1. Chapter 7 Electrochemistry What is electrochemistry? A science that studies the relation between electric and chemical phenomena and the disciplines that govern the conversion between electric and chemical energies.

2. Main contents Section 1: Electrolyte and electrolytic solution Section 2: Electrochemical Thermodynamics: Section 3: Irreversible electrochemical system Section 4: Applied electrochemistry Chapter 7 Electrochemistry

3. §7.1 Electrolyte and electrolytic solution Main contents: 1)Electrolyte: origin of the concept 2)Existence of ions in solution 3)Hydration theory: 4)Interionic interaction 5)Motion under electric field 6)Conducting mechanism 7)Faraday’s law and its application

4. 7.1.1 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

5. 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 expressed 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.

6. The equation for freezing point depression and boiling point elevation contains the letter i. i stands for the van’t Hoff Factor. ∆T = imK f Since freezing point depression and boiling point elevation depend only on the number of particles ( it does not matter what the particles are), we need only determine the total m of the particles. If a solution is 0.2 m NaCl, the i would be about 2. The true van’t Hoff factor is not exactly 2, but is close enough to call it 2. http://en.wikipedia.org/wiki/Van_'t_Hoff_factor

7. In 1887, Svant August 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

8. Therefore, the number of particles present in solution is actually larger than that predicted by van’t Hoff, which resulted van’t Hoff factor. New definitions: Dissociation, ionization Weak / strong electrolyte? True and potential? Theory of Electrolytic Dissociation Acid-base theory Greenhouse effect Cf. Levine p.295

9. Solvated (hydrated) ion +  7.1.2 State of ion in solution In what state do ions exist in solution?

10. 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. ion Primary hydration shell secondary hydration shell Disordered layer Bulk solution Solvation shells The interaction between ions and water molecules disturb the structure of liquid water.

11. 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+ : 10-20

12. 7.1.3 Hydration Theory / Solvation Theory  H / kJ mol -1 4 NaCl(s) Na + (aq) + Cl  (aq) Na + (g) + Cl  (g) 788 784 hydration energy: 784 kJ mol -1 1948, Robinson and Storks Why does NaCl only melt at higher temperature, but dissolve in water at room temperature?

13. 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

14. At low concentration At medium concentration At high concentration  + +  +  Cf. Levine, p. 304 In equilibrium -- Bjerrum 7.1.4 Interaction between cation and anion

15. 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.

16. solution present species 0.52 mol·dm -3 KCl95% K + + 5% KCl 0.25 mol·dm -3 Na 2 SO 4 76 % Na + + 24% 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

17. (1) Category of conductor: Charge carriers: 7.1.5 Conducting mechanism of electrolyte PbO 2, NiOOH Ion and electron Mixed conductor Superconductors electron pair 5 th Conducting polymers polaron 4 th Semiconductor Electron and hole 3 rd Electrolytic solution, solid-state electrolyte (Al 2 O 3, ZrO 2 ) ion 2 nd Metals, carbonous materials, some metal oxides electron 1 st samplesCharge carrierConductor electron; ion; hole; Cooper electron pair; polaron.

18. (2) Conducting mechanism 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

19. Cl  ee ee ee At cathode: 2H + + 2e   H 2  Cl  H+H+ ee H+H+ ee H+H+ ee 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.

20. 7.1.7 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 1791-1867 Invent the electric motor and generator, and the principles of electrolysis. Faraday’s Law Faraday’s constant F = (1.6021917  10 -19  6.022169  10 23 ) C·mol -1 = 96486.69 C·mol -1 usually round off as 96500 C·mol -1, is the charge carried by 1 mole of electron.

21. Current efficiency (  ) Current efficiency is lower than 100% due to side-reactions. For example, evolution of hydrogen occur during electro- deposition of copper.

22. 1) Definition of ampere: IUPAC: constant current that would deposit 0.0011180 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

23. A 0.100 molality (mol/kg) solution of NaCl has a freezing- point depression of -0.348 o C, whereas the expected decrease in the freezing point is -0.186 o C. The van’t Hoff factor in this case is 1.87. If there were no ion pairing, we would expect the van’t Hoff factor for NaCl to be 2.00. Similarly, acetic acid in a 0.100 molal solution has a van’t Hoff factor of 1.05. Calculate the concentration of NaCl ion pairs and also the percent ionization of acetic acid form the above information. Exercise-1:

24. A current of 2.34 A is delivered to an electrolytic cell for 85 min. how many grams of (a) Au from AuCl 3, (b) Ag form AgNO 3, and (c) Cu from CuCl 2 will be plated out? Exercise-3 Levine: p.317 10. exercise 48 Exercise -4 Yin: p. 217 exercise 1. Exercise-2:

25. Ira N. Levine, Physical Chemistry, 5 th Ed., McGraw-Hill, 2002. pp. 294-310 Section 10.6 solutions of electrolytes Section 10.9 ionic association pp. 512-515 Section 16.6 electrical conductivity of electrolyte solutions. Outside class reading