Electrochemistry -1 Puneet Jyoti G.C.G-42 Chd.

Electron Transfer Reactions  Results in the generation of an electric current (electricity) or be caused by imposing an electric current.  Electron transfer reactions are oxidation-reduction or redox reactions.  Therefore, this field is often called ELECTROCHEMISTRY.  Electrochemistry is a branch of chemistry that studies chemical reactions which take place in a solution at the interface of an electron conductor (a metal or a semiconductors) and an ionic conductor (the electolyte), and which involve electron transfer between the electrode and the electrolyte or species in solution.

Motion of ions in the solution: Only the transfer can cause net electricity 1) diffusion: due to difference in concentration 2) convection: due to the difference in density or temperature 3) transfer: due to electric field

Conductance and its measurement For metals: Ohm’s Law R: resistance Dimension: Ohm, 

Resistivity Dimension: Ohm m,  m

For electrolytic solution: conductivity (  ) or spedific conductance: Definition:  = 1/  Dimension: S m -1 electric conductance (G) : Definition: G = 1/R Dimension:  -1, mho, Siemens, S

Cell constant of a conductivity cell

Influential factors for conductivity 1) concentration – dependence of conductance

1.Acids and bases have higher conductance 2. C < 5 mol dm -3,  increases with C 3. For CH 3 COOH conductance does not depend on C

(2) temperature- dependence of conductance

Molar conductivity 1) Definition V: degree of dilution The conductivity of a solution is approximately proportional to the concentration  m is the conductivity contributed by 1 mole of electrolyte between electrodes of 1 m apart

Dependence of molar conductivity on concentration  m decreases with concentration. Kohlrausch replotted  m against C 1/2 Due to the interaction between ions: interionic attraction

For 1:1 electrolytes: C < 0.002~ mol dm -3 Linear relationship between  m and C 1/2 can be observed.

Kohlrausch empirical formula To extrapolate the linear part of  m ~ C 1/2 at low concentration to C = 0,  m  can be obtained.  m  the limiting value of  m at infinite dilution: limiting molar conductivity

Kohlrausch’s law of independent ionic mobilities At infinite dilution,  m  should be the sum of the separate contributions of the ions

limiting molar conductivity of weak electrolyte

Ionic mobility and transference number of ions 1) Ionic mobility Under unit potential gradient: dE/dl = 1 V m -1 : U = R, ionic mobility

I = I + + I - Q = Q + + Q - The fraction of the current transported by an ion is its transference number or transport number t = t + + t - = 1 2) Transference number

3) Relation between ionic mobility and transference number C -, Z -, U - ; C +, Z +, U + ; For time t: Q + = A U + t C + Z + F Q  = A U  t C  Z  F

Q = Q + + Q  = AtF ( U + C + Z + + U  C  Z  ) C + Z + = C  Z  Q = AtF C + Z + ( U + + U  )

Measurement of transference numbers 1) Hittorf method (1853) Electrolysis of HCl solution Anodic regioncathodic regionBulk solution

4 Cl - -4e -  2 Cl 2  4 H + +4e -  2 H 2  When 4 Faraday pass through the electrolytic cell 3 mol H +  1 mol Cl - 

C residual = C initial – C react + C transfer 3 = 6 – 4 + C transfer For anodic region: t - = 1 / 4 = 0.25t + = 3 / 4 = 0.75

Hittorf’s transference cell

2) The moving-boundary method MA, MA’ have an ion in common. The boundary, rather difference in color, refractivity, etc. is sharp. In the steady state, the two ions move with the same velocity.

When Q coulomb passes, the boundary moves x, the cross- sectional area of the tube is A: xACZ + F = t + Q

THANX