Thermodynamics Questions and Answers – Mixtures of Variable Composition

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This set of Thermodynamics Multiple Choice Questions & Answers (MCQs) focuses on “Mixtures of Variable Composition”.

1. For a system of variable composition, the internal energy depends on
a) entropy
b) volume
c) moles
d) all of the mentioned
View Answer

Answer: d
Explanation: If some substance is added to the system, then energy of the system increases.
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2. If the composition of system does not change, then dU=TdS-pdV .
a) true
b) false
View Answer

Answer: a
Explanation: If the composition changes, then the relation includes many other terms.

3. The molal chemical potential is given by
a) ∂U/∂S
b) ∂U/∂n
c) ∂U/∂V
d) all of the mentioned
View Answer

Answer: b
Explanation: The molal chemical potential signifies the change in internal energy per unit mole of a component when S,V and number of moles of all other components are constant.
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4. The Gibbs entropy equation is given by
a) TdS = dU – pdV – Σ(molal chemical potential)*dn
b) TdS = dU + pdV + Σ(molal chemical potential)*dn
c) TdS = dU + pdV – Σ(molal chemical potential)*dn
d) TdS = dU – pdV + Σ(molal chemical potential)*dn
View Answer

Answer: c
Explanation: Here summation is taken for all the components present in the system.

5. An equation in Gibbs energy is be given by
a) dG = Vdp + SdT + Σ(molal chemical potential)*dn
b) dG = Vdp – SdT – Σ(molal chemical potential)*dn
c) dG = Vdp + SdT – Σ(molal chemical potential)*dn
d) dG = Vdp – SdT + Σ(molal chemical potential)*dn
View Answer

Answer: d
Explanation: For this we use G=U+pV-TS and here the summation is taken for all the components present in the system.
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6. The equation written for Gibbs energy can also be written for
a) H
b) F
c) Both of the mentioned
d) None of the mentioned
View Answer

Answer: c
Explanation: From the equation dG = Vdp – SdT + Σ[(molal chemical potential)*dn], we can write similar equations for F and H..

7. Chemical potential is an extensive property.
a) true
b) false
View Answer

Answer: b
Explanation: Chemical potential is an intensive property.
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8. If the phase of a multi-component system is enlarged, which of the following will happen?
a) U,S and V will increase and T,p and chemical potential will remain same
b) U,S and V will decrease and T,p and chemical potential will remain same
c) U,S and V will increase and T,p and chemical potential will decrease
d) U,S and V will decrease and T,p and chemical potential will increase
View Answer

Answer: a
Explanation: This depends on the type of property.

9. The Gibbs-Duhem equation is given by
a) SdT + Vdp – Σ(n)*d(molal chemical potential)
b) -SdT + Vdp – Σ(n)*d(molal chemical potential)
c) SdT + Vdp – Σ(n)*d(molal chemical potential)
d) -SdT – Vdp – Σ(n)*d(molal chemical potential)
View Answer

Answer: b
Explanation: This equation shows the relationship for simultaneous changes in p,T and chemical potential.
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10. For a phase which has only one constituent,
a) chemical potential = n/G
b) chemical potential = 1/(G*n)
c) chemical potential = G*n
d) chemical potential = G/n
View Answer

Answer: d
Explanation: This means that chemical potential is the molar Gibbs function and is a function of p and T only.

11. If a closed system is in equilibrium, which of the following remains constant?
a) entropy
b) volume
c) internal energy
d) all of the mentioned
View Answer

Answer: d
Explanation: In a closed system, there is no interaction with surroundings hence these quantities along with mass remains constant.

12. At chemical equilibrium, G will be minimum subjected to the equations of constraint.
a) true
b) false
View Answer

Answer: a
Explanation: This minimum occurs at a constant p and T.

13. The Gibbs phase rule for a non-reactive system is given by
a) f = C + (number of phases) + 2
b) f = C – (number of phases) – 2
c) f = C – (number of phases) + 2
d) f = C – (number of phases) – 2
View Answer

Answer: c
Explanation: Here f is the variance or the degree of freedom and C is the number of constituents.

14. For a pure substance existing in a single phase,
a) C=1
b) number of phases = 1
c) f=2
d) all of the mentioned
View Answer

Answer: d
Explanation: Hence we need to know two properties to fix up the state of the system at equilibrium.

15. Which of the following statement is true?
a) if C=1 and number of phases=2, then f=1
b) if C=1 and number of phases=3, then f=0
c) both of the mentioned
d) none of the mentioned
View Answer

Answer: c
Explanation: This comes from the Gibbs phase rule.

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Manish Bhojasia - Founder & CTO at Sanfoundry
Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He is Linux Kernel Developer & SAN Architect and is passionate about competency developments in these areas. He lives in Bangalore and delivers focused training sessions to IT professionals in Linux Kernel, Linux Debugging, Linux Device Drivers, Linux Networking, Linux Storage, Advanced C Programming, SAN Storage Technologies, SCSI Internals & Storage Protocols such as iSCSI & Fiber Channel. Stay connected with him @ LinkedIn | Youtube | Instagram | Facebook | Twitter