Chapter 7 Electrolyte solution

1. Electrolysis Cell Chapter 7 Electrolyte solution Chemical energy Electrical energy

2. 7 Electrolyte solution Main content: • Concepts in electrochemistry and Faraday’s Law • Ions transference and its number • Electric conductance • Theory about Strong electrolyte solution

3. 7.1Concepts in electrochemistry （1）basic concept （2）Faraday’s Law • Research areas • Faraday’ Law • Two kinds of conductor • Faraday constant • Anode and cathode • Math expression of Faraday’ Law • Positive electrode and Negative electrode • Primary cell • Example • Electrolytic cell • Efficiency of current

4. Electrolysis Electrical energychemical energy Cell Research areas Electrochemistry principally researches the conversion of chemical energy into electrical energy and the relevant rules in this conversion.

5. Two kinds of conductor 1. The first kind of conductor Electron conductor (such as metals and graphite) A.Conduct by the moving of the free electron B.The conductor isn’t changed in the process of electronic conduction C.Resistance is raised with the raising of the temperature D.All the amount of the electricity conducted is undertook by electron

6. Two kinds of conductor ⒉ The second kind of conductor Ion conductor (such as electrolyte solution and melt electrolyte) A.Conduct by the inversely moving of the positive and negative ions. B.There are chemical reactions in the process of electronic conduction. C.Resistance is descended with the raising of the temperature D.All the amount of the electricity conducted is undertook by positive and negative ions * Solid electrolyte such as AgBr and PbI2 belong to ion conductor, but because its conduction mechanism is very complicated and the ability of its conduction is not high, so in this chapter we mainly talk about electrolyte solution.

7. Positive electrode、Negative electrode Positive electrode : The electrode which electric potential is higher is called anode, current flows from anode to cathode. Among primary cells the positive electrode is cathode and among electrolytic cells the positive electrode is anode. Negative electrode: The electrode which electric potential is lower is called cathode, current flows from cathode to anode. Among primary cells the negative electrode is anode and among electrolytic cells the negative electrode is cathode.

8. The cathode where the reduction occurred is called cathode, among primary cells cathode is positive electrode and among electrolytic cells cathode is negative electrode. Cathode: The cathode where the oxidation occurred is called Anode, among primary cells Anode is negative electrode and among electrolytic cells Anode is positive electrode. Anode: Cathode、Anode

9. Zn electrode： Zn(S)→Zn2++2e- Oxidation occurred, this electrode is anode, electrons flow from electrode Zn to electrode Cu, Zn electric potential is lower, and it is negative electrode. Galvanic cell Cu electrode： Cu2++2e-→ Cu(S) Reduction occurred, this electrode is cathode, current electrode Cu to electrode Zn, Cu electric potential is higher, and it is positive electrode.

10. ① ② Electrolytic cell Electrode①： The electrode that is connected with the positive electrode of external electrical power is negative electrode and it occurred reduction reaction, so it is cathode Cu2++2e-→Cu(S) Electrode②： The electrode (2) that is connected with the negative electrode of external electrical power is positive electrode and it occurred oxidation reaction, so it is anode. Cu(S)→ Cu2++2e-

11. Efficiency of current Formula（1） Efficiency of current = Formula（2） Efficiency of current =

12. Faraday Law ⒈ The quantity of substance, which occurred chemical reaction in the surface of the electrode, is in direct ratio with the given quantity of electricity. ⒉ Pass the electricity through the series connection circuit of some electrolytic cells, when electric charge of the particle gotten is the same, the substances occurred chemical reactions in the entire electrode have the same mole, the quantity of the substances separated out is in direct ratio with its mole quantity.

13. or Math expression of Faraday Law Select the gains or losses electrons as z, quantity of electricity as Q, and the moles n of the substance that occurs chemical reaction in the electrode are: The mass m of the substance that occurs chemical reaction in the electrode is:

14. Faraday constant Faraday constant is equal to the quantity of electricity of 1mol electric charge. F=L·e =6.022×1023mol-1×1.6022×10-19 C =96484.6 C·mol-1 ≈96500 C·mol-1

15. 7.2Ion transference and its number • The phenomenon of the ions electromigration • The electric transference ratio and number of ions • The method of measuring the transference number

16. The phenomenon of the ions electromigration

17. The phenomenon of the ions electromigration Three solution parts: anode, middle and cathode. Before passing through electricity, there are 5mol positive ions (+) and 5 negative irons (–) in every part. All ions are 1 valence, when passing through quantity of electricity of 4 mole electrons, 4 moles negative ions in the anode are oxidized, 4 moles positive irons in the cathode are reduced.

18. The phenomenon of the ions electromigration 1．Ifr+=r-, the task of conduction are both 2 moles, on the surface of the imaginable planes AA and BB, there are both 2 moles negative and positive ions passing through. When the electricity is over, the concentration of the solution in the anode and cathode parts are the same, but 2 moles less than the original solution, the concentration in the middle part is unchanged.

19. The phenomenon of the ions electromigration

20. The phenomenon of the ions electromigration 2． If r+=3r-, the task of conduction of positive ions are 3 moles, the negative ions is 1 mole. On the surface of AA and BB, there are 3 moles positive and 1mole negative ions passing through in inverse direction. When the electricity is over, in the concentration in anode parts 3 moles positive and negative ions decrease, and in cathode 1 mole positive and negative ions decrease, the concentration in the middle part is unchanged.

21. The phenomenon of the ions electromigration

22. The rule of the transference of the ions 1. All the moles of the positive and negative ions that transfer to the cathode and anode are equal to the total quantity of the electricity passing through. 2.

23. The electric transference ratio of ions The speed of ions in the electric field is expressed by formula: dE/dl: electric potential gradient U+electric transference rate of positive ions U-electric transference rate of negative ions Ionic mobility: the speed of the ions when the electric potential gradient is in 1 unit. Its unit is ㎡·s-1·V-1.

24. Transference number definition the transference number of ion B: The ratio between the current of ion B carrying and the total current Because the speeds of positive and negative ions are different, they carry different electric charge, so they undertake differentproportion of quantity of electricity when they transfer.

25. Transference number definition If the solution has only one electrolyte: If the solution has many electrolytes, :

26. Transference number definition There are two parallel electrodes, the distance is l, the surface area A, and the external voltage is E. Put the electrolyte MxNy (concentration c) between two electrodes, the dissolve degree is α, c unit: mol·m-3.

27. Transference number definition The speed of positive ion is r+, in unit time there is (cxαAr+) mol substance passing through any cross section, thequantity of electricity transferred is (cxαAr+)z+F, because it is in the unit time, so The same reason

28. （ ，Electric potential gradient is the same） Transference number definition Because the solution is electric neutral, so

29. Measuring methods of transference number • Hittorfmethod • Interface motion method • Electromotive force method

30. Hittorf method • Put the electrolyte solution into the Hittorf tube, connect the stable voltage and direct current power, then the electrodes occur in chemical reaction, positive and negative ions transfer to cathode and anode. • The concentration near the electrode is changed. • Weigh solution in cathode (or anode), calculate the transference number according to the quantity of electricity inputted and the change of the concentration near the electrode.

31. The method of measuring the transference number

32. 7.3 Electric conductance • Electricconductance, electrolyticconductivity, molar, electrolyticconductivity • Relationship of electrolytic conductivity and molar electrolyticconductivitywith concentration • Independent motion law of ions • Some useful relation formula • Application of the measurement of the electric conductance

33. Electric conductance is the reciprocal of the resistance, the unit is or s. Electric conductance G is in direct proportion with the area A and is in reverse ratio with the length l of the conductor. 7.3.1 electric condutance

34. 7.3Electric conductance

35. Because: 7.3.2Electric conductivity k: electrolytic conductivity, the electric conductance in unit length and unit area, the unit is Electrolytic conductivity is the reciprocal of the resistance rate:

36. 7.3.3Molar conductivity molar electrolytic conductivity Λm: the electric conductance of the solution, unit length, 1 mole electrolyte solution，unit is S·m2·mol-1. Λm=k Vm =k/c Vmis the volume of the 1mole electrolytic solution, unit is m3·mol-1, c is the concentration, unit is mol·m3.

37. 7.3.3Molar conductivity

38. The measurement of electric conductance Wiston electric bridge, measurement of resistance, AB is even slippery string resistance; it is the changeable resistance, M is the electric conductance pool where the measuring solution is put. The resistance will be measured. I is the high frequency intercourse electricity power. G is the earphone or the oscillograph.

39. The measurement of electric conductance

40. The measurement of electric conductance Connect the electricity power, adjust C point, and make the circuitry DGC has no current, the drop of electric potential of point D and C is equal, the electric bridge is in equilibrium. According to the relationship of some resistance,

41. Cell constant Cell constant:unit is • The distance L and the area A of Pt electrode can’t be measured accurately. • Measuring Kcell in known electrolytic conductivity • Measuring the electrolytic conductivity of unknown solution.

42. Relationship of electrolytic conductivity with concentration • The electrolytic conductivity of strong electrolytic solution increases with increasing of the concentration. When the concentration is raised to a certain degree, the dissociation degree is descend, the speed of ion and electrolytic conductivity decrease. Such as H2SO4 and KOH solution. • The change of electrolytic conductivity of weak electrolytic solution is not so obvious, such as acetic acid.

43. Relationship of electrolytic conductivity with concentration

44. Relationship of Λm with concentration Material mass (1 mole) in the solution is given. When the concentration reduces, interaction between particles weakens, the speed of positive and negative ions quickens. So Λmmust rise.

45. Relationship of Λm of strong electrolyte with c Λm increase with decreasing of concentration. when concentration descend below 0.001mol·dm-3, the relationship of Lm (Λm) and is linearity, Put the line outside to , we can get molar electrolytic conductivity of infinitely diluted solution.

46. Relationship of Λm of strong electrolyte with c

47. Relationship of Λm of weak electrolyte with c Lm slowly rises with decreasing of concentration, when the solution is very watery, the relationship of Lm and is linear. Below to a certain degree, Lm rapidly rises Such as CH3COOH.

48. Independent motion law of ions Germany scientist, Kohlrausch discovered a law. In the infinitely diluted solution, every ion moves independently and it is not effected by other ions. The molar electrolytic conductivity of the infinitely diluted solution can be regarded as the sum of the two kinds of ions.

49. some useful relation formula To the strong electrolyte, when the concentration is not so large, we can approximately use this formula.

50. some useful relation formula 4. To the strong electrolyte we can approximately use this formula: