1. Electrochemistry: Electric Cells
SCH 4U

2. Recall: Grade 9 Science Electrons!
Which subatomic particles are transferred in a chemical reaction?
Electrons!
Ex. 2 Na(s) + Cl2(g) → 2 NaCl(s)

3. Electron Transfer Theory
Zn(s) + 2 HCl(aq) → ZnCl2(aq) + H2(g) Zn(s) → Zn2+(aq) 2H+(aq) → H2(g)
Zn2+(aq) Cl-(aq)
+ 2e-
+ 2e-
*These are called half-reactions!

4. Zn → Zn2+ + 2e- 2H+ + 2e- → H2 Zn + 2H+ → Zn2+ + H2
What happened to the chlorine?
Zn → Zn e-
2H e- → H2
Zn H+ → Zn H2
Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)
*Notice half-reactions
are balanced by both mass and charge

5. Terminology: i.e. 2H+ + 2e- → H2 i.e. Zn → Zn2+ + 2e-
Reduction = process in which electrons are GAINED
i.e. 2H e- → H2
Oxidation = process in which electrons are LOST
i.e. Zn → Zn e-

6. LEO the lion says GER! Hmm, how will I remember this?
Lose e- = Oxidation
LEO the lion says GER!
Gain e- = Reduction

7. Ag+(aq) + e- → Ag0(s) Cu0(s) → Cu2+(aq) + 2e- 2 2 GER = Reduction
LEO = Oxidation
Cu(s)
2Ag+(aq) + Cu(s) → 2Ag(s) + Cu2+(aq)
*Total number of e- gained must equal the total number of e- lost
Ag(s)

8. Often called “redox” reactions
The reduction and oxidation reactions occur simultaneously ...
Often called “redox” reactions
Chemical reactions involving a transfer of electrons
Q: Are all reactions redox reactions? NO
Example of a non-redox rxn:
Pb(NO3)2(aq) + Na2SO4(aq) ---> 2NaNO3(aq) + PbS(s) Double displacement, oxidation states NOT changing

9. Ex. 1 Zn(s) → Zn2+(aq) + 2e- Pb2+(aq) + 2e- → Pb(s)
Write and label two balanced half-reaction equations for the rxn of zinc metal with aqueous lead (II) nitrate.
Zn(s) + Pb(NO3)2(aq) → Pb(s) + Zn(NO3)2(aq)
Zn(s) → Zn2+(aq) + 2e-
Pb2+(aq) + 2e- → Pb(s)
LEO = Oxidation
GER = Reduction

10. Oxidation States Oxidation Number
Useful (necessary!) when redox reactions involve molecules and polyatomic ions. → a method of “electron bookkeeping”
Oxidation Number
a positive or negative # corresponding to the apparent charge an atom in a molecule/ion would have if e- pairs in covalent bonds belonged entirely to the more electronegative atom
So far just looked at atoms and monatomic ions
To explain/describe oxidation and reduction of molecules and polyatomic ions

11. Example: Water -2
Oxygen atom: EN = 3.1
Hydrogen atom EN = 2.1 +1 +1

12. Note:. To avoid confusion with charges, oxidation
Note: To avoid confusion with charges, oxidation numbers are written with the sign BEFORE the number.
i.e oxidation number vs charge

13. Need to know: Atom or Ion Oxidation Number Examples
All atoms in elements
Na, Cl2
Hydrogen in all compounds +1
H in HCl
*Except H in hydrides -1
H in LiH
Oxygen in all compounds -2
O in H2O
*Except O in peroxides
O in H2O2
All monatomic ions
charge on ion
Na+ is +1; S2- is -2

14. Examples
*Remember: sum of oxidation numbers must equal the total charge
What is the oxidation number of carbon in methane, CH4?
CH4
What is the oxidation number of manganese in a permanganate ion, MnO4-?
MnO4- -4 +1
x + 4(+1) = 0
x = -4
*Remember: sum of oxidation numbers must equal the total charge
Because just a systematic way of counting electrons
x + 4(-2) = -1
x = +7 +7 -2

15. Oxidation Numbers in Redox Rxns
If the ox. number of an atom/ion changes in a chemical rxn, it is a redox reaction! → electron(s) transferred!
An oxidation (LEO) = increase in ox. number
A reduction (GER) = decrease in ox. number

16. Example: +6 -2 +1 -2 -2 +1 2x + 6(+1) + (-2) = 0 x = -2 +3 +1 -2 -2 +1
Identify the oxidation and reduction in the following reaction: 2Cr2O72-(aq) + 16H+(aq) + 3C2H5OH(aq) → 4Cr3+(aq) + 11H2O(g) + 3CH3COOH(aq) Which species’ ox. numbers have changed? +6 -2 +1 -2 -2 +1
2x + 6(+1) + (-2) = 0
x = -2 +3 +1 -2 -2 +1
2x + 7(-2) = -2
x = +6
Det. blood alcohol content from breathe or blood:
involves reaction of sample with dichromate ion in an acidic environment
If ethanol is present,
produces chromium (III) ions, water, and acetic acid
x + 4(+1) + 2(-2) = 0
x = 0
Chromium atoms in Cr2O72- are reduced (GER) while carbon atoms in C2H5OH are oxidized (LEO)

17. Homework
p. 656 # 7-11 p.659 # p. 662 # 18 & 19 p. 663 # 7