This set of Phase Transformation Questions and Answers for Aptitude test focuses on “Kinetics of Phase Transformation”.
1. Which among the following condition should be satisfied by homogeneous nucleation?
a) Occurs slowly
b) Occurs at the preferential site
c) Occurs in an orderly fashion
d) Occurs randomly
Explanation: Homogenous nucleation takes place randomly and it’s spontaneous in nature and for this nucleation, there are no preferential sites like the ones for heterogeneous nucleation where it occurs at the surface.
2. Which among the following condition should be satisfied by heterogeneous nucleation?
a) Occurs spontaneously
b) Doesn’t have a preferential nucleation site
c) Occurs randomly
d) Occurs at preferential sites
Explanation: It is associated with the surface rather than the bulk and occurs at preferential sites. And in case of homogeneous nucleation it occurs randomly and spontaneously without any preferential site.
3. Nucleation that takes place at the grain boundaries are known as___________
a) Heterogeneous nucleation
b) Homogeneous nucleation
c) Partial nucleation
d) Mixed nucleation
Explanation: Heterogeneous Nucleation occurs at preferential sites such as grain boundaries, dislocations or impurities. They rather prefer the surface than the bulk of the material unlike the homogenous nucleation.
4. Homogeneous nucleation occurs only when liquid is undercooled by___________
a) Critical undercooling for nucleation
b) Critical undercooling for growth
c) Layered undercooling for nucleation
d) Layered undercooling for growth
Explanation: It occurs only when liquid is undercooled by critical undercooling for nucleation. The following assumptions are made before the process. Liquid with nuclei is an ideal solution of various size clusters and each size cluster contains N atoms or molecules.
5. Phase transformations in metals/alloys occur by _____
a) Random motion
c) Rotation and vibration
d) Nucleation and growth
Explanation: During nucleation new phase appears at certain sites within the metastable parent phase and during growth, nuclei grows into the surrounding matrix.
6. During homogeneous for a spherical, the change in Gibbs free energy involve_______
a) Interfacial energy
b) Volume free energy and surface free energy
c) Rotational and Vibrational energy
d) Surface free energy and vibrational energy
Explanation: The ∆G of homogeneous nucleation depends on the Volume free energy and Surface free energy and both are dependent on radius of sphere. Furthermore, in the curve resulting from the sum of volume free energy and surface free energy, value of the volume free energy will be negative if the temperature is below the equilibrium solidification temperature and the surface free energy is always positive.
7. The magnitude of the contribution of volume free energy of a spherical particle increases with radius.
Explanation: The magnitude of the contribution of volume free energy associated with a spherical particle increases with increase in radius because it is directly proportional to the cube of radius. But the value of the volume free energy will be negative if the temperature is below the equilibrium solidification temperature.
8. The magnitude of surface free energy of a spherical particle increases with radius.
Explanation: Surface free energy increases with the increase in the radius and it is directly proportional to the square of the radius hence it is positive, furthermore, the magnitude of this contribution is the product of interfacial energy and the surface area of the nucleus. Finally, the total free energy change is equal to the sum of the surface free energy and volume free energy.
9. Gibbs free energy of a homogeneous nucleation is _____
a) Greater than heterogeneous nucleation
b) Less than heterogeneous nucleation
c) Greater than or equal to heterogeneous nucleation
d) Less than or equal to heterogeneous nucleation
Explanation: ΔG heterogeneous is equal to some S(ϴ)*∆G of homogeneous where S(ϴ) ≤ 1, is a function of the wetting angle. So hence the product of this will always be less than or equal to the homogeneous nucleation.
10. The rate at which a transformation occurs will depend on the frequency with which atoms________
a) Reach the activated state
b) Deviate from the activated stage
c) Vibrate in the activated stage
d) Rotate in the activated stage
Explanation: According to kinetic theory, the probability of an atom reaching the activated state is given by exp(∆G/KT) where K is Boltzmann’s constant, G is known as the activation free energy barrier. The rate at which a transformation occurs will depend on the frequency with which atoms reach the activated state.
11. Radius of the new nuclei formed during heterogeneous nucleation depends on___________
a) ∆T (undercooling)
b) S(ϴ) (Is a function of the wetting angle)
c) A (surface area of parent particle)
d) V (volume of parent particle)
Explanation: Radius of the new nuclei in case of a heterogeneous nucleation is inversely proportional to ∆T and this is the same when you take homogeneous nucleation but much lesser degree of super cooling is needed for heterogeneous nucleation.
Sanfoundry Global Education & Learning Series – Phase Transformation.
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