Chapter 7 Electrochemistry A science that studies the relation between electric and chemical phenomena and the disciplines that govern the conversion between electric and chemical energies.
Main contents Section 1: Electrolyte and electrolytic solution Section 2: Electrochemical Thermodynamics: Section 3: Irreversible electrochemical system Section 4: Applied electrochemistry Chapter 7 Electrochemistry
§7.1 Electrolyte and electrolytic solution Main contents: 1)Electrolyte: origin of the concept 2)Existence of ions in solution 3)Ion-dipole interaction--Hydration theory 4)Interionic interaction 5)Motion under electric field 6)Conducting mechanism 7)Faraday’s law and its application
1. Origin of the concept – electrolyte An electrolyte is a substance that, when dissolved in solvent, produces a solution that will conduct electricity. 1) Definition of electrolyte Progress of the definition: (1) molten salt; (2) solid-state electrolyte (3) room-temperature ionic liquids (RTIL).
In 1886, van’t Hoff published his quantitative research on the colligative properties of solution. For sucrose, the osmotic pressure ( ) can be expressed as: = c R T But for some other kind of solvates such as NaCl, the osmotic pressure had to be modified as: = i c R T i, van’t Hoff factor, is larger than 1. 2) Dissociation of substance In the paper written in Achieves Neerlandaises (1885) and Transactions of the Swedish. Academy (1886), van't Hoff showed analogy between gases and dilute solutions.
In 1887, Svant A. Arrhenius postulated that, when dissolved in adequate solvent, some substances can split into smaller particles, the process was termed as dissociation. AB A + + B – molecule cation anion positive ion negative ion The charged chemical species are named as ions and the process is termed as ionization. ++ + 3) Dissociation theory for weak electrolytes
New definitions: Ion, cation, anion; Dissociation, ionization Weak / strong electrolyte? True / potential electrolyte? Theory of Electrolytic Dissociation Acid-base theory Cf. Levine p.295
Solvated (hydrated) ion 2. Ions in solution In what state do ions exist in solution? ion Primary hydration shell secondary hydration shell Disordered layer Bulk solution Solvation shells
Hydration of ion Coordination number: Li + : 4, K + : 6 Primary solvation shell: 4-9, 6 is the most common number Secondary slovation shell: 6-8, for Al 3+ and Cr 3+ : The water molecules in the hydration sphere and bulk water have different properties which can be distinguished by spectroscopic techniques such as nuclear magnetic resonance (NMR), infrared spectroscopy (IR), and XRD etc.
3. Hydration Theory / Solvation Theory H / kJ mol -1 4 NaCl(s) Na + (aq) + Cl (aq) Na + (g) + Cl (g) hydration energy: 784 kJ mol , Robinson and Storks Why does NaCl only melt at higher temperature, but dissolve in water at room temperature?
The interionic distance for NaCl crystal is 200 pm, while for 0.1 mol dm -3 solution is 2000 pm. To draw Na + and Cl apart from 200 nm to 2000 nm, the work is: W (/kJ) = 625 / r for melting: r =1, W = 625 kJ, m.p. = 801 o C 。 for dissolution in water: r = 78.5, W = 8 kJ. Therefore, NaCl is difficult to melt by easy to dissolve in water at room temperature. Long-range forces
At low concentration At medium concentration At high concentration + + + Cf. Levine, p. 304 In equilibrium -- Bjerrum 4. Interaction between cation and anion
Owing to the strong interaction, ionic pair forms in concentrated solution. ionic pair vs free ion In an ionic pair, the cation and anion are close to each other, and few or no solvent molecules are between them. Therefore, HCl does not ionize and NaCl does not dissociate completely in solvents.
solution present species 0.52 mol·dm -3 KCl95% K + + 5% KCl 0.25 mol·dm -3 Na 2 SO 4 76 % Na % NaSO 4 ¯ 0.1 mol·dm -3 CuSO 4 44% CuSO 4 Some facts about strong electrolytes Degree of association Activity coefficient is essential for quite dilute solutions For concentration-dependence of ion pair, see Levine p. 305, Figure 10.10
Electric transfer of ion in solution under electric field + Motion of ions in the solution: 1) diffusion: due to difference in concentration 2) convection: due to the difference in density 3) transfer: due to the effect of electric field How can current cross the electrode / solution interface ? I E 5. Conducting mechanism of electrolyte
Cl ee ee ee At cathode: 2H + + 2e H 2 Cl H+H+ ee H+H+ ee H+H+ ee H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ At anode: 2Cl 2e Cl 2 H+H+ Cl Conducting mechanism: 1)Transfer of ion in solution under electric field; 2) electrochemical reaction at electrode/solution interface.
6. Law of electrolysis where m is the mass of liberated matter; Q the electric coulomb, z the electrochemical equivalence, F a proportional factor named as Faraday constant, M the molar weight of the matter. For quantitative electrolysis: Micheal Faraday Great Britain Invent the electric motor and generator, and the principles of electrolysis. Faraday’s Law Faraday’s constant F = ( ) C·mol -1 = C·mol -1 usually round off as C·mol -1, is the charge carried by 1 mole of electron.
Current efficiency ( ) Current efficiency is lower than 100% due to side-reactions. For example, evolution of hydrogen occur during electro- deposition of copper.
1) Definition of ampere: IUPAC: constant current that would deposit g of silver per second from AgNO 3 solution in one second: 1 ampere. Application of Faraday’s law 2) Coulometer: copper / silver / gas (H 2, O 2 ) coulometer 3) Electrolytic analysis – electroanalysis Q ↔m ↔ n ↔ c
7. Transfer of ion under electric field 1) Ionic mobility Ionic mobility (U) : the ionic velocity per unit electric field, is a constant. Rate of electric transfer: Ionic velocity How do we describe the motion of ions under electric field?
MA, MA’ have an ion in common. The boundary, rather different in color, refractivity, etc. is sharp. measure ionic mobility using moving boundary method
I = I ＋ + I － Q = Q ＋ + Q － t + + t - = ? 8. Transference number Transference number (transfer/ transport number), is the fraction of the current transported by an ion. plane A I－I－ I＋I＋ I Supporting electrolyte?
For time t: Q + = A U + t c + Z + F Q = A U t c Z F (1) Principle for measuring transference number Owing to electric migration, on the left side of plane A, there are more anions, while on the right side, more cations. Is this real? A I＋I＋ B C
(1) Principle of Hittorf method (1853) Example: Electrolysis of HCl solution When 4 Faraday pass through the electrolytic cell anodic region cathodic regionbulk solution + = 1 F 4Cl - -4e - 2Cl 2 4H + +4e - 2H 2 3 mol H + 1 mol Cl - 3 mol H + 1 mol Cl -
anodic region cathodic region bulk solution For anodic region: final result 4Cl - -4e - 2Cl 2 4H + +4e - 2H 2 3 mol H + 1 mol Cl - 3 mol H + 1 mol Cl -
EXAMPLE Pt electrode, FeCl 3 solution: In cathode compartment: Initial: FeCl mol·dm -3 Final: FeCl mol·dm -3 FeCl mol·dm -3 Calculate the transference number of Fe 3+ Hittorf’s transference cell Anode chamber Cathode chamber Cock stopper What factors will affect the accuracy of the measurement?
(2) Principle for the moving-boundary method When Q coulomb passes, the boundary moves x, the cross-sectional area of the tube is A, then: xAcZ + F = Q + = t + Q Why is the moving-boundary method more accurate that the Hittorf method? Are there any other methods for measuring transfer number?
(1) Temperature and (2) Concentration Transference number of K + in KCl solution at different concentration and temperature T / ℃ c /mol·dm -3 (3) Influential factors
(3) Co-existing ions ElectrolyteKClKBrKIKNO 3 t+ t ElectrolyteLiClNaClKClHCl t– t– Table transference number on co-existing ions Problem: Why does the transference number of certain ion differ a lot in different electrolytes?
Ira N. Levine, Physical Chemistry, 5 th Ed., McGraw-Hill, pp Section 10.6 solutions of electrolytes Section 10.9 ionic association pp Section 16.6 electrical conductivity of electrolyte solutions. Outside class reading