Compound formation with congruent melting point (Mg Zn) and incongruent melting point (NaCl H2O) system.

Compound formation with congruent melting point (Mg Zn) and incongruent melting point (NaCl H2O) system.

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  PRESENTED BY:- RJAFFRIN RESSICA ED24B1018 NITPY DEPARTMENT OF EDUCATION CHEMISTRY SEMINAR ED-1106 THERMODYNAMICS EQUILIBRIUM AND SOLUTION
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  MY TOPIC Compound formationwith congruent melting point (Mg-Zn) and incongruent melting point (NaCl-H2O) system.
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  It is anexample of a two component system in which two metals form a compound having Mg(Zn)2 having congruent melting point. Melting point of pure Zn is 420°C (point A), and that of pure Mg is 650°C (point B) whereas m.p. of compound Mg(Zn)2 is 590°C (point D) which is in between the melting point of two metals. This system has four phases i.e. solid Zn, solid Mg solid compound Mg(Zn)2 and liquid (L) - atoms of Mg and Zn in same composition as that of MgZn2. COMPOUND FORMATION WITH CONGRUENT MELTING POINT - Mg - Zn SYSTEM. Point A: It represents the m.p. of pure Zn (420°C) Curve AC: When Mg is added to Zn gradually the m.p. of Zn is lowered along the curve AC. Along this curve solid Zn is in equilibrium with melt (liquid). Thus there two phases along this curve and on applying the condensed phase rule F = C – P + 1 = 2 – 2 + 1 = 1 we see that it is a monovariant system. It means only one variable can be changed (either temperature or composition).
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  Point B: Thisrepresents the melting point of pure Mg (650°C) . Curve BE: When Zn is added to Mg gradually, m.p. of Mg is lowered along the curve BE. Along the curve BE solid Mg is in equilibrium with melt (liquid). Again this is also a monovarient curve having one degree of freedom. Curve DC: This curve represents the freezing point curve of solid Mg(Zn)2 when Zn is added, the m.p. of compound Mg(Zn)2 lowers along this curve. Curve DE: This curve represents the melting point curve of Mg(Zn)2 when Mg is added in it. Point D: At this point, the liquid and solid have the same composition i.e. Mg(Zn)2. Thus point D is the congruent point and temperature (590°C) corresponds to this point is known as congruent melting point of Mg(Zn)2. Point C: Two curve AC and DC intersect at point C. Three phases i.e. solid Zn, compound Mg(Zn)2 and melt are in equilibrium at this point. Thus, on applying phase rule F = C – P + 1 = 2 – 3 + 1 = 0 we see that point C is non variant and is known as eutectic point.
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  Point E: Thisis the point where curves DE and BE intersect. Here also three phases i.e. solid Mg, compound Mg(Zn)2 and melt are in equilibrium at this point. Again it is invariant and known as eutectic point. Cooling: When liquid melt is cooled at point X, it follows the path xy and Mg(Zn)2 seperates out at y and two phases i.e. solid Mg(Zn)2 and liquid are in equilibrium and the degree of freedom is one. On further cooling curve follows the path yC and at C, solid Zn also separates out. Thus there will be three phases at point C. These phases are solid Zn, solid Mg(Zn)2 and melt. Therefore C is monovariant.
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  D
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  FORMATION OF COMPOUNDSWITH INCONGRUENT MELTING POINT - NaCl-H2O SYSTEM The NaCl-H2O is a typical case of solid-liquid equilibria in a two-component system that forms compounds with an incongruent melting point. The phases involved in this case are solid NaCl, solid NaCl.2H2O, ice and liquid mixture. Point O, S and Q :- The point O and point S represent the freezing point of pure water and pure NaCl respectively. The point Q however is quite strange because it represents the transition temperature (incongruent melting point of NaCl.2H2O) where compound NaCl.2H2O decomposes into pure NaCl.
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  When NaCl isadded to pure water, ice will just start to separate out as solid along path OP. This will lead to a change in composition along the line OP. When point P is attained, the formation of compound NaCl.2H2O will start, and since three phases coexist at P ( ice, NaCl.2H2O and liquid), the reduced phase rule:- F' = 2-3+1= 0 Which means that there will be no degree of freedom at point P( P is non variant). Curve OP, SQ and QP:- When NaCl is added to water, its freezing point decreases regularly along OP. In other words, OP is the fusion curve of water along which the ice is in equilibrium with the solution. The line SQ is the fusion curve of pure NaCl along which solid NaCl is in equilibrium with the brine solution. Similarly, QP is the fusion curve of compound NaCl.2H2O along which solid NaCl.2H2O is in equilibrium with the solution.
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  On the otherhand, the curve SQ is the fusion curve of NaCl. The further cooling will lead to a change in composition along the line SQ. When point Q is attained, the following meritectic reaction will take place:- Solid NaCl + Solution ⇌Solid NaCl.2H2O Therefore the formation of compound NaCl.2H2O will be started, and since three phases coexist at this point ( solid NaCl, solid NaCl.2H2O and liquid), the point Q also becomes invariant. The transformation of compound NaCl to compound NaCl.2H2O occurs at constant temperature and therefore the point Q is called as peritectic point.
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  QnA Time QnA Time Welcometo 1. In a binary A–B system, a compound AB melts at a temperature at which the liquid has the same composition as the solid AB. This melting point is called the: (a) Eutectic melting point (b) Congruent melting point (c) Incongruent melting point (d) Peritectic melting point Answer: (b) Congruent melting point
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  2. In atwo-component system, the compound AB decomposes on heating before it fully melts and yields a different solid plus a melt. Such a compound has: (a) A congruent melting point (b) A peritectic reaction point (c) A eutectic melting point (d) An incongruent melting point Answer: (d) An incongruent melting point 3. Consider the phase diagram of the Magnesium-Zinc system where a compound MgZn₂ forms and melts at a point where the liquid and solid compositions are identical. What kind of melting point does MgZn₂ exhibit? (a) Eutectic (b) Peritectic (c) Congruent (d) Incongruent Answer: (c) Congruent
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  4. Which ofthe following statements is true about a compound having an incongruent melting point? (a) On heating, it melts completely into a liquid of the same composition. (b) On heating, it decomposes and gives one new solid phase + liquid of different composition. (c) It behaves like a eutectic mixture. (d) It always forms a solid solution. Answer: (b) On heating, it decomposes and gives one new solid phase + liquid of different composition. 5. In the application of the phase rule to a system at the congruent melting point of a binary compound, the degrees of freedom F = C – P + 1 reduces to 0. Here C = 1 (since solid and liquid have same composition) and P = 2 (solid and liquid phases) so free degrees = 0. Which of these conditions corresponds to that scenario? (a) Two phases in equilibrium, but composition fixed →invariant. (b) Three phases in equilibrium →invariant. (c) Two phases in equilibrium with variable composition →univariant. (d) One phase only →univariant. Answer: (a) Two phases in equilibrium, but composition fixed →invariant.
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  Thank You Thank You