ELECTROCHEMISTRY Lecture/Lession Plan -1

Transcription

1 Chapter 4 ELECTROCHEMISTRY Leture/Lession Plan -1 ELECTROCHEMISTRY 4.1 Conept of eletrohemistry Eletrohemistry is a branh of hemistry where we will study how hemial energy an be transformed into eletrial energy and vie versa. Substanes whih ondut eletriity or urrent are termed as ondutors. Metals and eletrolytes are good ondutor of eletriity; metal arries urrent by flow of eletron where no matter is being transported during urrent flow whereas in ase of eletrolytes, movement of ions (matter) is responsible for eletriity flow. During urrent flow an internal fore of the ondutor resist the flow of urrent through it whih is known as resistane of the ondutor. 4.2 Condutane Aording to Ohm s law, resistane (R) of any ondutor is diretly proportional to its length (l) and inversely proportional to its area of ross setion (a). Ohm s law an be expressed as: R = ρ l a ; where is speifi resistane or resistivity. When l=1 m and a=1 m 2 then ρ will be equal to R. Hene speifi resistane or resistivity of a ondutor an be defined as the resistane of the ondutor having 1 m length and 1 m 2 area of ross setion. Unit of resistane is ohm and unit of speifi resistane an be determined as: R = ρ l a or, ρ = R l a 7

2 8 CHAPTER 4. ELECTROCHEMISTRY LECTURE/LESSION PLAN -1 Unit of speifi resistane = ohm.m.m m = ohm.m Condutane (Λ) an be defined as reiproal of resistane Speifi ondutane speifi ondutane (κ) an be defined as reiproal of speifi resistane. Condutane is basially the ease of urrent flow through a liquid ondutor. speifi onduatane is defined by the ondutane of a solution of the dissolved eletryle and the whole solution is being plaed between two eletrode of 1 sq. m area and 1 m. Unit of ondutane will simply be reiproal of unit of resistane i.e., Unit of speifi ondutane = Equivalent ondutane 1 ohm = ohm 1 = mho. 1 ohm.m = mho.m 1 Form the expression of ondutane, we know that Λ = 1 R = 1 a ρ l = 1 a l ρ l l = κ V l 2 ; where V = volume of the solution = area x length= a x l Equivalent ondutane is defined by the ondutane of a solution ontaining 1 gm-equivalent of the dissolved eletrolyte and the whole solution is being plaed between two eletrode of 1 sq. m area and 1 m apart. Thus equivalent ondutane (Λ) = κv (as l = 1 m 2 ) Now, say gm-equivalent dissolved in 1000 of solution So, 1 gm-equivalent will be present in 1000 solution.

3 4.2. CONDUCTANCE 9 So, equivalent ondutane (Λ) = 1000κ Unit of equivalent ondutane = mho.m 1 gm-equivalent. = mho.m 1 gm-equivalent.m 3 = mho.m2.gm-equivalent Molar ondutane Molar ondutane is defined by the ondutane of a solution ontaining 1 gm-mole of the dissolved eletrolyte and the whole solution is being plaed between two eletrode 1 m apart. Molar ondutane (Λm) = κv (V = volume of a solution ontaining 1 gm-mole dissolved eletrolyte) Now, say gm-mole dissolved in 1000 of solution So, 1 gm-mole will be present in 1000 solution. So, equivalent ondutane (Λ) = 1000κ Unit of equivalent ondutane = mho.m 1 gm-mole. = mho.m 1 gm-mole.m 3 = mho.m 2.gm-mole 1. Relation between equivalent ondutane and molar ondutane Let us onsider an eletrolyte (moleular weight M) having weight W gm taken in between two eletrode (in the ubi ore);p and q are the valeny and number of ations are respetively. Now, equivalent weight of the eletrolyte is; E = M pq C gm equivalent equivalent = W E = W m pq = W pq M

4 10 CHAPTER 4. ELECTROCHEMISTRY LECTURE/LESSION PLAN -1 So, Now, equivalent ondutane = Λ = 1000κ = 1000κ W pq M 1000κ = M gram mole pq = Λ m pq Λm = Λ pq 4.3 Effet of temperature on equivalent ondutane In general equivalent ondutane should inrease with temperature as with inreased temperature, the dissoiation of eletrolyte and ioni mobility of ions inresaes. Condutane and temperature are orelated by this equation: Λ 0 T = Λ C [1 + α(t 250 C] ; where Λ 0 T and Λ C are the equivalent ondutane at T0 C and 25 0 C respetively; α is the temperature oeffiient. 4.4 Effet of onentration on equivalent ondutane Condutane of an eletrolyte depends on the number of ions present in the solution and the speed of the ions whih is known as ioni mobility. As strong eletrolyte already ompletely dissoiated in solution hene number of ions do not hanges with dilution. Therefore, the ondutane of a strong eletrolyte depends upon only the ioni mobility of the ions present in solution. In a onentrated solution, the density of ions are high as the the attration between ions are high. Aording to Debye and Hukel, at high onentration a partiular type of ion is surrounded by the opposite harged ions and thus a ioni loud is being reated. A ationi loud formed on an anion and vie versa. These ioni louds are generally spherial when no eletri fields is present. But in presene of an external eletrial field these ioni spheres get distorted. As the extend of ioni loud distortion inreases the the ioni mobility dereases. This is ommonly known as relaxation effet. As dilution inreases, the extend of this distortion dereases or the attration between ions dereases; as a result the ioni mobility or the ondutane of the solution inreases. With inrease in onentration, the equivalent ondutane of any eletrolyte dereases but the pattern of the graph is different for strong and weak eletrolyte. Condutane generally depends upon the number of ions and speed of ions (ioni mobility). Strong eletrolytes are ompletely dissoiated in solution and hene derease in onentration or inrease in dilution will inrease the ioni mobility only. Hene the with dilution equivalent ondutane will inrease slightly in a regular straight line. At infinite dilution this value will reah at its maximum whih is termed as limiting equivalent ondutane (Λ 0 ). Example of strong eletrolytes are NaCl, KCl, K 2 SO 4,HCl, H 2 SO 4, HNO 3, NaOH, KOH et.

5 4.4. EFFECT OF CONCENTRATION ON EQUIVALENT CONDUCTANCE 11 Equivalent ondutane vs onentration for strong eletrolyte The relationship between onentration of any eletrolyte and its equivalent ondutane is given by Debye-Hukel-Onsagar equation: Λeqv. = Λ 0 b ; where b is a onstant and its value depends upon stoihiometry of the eletrolyte of interest, is the onentration of the eletrolyte in gm-eqv./l. Weak eletrolytes are partially dissoiated in solution and hene with inrease in dilution or derease in onentration both the dissoiation of eletrolytes and the number of ions inreases. These two ombine effet inreases equivalent ondutane exponentially and beomes almost equal parrallel to y-axis. Equivalent ondutane vs onentration for weak eletrolyte Thus at infinite dilution, the equivalent ondutane of a weak eletrolyte an not be determined. But from Kohlraush s law, indiretly we an alulate that. So for weak eletrolyte when equivalent ondutane reahes maximum at almost omplete dissoiation, i.e.; dissoiation onstant (α) is lose to unity. α an be expressed as a ratio of equivalent ondutane at a ertain onentration of a eletrolyte(λ) with equivalent ondutane at infinite dilution Λ 0 of same eletrolyte.

6 12 CHAPTER 4. ELECTROCHEMISTRY LECTURE/LESSION PLAN Transport number and Ioni ondutane Transport number an be defined as fration of urrent transported by a partiular type of ion in a solution of eletrolyte. So, transport number for ation and transport number for aniion t + = I + I t = I I ; where I + is the amount of urrent arried by ation and I is the amount of urrent arried by anion. Current arried by any type of ion depends upon speed of ions (u), harge of the ions () and valeny of ion (z). Thus and Hene, I + u + + z + I u z and Thus, t + = t = u + + z + (u + + z + ) + (u z ) u z (u + + z + ) + (u z ) and t + = t = u + (u + ) + (u ) u (u + ) + (u ) Let us onsider at infinite dilution the ioni ondutane of ation is λ and ioni ondutane of anion is λ a. Now as ioni ondutane varies diretly proportional to speed of that partiular type of ionu + λ and u λ a Thus, t + = λ (λ + λ a ) and t λ a = (λ + λ a ) or, t + = λ Λ 0 and t = λ a Λ 0 So, transport number of a partiular type of ion an be expressed as fration of ondutane at infinite dilution offered by that partiular type of ion.

7 4.6. MOBILITY OF H + AND OH IONS IN AQUEOUS SOLUTION 13 Transport number by Hittroff s method 4.6 Mobility of H + and OH ions in aqueous solution The ioni mobility and hene the ondutane value of both H + and OH ion is very high. The ondutane value of H + and OH ions are 350 mho.m 2.mole 1 and 200 mho.m 2.mole 1 respetively. Earlier it was onsidered that high harge density of both the ion is responsible for this abnormal high ondutane. But in aqueous solution the H + ion basially exists as a hydroxonium ion H 3 O +. The high mobility an be explained by the fat that from hydroxonium ion the H + ion migrates to neighboring water moleule and thus the proton moleule is transported from one moleule to other moleule of water. Thus in presene of an external field water moleule migrates from one moleule to another moleule. A proton does not travel an entire distane up to the eletrode rather it only travels the distane between two water moleule. This mehanism is known as Grotthus-type mehanism. Hydroxyl ion are getting transported in similar manner. The major differene is that in proton transfer proton transported from hydroxonium ion to water moleule but in ase for hydroxide ion transport proton transported from water to adjaent hydroxide ion.