Anna University 2 Marks Questions and Answers

Anna University TWO MARKS Questions & Answers

3. Phase Rule

 

1. State phase rule and explain the terms involved. (A.U. Dec 2005)(A.U. June 2007)

If the equilibrium between any number of phases is not influenced by gravity, or electrical, or magnetic forces but are influenced only by pressure, temperature and concentration, then the number of degree of freedom(F) of the system is related to number of components (C) and number of phases (P) by the following phase rule equation.

F = C – P + 2

 

2. Define phase (P) with suitable example.

(Chen A.U. Dec 2009) Phase is defined as, “any homogeneous physically distinct and mechanically separable. portion of a system which is separated from other parts of the system by definite boundaries”.

Consider a water system consisting of three phases.

Ice(s) ⇌ Water(1) ⇌ Vapour(g)

Each phase is physically distinct and homogeneous and there are definite boundaries between phases. So this forms three phases.

 

3. What is degree of freedom (F)?

(Or)

Comment on the degree of freedom.

(A.U. (CEG) Chen A.U. Dec 2011, May 2017, Jan. 2018)

Degree of freedom is defined as, “the minimum number of independent variable factors such as temperature, pressure and concentration, which must be fixed in order to define the system completely”.

 

4. Define component with suitable example.

(Chen A.U. Dec 2009, Dec 2015, Jan. 2018)

Component is defined as, “the smallest number of independently variable constituents, by means of which the composition of each phase can be expressed in the form of a chemical equation”.

Consider a water system consisting of three phases.

Ice(s) ⇌ Water(1) ⇌ Vapour(g)

The chemical composition of all the three phases is H₂O, but are in different physical form. Hence the number of component is one.

 

5. How many phases and components are present in the following system? (Chen A.U.T. Dec 2012; June 2010)

CaCO₃(s) ⇌ CaO(s) + CO₂(g)

It consists of two solid phases and one gaseous phase.

P = 3; C = 2;

F = C – P + 2 = 2 – 3 + 2 = 1.

 

6. NH₄Cl(s)  NH_3(g) + HCl _(g). Write the values of P, F and C for this system.

(A.U. Dec 2016)

This system consists of two phases and one component

P = 2; C = 1;

F = C – P + 2 = 1 – 2 + 2 = 1.

 

7. Calculate the number of phases present in the following systems.

(Coim. A.U. July 2009)(A.U. June 2007, Dec 2016)

(a) MgCO_3(s) ⇌ MgO_(s) + CO_{2 (g)}

Three phases.

 (b) Rhombic sulphur_(s) ⇌ Monoclinic sulphur_(s)

Two phases.

(C) Ice_(s) ⇌ water₍₁₎ ⇌ water vapour _(g)

Three phases.

(d) An emulsion of oil in water

Two phases

(e) NH₄Cl_(s) ⇌ NH_3(g) + HCl_(g)

Two phases.

 

8. How many components are present in the system.

KCl + Water ↔ KCI • hydrate (A.U.T(Coim) July 2010)

Number of phases = 3 ; Components = 2.

 

9. What is Phase diagram? What is the use of studying such diagrams?

Phase diagram is a graph obtained by plotting one degree of freedom against another.

It is possible to predic from the phase diagrams whether an eutectic alloy or a solid solution is formed on cooling a homogeneous liquid containing mixture of two metals.

 

10. What are the types of phase diagrams?

(i) P-T diagram

If the phase diagram is plotted between temperature against pressure, the diagram is called P-T diagram. P-T diagram is used for one component system.

(ii) T-C diagram

If the phase diagram is drawn between temperature against composition, the diagram is called T-C diagram. T-C diagram is used for two component system.

 

11. What is triple point? (A.U. June 2014)

It is the point at which three phases namely solid, liquid and vapour are simultaneously at equilibrium.

 

12. What is metastable equilibrium?

Sometimes water can be cooled below 0°C without the formation of ice, this water is known as super-cooled water. The equilibrium between super-cool water and the vapour is known as metastable equilibrium.

 

13. What is the effect of pressure on the melting point of ice?

Melting point of ice decreases with increase of pressure.

 

14. How many phases and components are existing at triple point in ice - water - water vapour system?

Phases - 3; Components - 1.

 

15. State reduced phase rule. (or) Write down the condensed phase rule. Explain.

(Coim. &TNV A.U. July 2009 & A.U. Jan 2014, May 2015)

A solid-liquid equilibrium of an alloy has practically no gaseous phase and the effect of pressure is negligible. Therefore, experiments are conducted under atmospheric pressure. Thus the system in which only the solid and liquid phases are considered and the gas phase is ignored is called a condensed system. Since the pressure is kept constant, the phase rule becomes

F' = C – P + 1

This equation is called reduced phase rule or condensed phase rule.

 

16. A system consists of benzene and water. What is the number of phases? (A.U. Dec 2007)

Two liquid phases and one vapour phase.

 

17. State the number of degrees of freedom for the following systems:

(a) PC1₅ (s) ⇌ PCl₃ (g) + Cl₂ (g) at 50°C

(b) CaCO₃ (s) = CaC(s) + CO₂(g)

(A.U. May 2008)

(a) F = C – P + 1; 2 – 3 + 2 ; F = 1

(b) C – P + 1; 2 – 3 + 2 ; F = 1

 

18. State the conditions under which two substances can form a simple eutectic.

(i) They must be completely miscible in the liquid state but completely immiscible in the solid state.

(ii) They should not chemically react with each other.

 

19. What is eutectic point? (A.U. June 2006)

(or)

Define eutectic point.

It is the point at which two solid and one liquid phase are in equilibrium.

 

20. Eutectic is a mixture and not a compound explain.

Eutectic is an unique mixture of two solids which has the lowest melting point. Since it is completely immiscible in the solid state, it is a mixture and not a compound.

 

21. What is an eutectic alloy?

It is an unique composition of two solids, which has the lowest melting point, when compared to the melting point of two solids,

Since, it has the same composition both in the liquid and solid states, it behaves like a pure solid substance.

 

22. What are the uses (or) significance of eutectic mixture? (A.U. Dec 2008)

1. Suitable alloy composition can be predicted with the help of eutectic systems.

2. Eutectic systems are used in preparing solders, used for joining two metal pieces together.

 

23. Mention the merits of phase rule. (or) Mention the applications of phase rule.

(i). It is applicable to both physical and chemical equilibria;

(ii). It is convenient method of classifying the equilibrium states interms of phases, components and degree of freedom.

(iii) It helps in deciding whether the given number of substances remain in equilibrium or not.

 

24. Differentiate melting point, eutectic point and triple point.

Melting point: It is the temperature at which the solid and liquid phases, having the same composition, are in equilibrium.

Solid A ⇌  Liquid A

Eutectic point: It is the temperature at which two solids and a liquid phase are in equilibrium.

Solid A + Solid B ⇌ Liquid

Triple point: It is the temperature at which three phases are in equilibrium.

Solid ⇌ Liquid ⇌Vapour

 

25. Write the number of phases and components in the following heterogeneous system:

CuSO_{4 (s)} + 5H₂O (1) ⇌CuSO₄. 5H₂O _(s). (Chen A.U. June 2009)

Number of phases = 3

Number of components = 2

 

26. What is the degree of freedom at eutectic point in Lead-Silver system? (A.U. Dec 2014)

Degree of freedom is zero ie., F = 0

 

27. What is thermal analysis.

Thermal analysis is a method involving a study of the cooling curves of various compositions of a system during solidification. The shapes of the freezing point curves for any system (involving metals) can be determined by thermal analysis. The form of the cooling curve indicates the composition of the solid.

 

28. How is cooling curve drawn.

A pure substance in the fused state is allowed to cool slowly and the temperature is noted at different time interval. Then graph is plotted between temperature and time

 

29. What are the important uses of cooling curves.

1. Melting point and eutectic temperature can be noted from the cooling curve.

2. Percentage purity of the compounds can be noted from the cooling curve.

3. The behaviour of the compounds can be clearly understood from the cooling curve.

4. The composition corresponding to its freezing point yields the composition of the alloy.

5. The procedure of thermal analysis can be used to derive the phase diagram of any two component system.

 

30. What are the uses (applications) of phase diagrams. (A.U. May 2017, Jan. 2018)

1. It is possible to predict from the phase diagrams whether an eutectic alloy or a solid solution is formed on cooling a homogeneous liquid containing mixture of two metals.

2. . The phase diagrams are useful in understanding the properties of materials in the heterogeneous equilibrium system.

3. The study of low melting eutectic alloys, used in soldering, can be carried out using phase diagrams.