Phase Transformation Mcqs

Q:

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
 
Answer & Explanation Answer: A) G2-G1<0

Explanation: When ?G = 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.

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Q:

Solid phases are most stable at low temperatures.

A) True B) False
 
Answer & Explanation Answer: A) True

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).

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Q:

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
 
Answer & Explanation Answer: C) 0

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.

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78
Q:

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
 
Answer & Explanation Answer: D) Gibbs free energy

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.

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