Oxidation and Reduction Reactions (called “redox”) OBJECTIVES Define the terms oxidation and reduction.

Don’t Write This.. Early chemists saw “oxidation” reactions only as the combination of a material with oxygen to produce an oxide. But, not all oxidation processes that use oxygen involve burning. (Rust, H 2 O 2 ) A process called “reduction” is the opposite of oxidation, and originally meant the loss of oxygen from a compound.

Do Write This! Oxidation and reduction reactions always occur simultaneously – some element oxidizes while the other reduces. The definitions have become more broad over time. Oxidized is when a substance loses electrons. Reduced Is when the substance gains electrons. (Sounds backwards right?)

Oxidation and Reduction (Redox) Redox reactions mean that electrons are transferred between reactants Mg + S → Mg 2+ + S 2- The magnesium atom (which has zero charge) changes to a magnesium ion by losing 2 electrons, and is oxidized to Mg 2+. (The charge increased!) The sulfur atom (which has no charge) is changed to a sulfide ion by gaining 2 electrons, and is reduced to S 2- (The charge was reduced!) (MgS)

Each sodium atom loses one electron: Each chlorine atom gains one electron: Half Reactions: We’ll talk about this later OXIDIZIED! REDUCED!

How can you remember the terms? LEO the lion says GER : How can you remember the terms? LEO the lion says GER : Lose Electrons = Oxidation Gain Electrons = Reduction

Here’s where it get confusing… Here’s where it get confusing… - Magnesium is oxidized. But it causes the Sulfur to be reduced. Therefore Magnesium is the reducing agent. - Sulfur is being reduced. But it causes the Magnesium to be oxidized. Therefore Sulfur is the oxidizing agent. Mg (s) + S (s) → Mg 2+ (aq) + S 2- (aq) Mg is oxidized: loses e -, becomes a Mg 2+ ion S is reduced: gains e - = S 2- ion Mg is the reducing agent S is the oxidizing agent

Practice Problems Practice Problems Identify the substance being reduced and oxidized: Mg + Cl  Mg +1 (aq) Cl 2 -1 (aq) Magnesium = oxidized Chlorine = reduced Identify the reduction agent and oxidizing agent: Zn +2Ag 1+  Zn 2+ +2Ag Zinc = oxidized = Reducing agent Silver = reduced = Oxidizing agent

Oxidation Numbers OBJECTIVES Determine the oxidation number of each atom that make up a molecule or chemical compound.

Assigning Oxidation Numbers An “oxidation number” is a positive or negative number assigned to an atom to indicate its gain or loss of an electron. Most elements will be the same as they are on the periodic table. However, some elements will always have certain oxidation numbers. Some elements can have different oxidation numbers. (Transition Metals, C, N, S, P)

Rules for Assigning Oxidation Numbers 1)The oxidation number of any element by itself (not in a compound) is zero. 2)The oxidation number of a monatomic ion is the same as its charge.

3) The sum of the charges of all the atoms in the compound or molecule must equal 0. 2(+1) + (-2) = 0 H O (+2) + 2(-2) + 2(+1) = 0 Ca O H

4) Oxygen’s oxidation number is usually -2. EXCEPT in a peroxide molecules where it is -1. 5) Hydrogen is usually a +1 charge. EXCEPT in metal hydrides where it is -1. NORMALEXCEPTION EXCEPTION

6)The sum of the charges in a polyatomic ion is equal to its charge. X + 3(-2) = -1 N O thus N = +5 thus S = +6 X + 4(-2) = -2 S O

7) Not on your sheet! Halogens are normally -1, except when they are next to an oxygen in a polyatomic ion! Thus Cl = +3 (+1) + X + (-4) = 0 H Cl O

oxidation #s SO oxidation #s PO oxidation #s Na 2 C 4 O

oxidation #s Zn(OH) oxidation #s Na 2 O ? oxidation #s CaH 2 +1 EXCEPTION! +1 +2? EXCEPTION! +2

Not All Reactions are Redox Reactions - Reactions in which there has been no change in oxidation number are NOT redox reactions. Examples:

ASSIGN AN OXIDATION NUMBER / STATE TO EACH ATOM IN : Cl 2 CO 3 2- Ca 2+ SO 3 2- Al 3+ ClO - H2OH2O IO 4 - CO 2 CH 4 ClF MnO 4 - NO 3 - Na 2 S 4 O 6 CuCl CuBr 2 N2N2 C 2 O 4 2- BrF 5 Mn 2 O 3 SF 6 CO S 2- BrF VCl 2 Na 2 S NO 2 - BrO 3 - NH 4 + H 2 SO 4 SO 4 2- I-I- S 2 O 3 2- NH 3 CCl 4 Cr 2 O 7 2- Cl(0) Ca(+2) Al(+3) H(+1)  O(-2) O(-2)  C(+4) F(-1)  Cl(+1) O(-2)  N(+5) Cl(-1)  Cu(+1) N(0) F(-1)  Br(+5) F(-1)  S(+6) S(-2) Cl(-1)  V(+2) O(-2)  N(+3) H(+1)  N(-3) O(-2)  S(+6) O(-2)  S(+2) Cl(-1)  C(+4) O(-2)  C(+4) O(-2)  S(+4) O(-2)  Cl(+1) O(-2)  I(+7) H(+1)  C(-4) O(-2)  Mn(+7) Na(+1) & O(-2)  S(+2.5) Br(-1)  Cu(+2) O(-2)  C(+3) O(-2)  Mn(+3) O(-2)  C(+2) F(-1)  Br(+1) Na(+1)  S(-2) O(-2)  Br(+5) O(-2) & H(+1)  S(+6) I(-1) H(+1)  N(-3) O(-2)  Cr(+6)