1 Engineering Chemistry (KAS102/202) MODULE-3 : Electrochemistry, Corrosion, Phase Rule Question-Answer Q.1 Explain following with example.: The Gibb’s phase rule, Phase, Component, Degree of freedom. Ans. Gibb’s Phase Rule: The rule was given by W. Gibb’s. It is defined as “When the equilibrium in a heterogeneous system is influenced by only temperature, pressure, and concentration and is not influenced by gravitational or electrical or magnetic forces or by surface action, the number of phases (P) existing at equilibrium is related to the degrees of freedom (F) and number of components (C) by the rule ”: F=C−P+2 Explanation of terms with examples: (1) Phase (P): Phase is defined as, “any homogeneous physically distinct and mechanically separable portions of a system which is separated from other parts of the system by definite boundaries”. a) Gaseous phase (g):All gases are completely miscible and have no boundaries between them. Hence all gases constitute a single phase. E.g.: Air, a mixture of O2, H2, N2, CO2 and water vapour etc., constitutes a single phase. (b) Liquid Phase (l):The number of liquid phases depends on the number of liquids present and their miscibility. • If two liquids are immiscible, they will form two separate liquid phases. (e.g.) Benzene – Water system. • If two liquids are completely miscible, they will form only one liquid phase. (e.g.) Alcohol - Water system. (c) Solid Phase (s): Every solid constitutes a separate single phase. (e.g.) Decomposition of CaCO3 CaCO3(s) CaO (s) + CO2(g) It involves 3 phases namely solid CaCO3, solid CaO and gaseous CO2. 2. Component (C): Component is defined as, “the minimum number of independent chemical constituents taking part in the equillibrium by means of which the composition of each phase can be expressed either directly or indirectly in the form of a chemical equation”. Examples: (a) Consider a water system consisting of three phases: Ice(s) Water(l) Vapour(g) Phase = 3 , Component = 1 (b) Sulphur exists in 4 phases namely rhombic, monoclinic, liquid and vapour, but the chemical composition is only sulphur (S8). Hence , Phase = 4 , Component = 1 (c) Thermal decomposition of CaCO3: Phase = 3 , Component = 2 CaCO3(s) CaO (s) + CO2 (g) (d) An aqueous solution of NaCl . The constituents are NaCl and H2O. Phase = 1 , Component = 2 (e) In the dissociation of NH4Cl , the following equilibrium occurs. NH4Cl (s) NH3 (g) + HCl (g) [NH3 = HCl] Phase = 2 , Component = 1
2 3. Degree Of Freedom (F): It is defined as, “the minimum number of independent variable factors like temperature, pressure and concentration, which must be specify , then remaining variables are fixed automatically and the system is completely define”. A system having 1, 2, 3 or 0 degrees of freedom are called as univariant, bivariant, trivariant and non-variant systems respectively. Examples: (a) Consider the following equilibrium Ice(s) Water(l) Vapour(g) P = 3 , C = 1, F = 0 These 3 phases will be in equilibrium only at a particular temperature and pressure. Hence, this system does not have any degree of freedom, so it is non-variant/ zero-variant/ in-variant system. (b) Consider the following equilibrium: Liquid Water (l) Water- vapour (g) P = 2 , C = 1, F = 1 Here liquid water is in equilibrium with water vapour. Hence any one of the degree of freedom such as temperature (or) pressure has to be fixed to define the system. Therefore the degree of freedom is one. F = C – P + 2; = 1 – 2 + 2 = 1 Q.2 Discuss the salient features of phase diagram of water system. Ans. The Water System: • Water system is a one component system. • Water exists in 3 possible phases, namely solid ice, liquid water and water-vapour. • ice(s) water(l) vapour(g) • The phase diagram for the water system contains curves, areas, and triple point.
3 S.No. 1. 2. 3. 4. 5. 6. 7. Areas/Curves /Points Area AOB Area AOC Area BOC Curve OA Curve OB Curve OC Curve OA’ 8. Triple Point O Phase/Equilibrium F= C-P+2 Remarks Vapour, P = 1 Liquid, P = 1 Ice, P = 1 Liquid = Vapour , P = 2 Solid = Vapour , P = 2 Liquid = Solid , P = 2 Super cooled liquid = Vapour , P=2 Liquid = Vapour = Solid , P=3 F=2 F=2 F=2 F=1 F=1 F=1 F=1 Bivariant Bivariant Bivariant Univarient Univarient Univarient Metastable curve Invarient F=0 Curves: i. Vapourisation curve OA, water and vapour are in equilibrium. water(l) vapour(g) ii. Sublimation curve OB, ice and vapour are in equilibrium. ice(s) vapour(g) iii. Melting point curve OC, ice and water are in equilibrium. ice(s) water(l) Along all the curves, two phases are in equilibrium. Applying phase rule, F=C-P+2 =1-2+2 =1 The degree of freedom is one or univariant. To define any point along the curve, it is enough to mention either pressure or temperature. Areas: Areas AOC = Liquid Area BOC = Ice Area AOB = Vapour In any area, only one phase is present. Applying phase rule, F=C-P+2 =1-1+2 =2 The degree of freedom is two or bivariant. To define any point in an area, we have to mention both pressure and temperature. Triple Point: The three curves OA, OB and OC meet at the tripe point ‘O’. At ‘O’ ice, water and vapour are in equilibrium, ice(s) water(l) vapour(g) Applying phase rule, F = C − P+2 = 1 −3 + 2 = 0 The degree of freedom is zero i.e., invariant. The triple point is defined only at fixed temperature 0.0075C and pressure 4.58mm at ‘O’ . Metastable curve OA’ It is called vapour pressure curve of the super-cool water (or) metastable equilibrium. Some times water can be cooled below 0C without the formation of ice crystals; this water is known as super-cool water. Super-cooled water vapour The equilibrium is unstable. Water can be converted into ice by slight disturbance.The degree of freedom of the system is one i.e., univariant. Q.3 Define corrosion. Describe Wet or Electrochemical Corrosion with oxygen absorption and hydrogen evolution mechanisms.
4 Ans. Corrosion is the slow process of decay of the metal, due to the attack of the atmosphere gases on the surface of the metal, resulting in the formation of metallic compounds such oxides, hydroxides, carbonates, sulphides etc. For e.g., Rusting of iron .Rust is hydrated oxide of Fe2O3.xH2O. Wet or Electrochemical Corrosion: • • This type of Corrosion occurs where a conducting liquid is in contact with the metal or when two dissimilar metals or alloys are dipped partially in a solution. This corrosion occurs due to the existence of separate anodic and cathodic parts, between which current flows through the conducting solution. Mechanism: • At anodic area :At anodic area, oxidation reaction occurs, thereby, destroying the anodic metal either by dissolution or formation of compounds. Hence corrosion always occurs at anodic parts. At Anode: • M Mn+ + ne- At cathodic area:The cathodic reaction consumes electrons with either evolution of hydrogen or absorption of oxygen which depends on the nature of corrosive environment. Evolution of hydrogen: • • • In acidic medium, the cathodic reaction takes place by the evolution of H2. Iron undergoes oxidation at anode to form Fe2+ ions with the liberation of electrons. The electrons released flow through the metal from anode to cathode, whereas H+ ions of acidic solution are eliminated as hydrogen gas. • The anodes are large areas, whereas cathodes are small areas. Fe Fe2+ + 2e- (Oxidation + 2H + 2e H2 The overall reaction is: Fe + 2H+ Fe2+ + H2