This set of Phase Transformation Multiple Choice Questions & Answers (MCQs) focuses on “Thermodynamics and Phase Diagrams – Equilibrium”.
1. The relative stability of a system for transformations that occur at constant temperature and pressure is determined by its ____________
a) Exergy
b) Helmholtz free energy
c) Entropy
d) Gibbs free energy
View Answer
Explanation: Gibbs free energy decides the stability of a system for transformations that occur at constant temperature and pressure. Helmholtz free energy decides the stability of a system for transformations that occur at constant volume and temperature. The amount of exergy a system has is not dependent on whether or not it’s an isothermal or isobaric process.
2. Based on the Gibbs phase rule, how many degrees of freedom are present at the triple point of water?
a) 1
b) 2
c) 0
d) 3
View Answer
Explanation: According to the Gibbs phase rule, a single component system has no degrees of freedom when the three phases exist in equilibrium and the system is invariant.
3. Solid phases are most stable at low temperatures.
a) True
b) False
View Answer
Explanation: From the concept of Gibbs free energy it is clear that the state with highest stability will be that with the best compromise between low enthalpy and high entropy. Thus, at low temperature solid phases are more stable since they have the strongest atomic binding and therefore the lowest internal energy (enthalpy).
4. Which of the following is a necessary criterion for any transformation (G1, G2 are the free energies of initial and final state respectively)?
a) G2-G1<0
b) G1=G2
c) G2>G1
d) G2=0
View Answer
Explanation: When ΔG = G2-G1 is less than zero the reaction is spontaneous in the forward direction. Any transformation that results in a decrease in Gibbs energy is possible.
5. Which of the thermodynamic functions can be considered as an intensive property?
a) Volume
b) Pressure
c) Enthalpy
d) Entropy
View Answer
Explanation: Pressure is an intensive property. Intensive properties are those which are independent of the size of the system such as temperature and pressure.
6. Which among the following points represents metastable equilibrium?
a) A
b) B
c) C
d) D
View Answer
Explanation: Point A represents the metastable state. Point A is a local minimum of free energy and satisfies the condition ΔG=0, but do not have the lowest value among these points hence it is in the metastable state.
7. When the system is at local equilibrium, its Gibbs free energy (G) function has reached its minimum value.
a) True
b) False
View Answer
Explanation: When the system is at complete equilibrium or global equilibrium, its Gibbs free energy (G) function has reached its minimum value. Local equilibrium, on the other hand is defined in such a way that the equilibrium exists only at the interfaces between the different phases present in the system.
8. Graphite and diamond at room temperature and pressure are examples of _____________
a) Unstable and stable equilibrium states
b) Stable and unstable equilibrium states
c) Metastable and stable equilibrium states
d) Stable and metastable equilibrium states
View Answer
Explanation: Graphite and diamond at room temperature and pressure are examples of stable and metastable equilibrium states. Given time, diamond under these conditions will transform to graphite.
9. The quantum of energy in an elastic wave is called a ___________
a) Debye model
b) Sonar
c) Phonon
d) Photon
View Answer
Explanation: Phonon is the name for the quantum of energy in an elastic wave. Sonar is method or technique to detect object on or under the surface of water. Debye model is a technique to estimate the phonon contribution to the specific heat.
10. What happens to the entropy in a system with constant volume and constant internal energy during a spontaneous process?
a) Remains same
b) Decreases
c) Increases
d) First decreases then increases
View Answer
Explanation: At either constant internal energy (ΔE = 0) or constant entropy (ΔS = 0).
In the Clausius inequality when we apply the above mentioned condition, finally we get the equation which states that the entropy in a system decreases when the volume and internal energy remains constant during a spontaneous process.
11. Two crystal structures are in equilibrium and their Gibbs energies are the same. Thus, the driving force for the transformation is_________
a) Less than zero
b) Zero
c) Greater than Zero
d) One
View Answer
Explanation: Since the Gibbs energies are the same that means, ΔG is 0, hence the driving force will be zero, if the driving force is not zero then it violates the basic rule of phase transformation.
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