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Chemical Engineering Thermodynamics [ Question No: 1 To 20 ] :
A refrigerant with too low a boiling point at the evaporator temperature would require
All of the following are desirable properties of a refrigerant EXCEPT
The coefficient of performance of a refrigerator is defined as
The equilibrium constant is related to the standard Gibbs energy change by
Assertion (A): A Txy diagram must be labelled with the pressure at which it was constructed.
Reason (R): For a binary two-phase system the compositions are fixed only once both temperature and pressure are given.
The K value of a component in vapour liquid equilibrium is
Assertion (A): Thermodynamic consistency tests are applied to published activity coefficient data.
Reason (R): The Gibbs-Duhem equation places a constraint that consistent data must satisfy.
Three of the following describe the same law. Which is the odd one out?
What is the entropy of mixing per mole for an equimolar ideal binary mixture, taking the gas constant as 8.314 joules per mole kelvin?
Assertion (A): For a real fluid the difference between the two heat capacities is expressed using expansivity and compressibility.
Reason (R): The general relation follows from a Maxwell relation and reduces to the gas constant for an ideal gas.
Assertion (A): The Clapeyron equation relates the slope of a phase boundary to the latent heat.
Reason (R): Along a coexistence line the molar Gibbs energies of the two phases remain equal.
The chief practical value of a Maxwell relation is that it
Consider the following statements about a real adiabatic turbine.
1. Its exit entropy is higher than its inlet entropy.
2. Its work output is less than the isentropic value.
3. Its exit temperature is lower than the isentropic exit temperature.
Iterative solution of a cubic equation of state for the vapour root is usually started from
Below the critical temperature a cubic equation of state gives three real roots for volume. The correct interpretation is that
The latent heat of vaporisation as the critical point is approached
State whether the following is true or false.
The second virial coefficient has a firm interpretation in terms of interactions between pairs of molecules.
Consider the following statements about the compressibility factor.
1. It equals one for an ideal gas at all conditions.
2. It falls below one at moderate pressures for most gases.
3. It can never exceed one for any real gas.
Assertion (A): Departure or residual functions are used for real fluids.
Reason (R): They express the difference between the real property and the ideal gas value at the same temperature and pressure.
Water is pumped from 100 to 3000 kilopascals. Taking its specific volume as 0.001 cubic metres per kilogram, what is the ideal pump work?
Total number of questions : 20
Number of answered questions : 0
Number of unanswered questions : 20
Chemical Engineering Thermodynamics [ Question No: 1 To 20 ] :
A refrigerant with too low a boiling point at the evaporator temperature would require
Your Answer : (Not Answered)
Correct Answer : D
The saturation pressure at the working temperature sets the operating pressures, so a fluid that is too volatile drives the whole cycle to high pressure. One that is not volatile enough puts the evaporator under vacuum, where air leaks in.
All of the following are desirable properties of a refrigerant EXCEPT
Your Answer : (Not Answered)
Correct Answer : C
Correct answer: a high global warming potential
A high global warming potential is precisely what a refrigerant should not have, and it is what has driven the successive phase-outs - CFCs for ozone depletion, then HFCs for their warming potential. The other three are genuine requirements: the boiling point must suit the operating pressures, the fluid must be stable and safe to handle, and a high latent heat means less refrigerant circulated for the same duty.
The coefficient of performance of a refrigerator is defined as
Your Answer : (Not Answered)
Correct Answer : D
It measures what you want against what you pay for, which is why it exceeds one for a working machine. The second definition belongs to a heat pump, whose coefficient is larger by exactly one. Both describe the same machine viewed from different ends.
The equilibrium constant is related to the standard Gibbs energy change by
Your Answer : (Not Answered)
Correct Answer : C
The relation converts a tabulated thermodynamic quantity into the composition ratio at equilibrium. The negative sign means a negative standard Gibbs energy change gives a constant greater than one. Products are then favoured at equilibrium.
Assertion (A): A Txy diagram must be labelled with the pressure at which it was constructed.
Reason (R): For a binary two-phase system the compositions are fixed only once both temperature and pressure are given.
Your Answer : (Not Answered)
Correct Answer : D
Both are true and the reason gives the necessity: the same pair of components produces a different diagram at every pressure, so an unlabelled diagram cannot be used for any quantitative work. The pressure belongs in the caption of every such plot.
The K value of a component in vapour liquid equilibrium is
Your Answer : (Not Answered)
Correct Answer : D
It is the distribution ratio between the phases and is the working quantity in every flash calculation. For an ideal system it reduces to the vapour pressure divided by the total pressure, but in general it depends on composition too.
Assertion (A): Thermodynamic consistency tests are applied to published activity coefficient data.
Reason (R): The Gibbs-Duhem equation places a constraint that consistent data must satisfy.
Your Answer : (Not Answered)
Correct Answer : D
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason gives the basis. The Gibbs–Duhem equation relates the activity coefficients of the components at constant temperature and pressure, so they cannot vary independently. Data that violate it are internally inconsistent and must contain experimental error, which the area test and the point test detect - and which is why published data are screened before being correlated.
Three of the following describe the same law. Which is the odd one out?
Your Answer : (Not Answered)
Correct Answer : B
The odd statement is Dalton's law applied to the vapour phase, while the other three all describe Raoult's law for the liquid. The two are used together and are constantly confused. One describes the liquid phase and the other the vapour.
What is the entropy of mixing per mole for an equimolar ideal binary mixture, taking the gas constant as 8.314 joules per mole kelvin?
Your Answer : (Not Answered)
Correct Answer : C
The expression is minus the gas constant times the sum of each mole fraction times its logarithm, which for two halves gives the gas constant times the logarithm of two. That is about five and three quarters joules per mole kelvin.
Assertion (A): For a real fluid the difference between the two heat capacities is expressed using expansivity and compressibility.
Reason (R): The general relation follows from a Maxwell relation and reduces to the gas constant for an ideal gas.
Your Answer : (Not Answered)
Correct Answer : D
Both are true and the reason connects the general and the special case: the ideal gas result is not a separate rule but the limit of the general one when the equation of state is simple. Expansivity and compressibility are both measurable quantities.
Assertion (A): The Clapeyron equation relates the slope of a phase boundary to the latent heat.
Reason (R): Along a coexistence line the molar Gibbs energies of the two phases remain equal.
Your Answer : (Not Answered)
Correct Answer : B
Both are true and the reason gives the derivation: differentiating that equality along the line produces the relation directly, with the volume change of the transition appearing in the denominator. It holds for melting and sublimation lines equally well.
The chief practical value of a Maxwell relation is that it
Your Answer : (Not Answered)
Correct Answer : B
Entropy cannot be measured directly, but pressure, volume and temperature can, so a relation converting one into the other is exactly what a property calculation needs. Heat capacity data are still required for the temperature dependence.
Consider the following statements about a real adiabatic turbine.
1. Its exit entropy is higher than its inlet entropy.
2. Its work output is less than the isentropic value.
3. Its exit temperature is lower than the isentropic exit temperature.
Your Answer : (Not Answered)
Correct Answer : B
The first two are correct and follow from irreversibility. The third is inverted: less work extracted means more energy left in the stream, so the real exit temperature is higher. That extra enthalpy is the work that was never extracted.
Iterative solution of a cubic equation of state for the vapour root is usually started from
Your Answer : (Not Answered)
Correct Answer : C
The vapour root lies close to the ideal value at moderate pressure, so the iteration converges in a few steps. Starting from b, which is near the liquid volume, is the standard way to converge on the liquid root instead.
Below the critical temperature a cubic equation of state gives three real roots for volume. The correct interpretation is that
Your Answer : (Not Answered)
Correct Answer : B
The middle root lies on a branch where volume increases with pressure, which is mechanically unstable and cannot be realised. The two outer roots are the saturated phases at that pressure. Their difference vanishes as the critical point is approached.
The latent heat of vaporisation as the critical point is approached
Your Answer : (Not Answered)
Correct Answer : B
The two phases become identical there, so no energy is required to convert one into the other. This is the same fact seen through enthalpy that the merging densities show through volume. Both curves terminate together at the same point.
State whether the following is true or false.
The second virial coefficient has a firm interpretation in terms of interactions between pairs of molecules.
Your Answer : (Not Answered)
Correct Answer : B
Correct answer: True
The second virial coefficient arises exactly from interactions between pairs of molecules, and it can be calculated from the intermolecular potential rather than merely fitted. The third coefficient corresponds to triplets, and so on. That rigorous foundation is what distinguishes the virial equation from the cubic equations of state, whose parameters are empirical.
Consider the following statements about the compressibility factor.
1. It equals one for an ideal gas at all conditions.
2. It falls below one at moderate pressures for most gases.
3. It can never exceed one for any real gas.
Your Answer : (Not Answered)
Correct Answer : D
The first two are correct. The third is wrong, because at high pressure the molecules are pushed close enough that repulsive forces dominate and the factor climbs above one, sometimes well above. Hydrogen and helium show this at quite ordinary pressures.
Assertion (A): Departure or residual functions are used for real fluids.
Reason (R): They express the difference between the real property and the ideal gas value at the same temperature and pressure.
Your Answer : (Not Answered)
Correct Answer : C
Both are true and the reason gives the construction: the ideal gas part is easy from heat capacity data, so isolating the departure lets an equation of state supply only the correction needed. Heat capacity data and an equation of state together suffice.
Water is pumped from 100 to 3000 kilopascals. Taking its specific volume as 0.001 cubic metres per kilogram, what is the ideal pump work?
Your Answer : (Not Answered)
Correct Answer : D
For an incompressible liquid the reversible work is the specific volume times the pressure rise, which is two thousand nine hundred kilopascals times one thousandth of a cubic metre. That is why pump work is small beside turbine work in a steam cycle.
Marks
Total number of questions : 20
Number of answered questions : 0
Number of unanswered questions : 20