1. Oxidation and Reduction (REDOX) reactions?
A reaction in which electrons are transferred from one atom to another is called an oxidation and reduction reaction or REDOX reaction.

2. Rules for Assigning Oxidation Numbers
The charge the atom would have in a molecule
(or an ionic compound) if electrons were completely transferred.
Free elements (uncombined state) have an oxidation number of zero. All Diatomics have an oxidation state of zero.
Na, Be, K, Pb, H2, O2, P4 = 0
In monatomic ions, the oxidation number is equal to the charge on the ion.
Li+, Li = +1; Fe3+, Fe = +3; O2-, O = -2
The oxidation number of oxygen is –2.
Exception: In H2O2 it is –1. 2

3. The oxidation number of hydrogen is +1 except when it is bonded to metals in binary compounds. In these cases, its oxidation number is –1.
Group IA metals are +1, IIA metals are +2 and fluorine is always –1.
6. The sum of the oxidation numbers of all the atoms in a molecule or ion is equal to the charge on the molecule or ion.
HCO3-
Oxidation numbers of all the atoms in HCO3- ? 3

4. How to write oxidation numbers using rules?:Biochemhelp

5. Find the oxidation state of Cr in K2Cr2O7
Element K Cr O
Total Charge of the compound has to equal ZERO
Subscript
Oxidation State of 1 atom
Sum of oxidation states

6. Oxidation-Reduction Reactions “REDOX” reactions
Determine the oxidation number of the boldface underlined element in the following formulas:
NaClO4 NH4+ HCO3- 6

7. Concept Check Give the oxidation number for the following atoms:
Cr3+ Cr = _____ O2 = _____
Cl-1 Cl = _____ Mg = _____
AgNO3 N = _____
NH3 N = _____

8. Concept Check Give the oxidation numbers for the atoms in (NH4)2CrO4
NH4NO3

9. How would you classify most metals and non- metals?
Redox Reactions
OXIDATION — loss of electron(s) by a substance. Increase in oxidation number. (Mg  Mg e-)
REDUCTION — gain of electron(s) by a substance. Decrease in oxidation number. (S+ 2e-  S2-)
OXIDIZING AGENT — electron acceptor and the substance is reduced. (Non-metals, Cl atom gains 1 e-)
REDUCING AGENT — electron donor and the substance is oxidized. (Metals, Na atom loses 1 e-)
How would you classify most metals and non- metals? 9

11. You can’t have one… without the other!
Reduction (gaining electrons) can’t happen without an oxidation to provide the electrons.
You can’t have 2 oxidations or 2 reductions in the same equation. Reduction has to occur when there is oxidation.
LEO the lion says GER!
ose
lectrons
xidation
ain
lectrons
eduction
GER!

12. Another way to remember
OIL RIG
Oxidation is loss
(oxidation number increases)
Reduction is gain
(oxidation number decreases)

13. Zn + 2HCl  ZnCl2 + H2 LEO OXIDATION Loss of e-
Increase in oxidation number
Reducing Agent
GER
REDUCTION
Gain of e-
Decrease or reduction in oxidation number
Oxidizing agent
Reaction of zinc with hydrochloric acid - YouTube
Zn + 2HCl  ZnCl2 + H2

14. Examining Redox Reactions
After oxidation numbers have been assigned, it can be determined that the reaction may or may not have been redox.
Is this reaction a redox reaction? Explain why or why not.
NaCl + AgNO3  AgCl + NaNO3

15. Redox Reactions Oxidation-Reduction Basics - YouTube
4 Fe(s) + 3 O2(g)  2 Fe2O3(s)
Assign oxidation numbers to all atoms.
Identify what is oxidized and what is reduced.
Identify the oxidizing and reducing agent.

16. Concept Check Cu + 2HNO3  Cu(NO3)2 + H2
Give the oxidation number and identify what is oxidized, reduced, and identify the oxidizing and reducing agents.
Cu + 2HNO3  Cu(NO3)2 + H2 16

17. Oxidation and Reduction half-reactions
Shows either the oxidation or reduction part of a redox reaction.
Shows the electrons gained or lost.
Follow the law of conservation of matter:
a) Mass of atom(s) on reactant side = mass of atom(s) on the product side.
b) Conservation of charge: net charge must be the same on both sides of the equation, but does not necessarily equal zero.

18. Oxidation half-reaction Reduction half-reaction
Half Reactions
Oxidation half-reaction
Shows an atom or ion losing 1 or more electrons while its oxidation number increases.
Fe(s)  Fe3+(aq) + 3e-
For oxidation, the e- is on the product side.
Reduction half-reaction
Shows an atom or ion gaining 1 or more electrons while its oxidation number decreases.
Fe3+(aq) + 3e-  Fe(s)
For reduction, the e- is on the reactant side.

19. Write the Half-reaction from an equation
Cu(s) + AgNO3(aq)  Cu(NO3)2(aq) + Ag(s)
Step 1: Assign an oxidation number to each element.
Step 2: Write a partial half-reaction to show the change in oxidation state. (no e- yet)
Step 3: Now show the number of e- needed to explain how the oxidation number changed.
Step 4: Keep conservation of mass and charge for ½ reactions.
Step 5: There must be a balance between the number of electrons lost and gained. Balance by multiplying through by the lowest common denominator.
Step 6: Once the e- lost and gained are equal, we can cancel the e- on both sides and add the 2 half reactions.

20. A half reaction does not occur by itself.
At least two such reactions must occur so that the electron released by one reactant is accepted by another in order to complete the reaction.
Thus, oxidation and reduction reactions must take place simultaneously in a system.
2Mg (s) + O2 (g) MgO (s)
2Mg Mg2+ + 4e-
Oxidation half-reaction (lose e-)
O2 + 4e O2-
Reduction half-reaction (gain e-)
19.1

21. Zn(s) + Cu2+(aq)  Zn2+(aq) + Cu(s)
Half reactions
Zn(s) + Cu2+(aq)  Zn2+(aq) + Cu(s)
Zn(s)  Zn2+(aq) + 2e- OXIDATION ½ reaction
Cu2+(aq) + 2e-  Cu REDUCTION ½ reaction

22. Concept Check
Balance the following reactions using the half-reaction method. Li(s) + CuSO4(aq)  LiSO4(aq) + Cu(s) Ag(s) + S(s)  Ag2S(s) AgS animation

23. The Galvanic or Voltaic Cell Zn/Cu
Cu/Ag

24. Cell Construction KCl in agar Provides conduction between half-cells
Salt bridge –
KCl in agar
Provides conduction
between half-cells
Observe the electrodes to see what is occurring. Cu Zn
1.0 M CuSO4
1.0 M ZnSO4

25. - + What about half-cell reactions?
What about the sign of the electrodes? - +
Why?
cathode half-cell
Cu e-  Cu
anode half-cell
Zn  Zn e- Cu
plates out or deposits on electrode
Zn electrode erodes
or dissolves
What happened at each electrode? Cu Zn
1.0 M CuSO4
1.0 M ZnSO4

26. Galvanic cell cathode half-cell (+) REDUCTION Cu+2 + 2e-  Cu
anode half-cell (-)
OXIDATION Zn  Zn e-
overall cell reaction
Zn + Cu+2  Zn Cu
Spontaneous reaction that produces electrical current!

27. Standard Conditions Temperature - 25oC All solutions – 1.00 M
Now for a standard cell composed of Cu/Cu+2 and Zn/Zn+2, what is the voltage produced by the reaction at 25oC?
Standard Conditions
Temperature - 25oC
All solutions – 1.00 M
All gases – 1.00 atm

28. - + Now replace the light bulb with a volt meter. 1.1 volts
cathode half-cell
Cu e-  Cu
anode half-cell
Zn  Zn e- Cu Zn
1.0 M CuSO4
1.0 M ZnSO4

29. We need a standard electrode to make measurements against!
The Standard Hydrogen Electrode (SHE)
H2 input
1.00 atm
25oC
1.00 M H+
1.00 atm H2 Pt
Half-cell
2H e-  H2
inert
metal
EoSHE = 0.0 volts
1.00 M H+

30. Electrode - an electrical conductor which carries charge to or from a liquid undergoing electrolysis.
Electrolysis
Electrolysis is the break-down of a substance by electricity
Electrolyte - a molten or aqueous solution through which an electrical current can flow.
Lead bromide

31. Electrolysis experiments
Electrolysis only happens in:
- molten ionic liquids or
- aqueous solutions containing ions.
There must be a complete circuit.
A lamp or ammeter shows that electricity is flowing around the circuit.
This slide shows the basic equipment needed for electrolysis. If you have an interactive white board or the image is projected onto a normal white board, a student could be selected to label the diagram. A heat arrow could be added to show a molten electrolysis (which reiterates that ionic compounds have high melting points). No heat arrow is needed for solution electrolysis (which reiterates that many ionic compounds are soluble in water).
It is important to reinforce that a circuit must be complete. This could be extended by a teacher drawing a wire between the 2 electrodes, which would make a short circuit, and posing the question as to whether electrolysis would happen. It would not, as the electricity would flow through the wire rather than the solution as the wire would have less resistance. It is vital that the electrodes are below the liquid surface, or again the circuit would not be complete.
Please note that the circuit symbol for a lamp may be different, see your specification.
Question to support this slide:
Question 8 on page 117.

32. Electrolysis of zinc chloride

33. At the electrodes Anode (+) Cathode (-) (positive electrode)
Negative ions go here (anions).
As non-metal ions are negative, they go to the anode.
Ions lose electrons. They are oxidised and become neutral atoms
(which react together to form molecules).
Cathode (-)
(negative electrode)
Positive ions go here (cations).
As metal ions are positive, they go to the cathode.
Ions gain electrons. They are reduced and become neutral atoms.
This is an animated slide: when you click the left-hand mouse button once, the cathode statement will appear; click the button again and the contrasting anode statement will appear. This animation can be removed to have all text displayed by going into Slide Show, Custom Animation, then by highlighting each piece of text in turn and clicking (left-hand mouse key) Remove.
This slide is colour-coded: red for anything which is positive (anode and cation); blue for anything which is negative (cathode and anion); and green for anything neutral.
Questions to support this slide:
Question 5 on page 117.
Question 6 on page 117.
Question 16 on page 138.
Question 18 on page 139.
Question 19 on page 139.
Question 20 on page 139.