1. Electrochemistry

2. both atoms and the electrons must be balanced in redox reactions
gains 1e-
x 2 atoms = 2e-
oxidation numbers: +1 -1
Cu Cl2 → Cu Cl- 2 2 2
loses 1e-
x 2 = 2e-
done by assigning oxidation numbers to all elements

3. Mg (s) + Cl2 (g) → MgCl2 (s) Mg → Mg2+ + 2e- Cl + 1e- → Cl1-
**There is a written difference between ion charge and oxidation number**
Mg (s) + Cl2 (g) → MgCl2 (s)
Mg → Mg2+ + 2e-
Cl + 1e- → Cl1-
Magnesium ion
ion charge: 2+

oxidation #: +2
Chlorine ion
ion charge: 1-

oxidation #: -1
Sometimes these numbers are the same (like above) sometimes they are very different – which is why we write them differently

4. O#s are based on the electronegativity of the atoms
Oxidation number represents the charge the atom would have if every bond were ionic.
O#s are based on the electronegativity of the atoms
1. Assign known numbers first, calculate others.
All uncombined elements (& diatomics) – zero Cu O2
Monatomic ion - equals ion charge
Fe3+ +3
Cl- -1

5. Alkali metals (IA) – always +1 Earth metals (IIA) – always +2
Halogens (VIA) – usually -1
Hydrogen – usually +1
Oxygen – usually -2
Except metal hydrides (NaH)
Except peroxides (H2O2)
2. Neutral compound:
Sum of O#s must equal zero
3. Charged compound:
Sum of O#s must equal charge of compound

6. Multiple the O#s by the subscripts for each atom to get the sum
2. Neutral compound:
Sum of O# must equal zero
Assign O#s for each atom in K2Cr2O7
Step 1: Start with atoms which are known.
O: –2
 K: +1
Step 2: Solve for other atoms. +1 +6 -2
Multiple the O#s by the subscripts for each atom to get the sum
K2Cr O7 2 +2
+12 ??
-14
= 0
K = Cr = +6 O = –2.

7. NO3- Assign O#s for each atom in NO3- O: –2 N = +5 O = –2. +5 -2 +5 ??
3. Charged compound:
Sum of O# must equal charge of compound
Assign O#s for each atom in NO3-
Step 1: Start with atoms which are known.
O: –2

Step 2: Solve for other atoms. +5 -2
NO3- +5 ?? -6
= -1
N = O = –2.

8. Use ox.numbers to determine if reaction is a redox reaction.+4 -2 +1 -2 +1 +4 -2
SO H2O → H2SO3 +4 -4 +2 -2 +2 +4 -6
Remember: Redox involves the transfer of electrons – no change in O#s during the reaction means NO redox
NOT a redox reaction

9. Cu(s) + 2 AgNO3(aq) → CuNO3(aq) + 2 Ag(s)
Is the following reaction a redox reaction? +1 +5 -2 +1 +5 -2
Cu(s) + 2 AgNO3(aq) → CuNO3(aq) + 2 Ag(s) +1 +5 -6 +1 +5 -6
Oxidation cannot occur without reduction
Redox reaction
• Cu – oxidized (loss of electrons)

• Ag – reduced (gain of electrons)

10. Cu(s) + 2 AgNO3(aq) → CuNO3(aq) + 2 Ag(s)
Oxidizing agent - causes the oxidation of another substance.
AgNO3 is the oxidizing agent
Reducing agent - causes the reduction of another substance.
Cu is the reducing agent
Think opposites: if the compound is the “oxidizing agent” of the reaction, then it is reduced in the process and vice versa +1
Cu(s) + 2 AgNO3(aq) → CuNO3(aq) + 2 Ag(s) +1

11. 2 HNO3(aq) + 3 H2S(g) → 2 NO(g) + 3 S(s) + 4 H2O(l)
Identify the substance oxidized, the substance reduced, the oxidizing agent and the reducing agent. +1 +5 -2 +1 -2 +2 -2 +1 -2
2 HNO3(aq) + 3 H2S(g) → 2 NO(g) + 3 S(s) + 4 H2O(l) +1 +5 -6 +2 -2 +2 -2 +2 -2
S – oxidized
N – reduced
H2S – reducing agent
HNO3 – oxidizing agent
Substance - means element
Agent – means compound

12. How many electrons are transferred in the reaction below:
gains 3e- +1 +5 -2 +1 -2 +2 -2 +1 -2
HNO3(aq) + H2S(g) → 2 NO(g) + 3 S(s) + 4 H2O(l) 2 3
loses 2e-
Stoichiometry (coefficients and subscripts) used to determined
total electrons transferred
S: (3 atoms) x (2e- lost) = 6 electrons lost
N: (2 atoms) x (3e- gained) = 6 electrons gained

13. CAN YOU / HAVE YOU?
Determine the oxidation numbers for atoms in compounds and ions
Identify reactions as redox or non-redox
Include: oxidizing agent, reducing agent
Additional KEY Terms