1. Redox Reactions

2. Electron Transfer Reactions
Electron transfer reactions are oxidation-reduction or redox reactions.
Results in the generation of an electric current (electricity) or be caused by imposing an electric current.
Therefore, this field of chemistry, in which electron transfer reactions are investigated is often called ELECTROCHEMISTRY.

3. Oxidation and Reduction MUST go TOGETHER!!!

4. 2Mg (s) + O2 (g)  2MgO (s) Mg: has gained oxygen. has been oxidized.
is the reducing agent (since it removes the oxygen---OXYGEN REMOVER)
O 2:
has been removed.
has been reduced.
is the oxidizing agent (since it gives / is the source of oxygen– OXYGEN DONATOR)

5. REDOX REACTIONS CAN OFTEN BE OBSERVED AS SIGNIFICANT COLOR CHANGES.

6. e.g. Fe(s) + CuSO4(aq)  FeSO4(aq) + Cu(s)

7. Another way to define oxidation and reduction
OIL RIG s s
xidation
ose
eduction
ain

8. 2Mg (s) + O2 (g) 2MgO (s) 2Mg 2Mg2+ + 4e- Oxidation (lose e-)2+ 2-
2Mg (s) + O2 (g) MgO (s)
2Mg Mg2+ + 4e-
Oxidation (lose e-)
O2 + 4e O2-
Reduction (gain e-)

9. Terminology for Redox Reactions
OXIDATION—loss of electron(s) by a species; increase in oxidation number; increase in oxygen.
REDUCTION—gain of electron(s); decrease in oxidation number; decrease in oxygen; increase in hydrogen.
OXIDIZING AGENT—electron acceptor; species is reduced. (an agent facilitates something; ex. Travel agents don’t travel, they facilitate travel)
REDUCING AGENT—electron donor; species is oxidized.

12. e.g. Formation Silver Crystals
Give example of the reaction happening between Zn + H2SO4
Solve problems from the worksheet.

13. All “combustion” reactions and “metal+acid” reactions are REDOX!

14. Examples of Redox reactions
Photosynthesis

15. The reactions in film photography
In black-and-white film photography, the film is covered with a coating of gel that contains tiny grains of AgBr. Light causes this to break down:
2AgBr(s)  2Ag(s) + Br2(l)
It is a photochemical and redox reaction.

16. ELECTROLYSIS
Electrolysis is the process in which electrical energy is used to break down a substance.
In electrolysis, any liquid that contains IONS is used since it conducts the electricity. This liquid is called ELECTROLYTE. By means of the movement of both anions and cations, the electrical current is carried in the electrolyte.

17. Source of direct electrical current: pushes e- into cathode & pulls e- from anode
An electrolytic cell consists of two electrodes in a molten salt or solution. + -
(Where reduction occurs)
(Where oxidation occurs) + -
PA !
PA !
Positive Anode
electrolytic cell
Electrodes (usually GRAPHITE or PLATINIUM) are inert solids that conduct the electricity but do NOT react.

18. Electrolysis can be conducted for molten forms and aqueous solutions of ionic compounds.

19. ELECTROLYSIS OF MOLTEN IONIC COMPOUNDS
The compound is broken down to its constituent elements.

20. GENERAL RULE FOR THE ELECTROLYSIS OF MOLTEN IONIC COMPOUNDS…
At the ANODE----ELEMENTAL FORM OF THE NONMETAL
At the CATHODE---- ELEMENTAL FORM OF THE METAL
will be obtained.

21. Electrolysis of molten lead(II) bromide

22. Overall electrolytic cell rxn: PbBr2(l) Pb(s) + Br2(g)
In the liquid, the ions move to the electrode of opposite charge.
At the cathode (-), Pb2+ ions accept electrons and form Pb(s) on the electrode.
At the anode(+), Br- ions lose electrons and form Br2 gas.
Electrons flow from anode to the the (+) terminal of the battery and from (-) terminal of the battery to the cathode.
Electrons carry the current in the external circuit (the wires and electrodes).
Ions carry the current in the liquid.
The electrodes remain unchanged.
PbBr2(l)  Pb(s) +Br2(g)
ANODE CATHODE
2Br-  Br2 + 2e- Pb2+ + 2e-  Pb
Br2(g) Pb(s)
Overall electrolytic cell rxn: PbBr2(l) Pb(s) + Br2(g)

23. Because of high melting points of metals, electrolysis of molten salts requires very high temperatures.

24. Electrolysis of molten salts
Overall electrolytic cell rxn: AlCl3(l) Al(s) + 3/2Cl2(g)
C(graphite)
electrode
C(graphite)
electrode
Al deposits
Cathode:Al+3(l)+3e-Al(s)
Al+3
Molten AlCl3

25. Exercise Which substances will be obtained at anode and cathode?
C(graphite)
electrode
C(graphite)
electrode
Ansswer the questions 1 and 2 on page 105 after this example.
Molten KF

26. ELECTROLYSIS OF AQUEOUS SOLUTIONS
In aqueous solutions, we have to consider WATER as well since water itself produces ions.

27. Tiny % of water split up into ions…

28. General Principle At Cathode: Metals or Hydrogen gas is obtained.
At Anode:
- Nonmetals (other than Hydrogen) are produced.

29. Rules for the electrolysis of a solution
Tendency to be reduced at cathode (-):
Cu2+, Ag+ > H2O > Ions of of other metals
When metal ions are reduced at the cathode, their elemental forms (Cu, Ag, Na) are obtained.
- When water is reduced at the cathode, hydrogen is obtained.
IMPORTANT!!! The more reactive the metal, the more it likes to exist as ions.

30. Rules for the electrolysis of a solution
Tendency to be oxidized at anode (+):
Halides in concentrated solutions (Cl-, Br- ,l-) > H2O >other ions
When halides (Cl-, Br- ,l-) are oxidized at the anode, their elemental forms are obtained (Cl2, Br2, I2) .
- When water is oxidized at the anode, Oxygen is obtained.

31. RAC! Reduction at Cathode…

32. The electrolysis of aqueous NaCl(brine) is an important for the production of chlorine and NaOH.
Look at the examples given on page 105 and also the # 3 (the questions).

33. Name the products at each electrode when brine (concentrated aqueous solutions of NaCl) is electrolysed using inert electrodes.
They should take notes here in the class.

34. Name the products at each electrode when concentrated HCl(aq) is electrolysed using inert electrodes.
Solve some examples from the aqueous solutions worksheet.

35. ELECTROPLATING One of the uses of electrolysis.
In electroplating, the electrolysis is conducted to coat one metal with another to make the metal look better or to prevent corrosion.

36. Watch the MOVIE

37. Electroplating is used for …
Coating the steel car bumpers with chromium metal.
Cheap metal jewellery or cutlery with silver or gold.

38. How does it work? Silverplating: electroplating with silver.
Cathode: object to be electroplated.
Anode: metal (Ag in this example) to be used for plating.
Electrolyte: Solution of the compound of the metal to be used for electroplating
Answer question 6 on page 111.

39. Copperplating

40. Watch the 1st two MOVIES

41. References https://chemistry58.wikispaces.com/Electroplating