UNIT 6: ELECTROCHEMISTRY

REDOX REACTIONS Redox is short for ‘oxidation and reduction’ Oxidation refers to substances that combine with oxygen Iron rusting, campfire burning Reduction refers to converting metal ore to pure metal Oxidation-reduction reactions are reactions that exchange electrons (one loses and one gains them)

DEMO Copper wire in silver nitrate Cu(s) + 2AgNO 3 (aq) Cu(NO 3 ) 2 (aq) + 2 Ag(s) Ionic Equation Cu (s) + 2Ag + (aq) + 2NO 3 - (aq) → Cu 2+ (aq) + 2NO 3 - (aq) + 2Ag (s) Overall Net Ionic Equation Cu (s) + 2Ag + (aq) → Cu 2+ (aq) + 2Ag (s) What changed? Cu (s) → Cu 2+ (aq) + 2e - - Copper lost electrons and was OXIDIZED Ag + (aq) + e - → Ag (s )- Silver gained electron and was REDUCED

Oxidation the loss of electrons Reduction the gaining of electrons One can’t happen without the other

LEO GER LEO Loss of Electrons is Oxidation the Lion Says GER GER Gain of Electrons is Reduction

OXIDATION NUMBERS Electrons are transferred from one substance to another We use oxidation numbers aka electronegativity to track this Electronegativity The ability of a bonded atom to attract shared electrons towards itself Oxidation Numbers A positive or negative number assigned to an atom in a molecule or ion that reflects a partial gain or loss of electrons

NOTES 1.The oxidation number of a pure element (by itself, and not an ion) is zero Element Oxidation Number Na0 H2H2 0 O2O2 0 P4 P4 0

Ionic Compound IonsCharge Oxidation Number NaCl Na + +1 Cl - Mg 3 N 2 Mg N Oxidation number of anion is equal to its charge

3. oxidation number is hydrogen is always +1 Compoundelement Oxidation Number HCl H+1 Cl H2SH2S H+1 S-2

4. OXIDATION NUMBER IS OXYGEN IS ALMOST ALWAYS -2 The exceptions: peroxides, such as hydrogen peroxide. In peroxides oxygen has an oxidation number of -1. when oxygen is combined with fluorine it's oxidation number is +2 Compoundelement Oxidation Number MgO magnesium oxide Mg+2 O-2 Na 2 O sodium oxide Na+1 O-2 Na 2 O 2 sodium peroxide Na+1 O

5. sum of oxidation numbers in a compound is zero Compoundelement Oxidation Number Number atoms Total Mg 3 N 2 Mg+23+6 N-32-6 SUM 0 Mn 2 O 7 Mn O SUM0 Cl 2 O 3 Cl+32+6 O-23-6 SUM0

6. oxidation number of polyatomic ions are equal to it’s charge Compoundelement Oxidation Number Number atoms Total NO 3 - N+51 O-23-6 SUM Cr 2 O 7 2- Cr O SUM-2 SO 4 2- S+61 O-24-8 SUM-2

It is important to note that oxidation number always refers to each individual atom in the compound, not to the total for that element. For example in H 2 O - the total positive "charge" for both hydrogen atoms will be +2 (which balances with the -2 from oxygen), but EACH HYDROGEN has an oxidation number of +1.

PRACTICE PROBLEMS practice Being able to determine oxidation numbers for elements in a compound is critical before you move on to the next step