exists

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Explanation: The molecule’s ground state refers to the energy that the molecule would have if the gas were theoretically at absolute zero temperature. Therefore, the values for the translational, rotational, vibrational, and electronic modes are often referred to as zero – point energies.

10 - Question

In a boson particle, how many elementary particles are present inside a molecule?
a) Odd number
b) Even number
c) Zero
d) Imaginary number

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Explanation: There are certain atoms and molecules which have even number (2, 4, 6, ..) of elementary particles (quarks, leptons) that obey Bose – Einstein statistical distribution. These particles are known as Bosons.

11 - Question

What are the particles which obey Fermi – Dirac statistics called?
a) Bosons
b) Fermions
c) Leptons
d) Quarks

View Answer

Explanation: Molecules and atoms whose elementary particles present are in odd number usually follow a different statistical distribution known as Fermi – Dirac statistics. These atoms and molecules are commonly referred to as Fermions

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