Transcript Redox Reaction and Oxidation Numbers

Pgs 636 - 645
OXIDATION-REDUCTION
REACTIONS
Oxidation-Reduction
Reactions
 Reactions that occur when electrons are
transferred between atoms
 Also called Redox Reactions
 Oxidation = LOSS of electrons
 Reduction = GAIN of electrons
 LEO the lion goes GER
Oxidation – Reduction
Reactions
 Reaction of sodium metal with chlorine gas:
2Na (s) + Cl2 (g)  2NaCl (s)
 What’s really happening in this reaction?
Na
Cl
Na+ClNa
Cl
Na+ClEach sodium gives-up an electron to the chlorine resulting in ions!
Practice!!
 In each of the following reactions, identify
what is being oxidized, and what is being
reduced?
2Mg (s) + O2 (g)  2MgO (s)
2Al (s) + 3I2 (s)  2AlI3 (s)
Can reactions between
nonmetals be redox reactions?
 Yes, but it’s harder to see
CH4 (g) + 2O2 (g)  CO2 (g) + H2O (g) + energy
 Use oxidation states to determine where the
electrons are going!
Oxidation States
 Also called oxidation numbers
 Positive and negative numbers assigned to an
INDIVIDUAL atom to help of keep track of
electrons during redox reactions
Rules for Assigning
Oxidation Numbers
1. Oxidation number of a free element = zero
Examples  HONClBrIF, solid metals (Na, Ag)
2. Oxidation number of a monatomic ion is the
same as it’s charge
Examples  NaCl
CaCl2
3. Oxygen’s oxidation number in compounds is
-2, EXCEPT for peroxides where it is -1
Examples  O2
H2O
H2O2
Rules for Assigning
Oxidation Numbers
4. Hydrogen’s oxidation number = +1 in
covalent compounds
Examples  H2O
HI
NH3
5. For a neutral compound, the sum of the
oxidation states must be ZERO
Examples  CuCl2
Fe2O3
6. For polyatomic ions, the sum of the
oxidation numbers must equal the charge of
the ion
Examples  SO42-
Practice!!
 Assign oxidation numbers to each of the
following atoms:
SO3
N2O5
SO32-
PF3
C2H6
What do we use oxidation
numbers for?
 To determine what is being oxidized and
what is being reduced in a redox reaction
 Example:
 Identify what is oxidized and reduced in the
following reaction:
HINT  Oxidation = INCREASE in oxidation #
Reduction = DECREASE in oxidation #
CH4 (g) + 2O2 (g)  CO2 (g) + H2O (g)
Practice!!
 Determine what is oxidized and reduced in
the following reactions:
Cu (s) + 2AgNO3 (aq)  Cu(NO3)2 + 2Ag
Br2 (g) + 2NaF (aq)  2NaBr (aq) + F2 (g)
CH3OH (g) + O2 (g)  CO2 (g) + H2O (g)
Two more terms to know…
 Reducing Agent electron donor
 What is oxidized in the reaction
 Oxidizing Agent  electron acceptor
 What is reduced in the reaction
 Identify the reducing agent and oxidizing
agent in the previous 3 reactions:
Cu (s) + 2AgNO3 (aq)  Cu(NO3)2 + 2Ag
Br2 (g) + 2NaF (aq)  2NaBr (aq) + F2 (g)
CH3OH (g) + O2 (g)  CO2 (g) + H2O (g)