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ELECTROCHEMICAL CELLS Writing of equations representing oxidation and reduction half reactions and redox reactions Electrolytic and galvanic cells; Writing.

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Presentation on theme: "ELECTROCHEMICAL CELLS Writing of equations representing oxidation and reduction half reactions and redox reactions Electrolytic and galvanic cells; Writing."— Presentation transcript:

1 ELECTROCHEMICAL CELLS Writing of equations representing oxidation and reduction half reactions and redox reactions Electrolytic and galvanic cells; Writing Equations Relation of current and potential to rate and equilibrium; Understanding of the processes and redox reactions taking place in cells; Standard electrode potentials; Calculating Ecell Electrolysis Rust Industrial applications

2 ________________ Metal Electrodes. Metal atoms can leave their electrons (weakly held) and go into solution. _M___M__ O H H O H H O H H M (s)  M + (aq) + e - e-e- e-e- e-e- e-e- e-e- e-e- e-e- M+M+ M+M+ An electric potential develops between the solution(….) and electrode(……). Move the red charges to show the electric potential. O-O- H+H+ H+H+ M+M+ M+M+ - +

3 ________________ Direct Contact. _ Zn _ O H H O H H O H H Zn (s)  Zn 2+ (aq) + 2e - e-e- e-e- e-e- e-e- e-e- e-e- e-e- Cu 2+ The copper ions are reduced to copper metal and the zinc metal is oxidised to zinc ions. Cu 2+ (SO ) Cu 2+ O H H O H H O H H Cu 2+ (aq) + 2e -  Cu (s) The zinc rod would become layered with copper The blue solution would become colourless Copper has a STRONGER attraction for electrons than Zinc. State which ions would be OXIDISED AND WHICH WOULD BE REDUCED

4 Electrochemical Cell. Two metals can react indirectly. __________ Zn ____ Cu V - KNO 3(aq) + > e - Zn  Zn e - Cu e -  Cu CuSO 4 ZnSO 4 Salt bridge anode cathode Electrons flow through the external circuit from Zn to Cu. Ions flow through the salt bridge (barrier). Zinc metal is being Copper ions are being …………... Observations: The zinc electrode would be ………….- the copper electrode would ………………. Decide which electrode would be negative and which positive and put the charges over the correct electrode.

5 Electrochemical Cell. Two metals can react indirectly. __________ Zn ____ Cu V _ KNO 3(aq) + > Zn  Zn e - Cu e -  Cu Zn e - --> Zn Cu e - --> Cu Attract elelctons strongly Attract elelctons weakly CuSO 4 ZnSO 4 Salt bridge anode cathode Zn + Cu 2+  Zn 2+ + Cu oxidation reduction REDUCING AGENT OXIDISING AGENT REDUCING AGENT Top right reacts with bottom left. e- Move the labels until they show the correct locations of oxidizing & reducing agents.

6 ELECTROCHEMICAL CELLS GENERAL A METAL IN CONTACT with a SOLUTION OF ITS IONS is known as A HALF-CELL. M  M + + e - N + (aq) + e  N (s) When TWO HALF – CELLS are COMBINED an ELECTROCHEMICAL CELL IS OBTAINED. OXIDATION takes place at the POSITIVE/NEGATIVE (delete incorrect one) electrode. PLACE THE TWO SYMBOLS AT THE APPROPRIATE PLACE ON THE DIAGRAM M + (aq) M + (s) N + (aq) N + (s) V - e-e-

7 CELL DIAGRAMS The cell undergoing reduction (more +ve E θ value) is assigned as the R.H.S. electrode. M 1 (s)  M 1 n+ (aq)  M 2 n+ (aq)  M 2 (s) The solid line represents the phase boundary solid metal & the aqueous ions. The double dashed line represents the salt bridge. The cell potential is calculated by: E θ cell = E θ RHS  E θ LHS (do not change the signs of the E θ values ) Phase boundary salt bridge Phase boundary + -

8 ELECTROCHEMICAL CELLS Electrons flow from –ve to +ve in the external circuit. The salt bridge allows the flow of ions and completes the circuit. ……..  ……….. // ………….  …………. ……..  ……….. // ………….  …………. Zinc is ……………& copper is ……………. Zinc is ……………& copper is ……………. The Daniell cell : Galvanic cell converts chemical energy to electrical energy E ?

9 Reactions written as ………………….. Positive potentials ………….. electrons are good ……………… AGENTS. Negative potentials …………… electrons are good …………………… AGENTS. ELECTROCHEMICAL SERIES Electrochemical half-cell potentials (E θ ) are listed from +ve to –ve values. Zn e -  Zn Cu e -  Cu ElectricPotentials ( …. ) Zn ………………. so is MORE ………………. (….)

10 Reactions written as reductions. Positive potentials accept electrons are good OXIDISING AGENTS. Negative potentials donate electrons are good REDUCING AGENTS. ELECTROCHEMICAL SERIES Electrons flow in external circuit Electrons flow in external circuit. Electrochemical half-cell potentials are listed from +ve to –ve E θ values. Oxidizing Agents REDUCING AGENTS AGENTS Reactions take place Top LEFT to bottom RIGHT

11 ELECTROCHEMICAL SERIES Difference = 0.8 – (-2.71)

12 STANDARD HYDROGEN ELECTRODE ……….. electrode H 2 gas pressure of ………. (………. kPa) Solution of …..M H + ions (HCl) …………K E θ = ……………v S.H.E. is ………. to set up. ……………….. standard electrodes used.

13 DETERMINATION OF CELL POTENTIALS E θ values are determined using a ……………………………… electrode. E θ values are determined using a ……………………………… electrode. The S.H.E is always the …………………. electrode. The S.H.E is always the …………………. electrode. Standard Conditions : …………………………………………………. Standard Conditions : …………………………………………………. A secondary standard electrode the ………………. electrode can be used. A secondary standard electrode the ………………. electrode can be used. E θ cell = ………………………………………… E θ cell = ………………. – ………. N + (aq) N + (s) Unknown electrode Standard electrode High resistance voltmeter Salt bridge If a cell potential of 1.45v is obtained with metal N. Then the electrode potential of N + /N is

14 DETERMINATION OF CELL POTENTIALS If the calomel electrode (is used as a standard with another electrode the cell potential is V. If the calomel electrode ( V ) is used as a standard with another electrode the cell potential is V. By means of a calculation, identify the other metal. By means of a calculation, identify the other metal. M + (aq) M + (s) Unknown electrode Calomel electrode Salt bridge High resistance voltmeter

15 SPONTANEOUS REACTIONS N + (aq) N + (s) Positive electrode M + (aq) M + (s) Negative electrode Salt bridge High resistance voltmeter E 0 M - ve E  0.15 VE  V

16 NON SPONTANEOUS REACTIONS N + (aq) N (s) M + (aq) M (s) Salt bridge High resistance voltmeter N + (aq) E  0.15 V E  0.25 V

17 NON SPONTANEOUS REACTIONS N + (aq) N (s) M + (aq) M (s) Salt bridge High resistance voltmeter N + (aq)

18 WORKED EXAMPLES 1. The redox couples Mn 2+ (aq) Mn(s) and U 3+ (aq) U(s) have E θ values of –1.18 and –1.79 respectively. Determine E θ cell when they are combined and write an equation for the reaction that takes place. 2.The redox couples Co 2+ (aq) Co(s) and Ni 2+ (aq) Ni(s) have E θ values of –0.28v and –0.23v respectively. Write an equation for the spontaneous reaction and use the E θ values to justify your answer.

19 EXAMINATION QUESTIONS Use the standard E θ values for the half-equations shown below to answer the questions that follow. MnO 4 - (aq) +8H + (aq) +5e - Mn 2+ (aq) + 4H 2 O (l) E θ = +1.70v Cl 2 (g) + 2e - 2Cl - (aq) E θ = +1.36v Tl 3+ (aq) + 2e - Tl + (aq) E θ = +1.25v AgCl(s) + e - Ag (s) + Cl - (aq) E θ = v Determine E θ cell for the reaction of MnO 4 - (aq) with Tl + (aq) in acidic solution and write an equation for the reaction. Why is it not possible to use HCl acid in the reaction above. Use the E θ values to justify your answer. Construct an equation for the reaction of Cl 2 (g) with Ag(s) and calculate the E θ value for the reaction.

20 Electrolytic cell: Converts… Electrolysis Copper chloride

21 Electrolysis - Lead Bromide PbBr 2

22 Electrolysis - Lead Bromide PbBr 2

23 Purifying Copper Identify the anode cathode etc -+

24 Extraction of Aluminium

25 ELECTROCHEMICAL CELLS Writing of equations representing oxidation and reduction half reactions and redox reactions Electrolytic and galvanic cells; Writing Equations Relation of current and potential to rate and equilibrium; Understanding of the processes and redox reactions taking place in cells; Standard electrode potentials; Calculating Ecell Electrolysis Rust Industrial applications


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