Question Set 96

1.What is the difference between heat capacity and specific heat of a material ?

Ans: The heat capacity of a material is the amount of heat transformed to raise unit mass of a material 1 degree in temperature.The specific heat of a material is the ratio of the amount of heat transferred to raise unit mass of a material 1

degree in temperature to that required to raise unit mass of water 1 degree of temperature at some specified temperature. For most engineering purposes, heat capacities may be assumed numerically equal to;specific heats.

2.Explain the rule to find specific heat of aqueous solutions.

Ans: For aqueous solutions of salts, the specific heat can be estimated by assuming the specific heat of the solution equal to that of the water alone. Thus, for a 15% by weight solution of sodium chloride in water, the specific heat would be approximately 0.85.

3.What do you understand by latent heat ? Give four examples of latent heats.

Ans: For pure substances, the heat effects accompanying changes in state at constant pressure (no temperature change being evident) are known as latent heats. Examples of latent heats are : heat of fusion, vaporisation, sublimation, and change in crystal form.

4.Define the terms free energy and free enthalpy. What is their significance and importance ?

Ans: Free energy (or Helmholtz function) is defined as/= u -Ts.

It is equal to the work during a constant-volume isothermal reversible nonflow process.

Free enthalpy (or Gibbs function) is defined as g = h – Ts

(where u = internal energy, h = enthalpy, T = temperature, s = entropy)

Gibbs function is of particular importance in processes where chemical changes occur. For reversible

isothermal steady-flow processes or for reversible constant-pressure isothermal nonflow processes, change

in free energy is equal to net work.

5.Which parameter remains constant in isochoric process ?

Ans: Volume.

6.What is polytropic process ? Under what conditions it approaches isobaric, isothermal, and isometric process ? In which reversible process no work is done ?

Ans: A polytropic process is one that follows the equation pun = constant (index n may have values from –

oc to + oo. This process approaches isobaric when n = 0, isothermal when n = 1, and isometric when n =

<x>. No work is done in isometric process.

7.Whether superheated steam can be treated like ideal gas ?

Ans: Yes.

8.Out of constant pressure and constant volume lines on TS diagram which line has higher slope ? Andwhether slope is constant or variable ?

Ans: Constant volume line. Slope is variable.

9.Whether entropy is intensive property or extensive property ?

Ans: Entropy is extensive property.

 10.In which process fluid expands but does no work ?

Ans: Throttling process.