CHEMISTRY III 4TH QUARTER Topic: Oxidation-Reduction Reaction RedOx

How does electron transfer occur?

Applicable to IONIC COMPOUNDS

Oxidation-Reduction Reaction (RedOx)
examples: photosynthesis, rusting of iron, bleaching, burning

- transfer of electrons reaction
- produces electricity [ELECTROCHEMISTRY] (electricity is produced because of the electron transfer)

If you dip the metal cylinder connecting the bulb of the light bulb to a mixture of water(H2O) and salt (NaCl; the light bulb produce light.

RedOx Reaction -> a loss and gain of electrons

Reduction -> removal of O2 (Oxygen gas)
example: 2 (Fe2 O3) -> O2 + 2 Fe [2:1:2]

Oxidation -> reaction with O2 (oxygen gas)
example: 2 (H2) + O2 -> 2(H2O) [2:1:2]

Reduction & Oxidation [RedOx] are complimentary processes.

Terminology:
1)Oxidation ~ loss of electrons ~ the charge becomes more positive
2)Reduction ~ gain of electrons ~ the charge becomes more negative
3)Oxidizing Agent ~ electron acceptor ~ the substance that is reduced
4)Reducing Agent ~ electron giver ~ the substance that is oxidized

LEO (Losing Electrons is Oxidation) says GER (Gaining Electrons is Reduction)

Oxidation Numbers/Oxidation State:
METAL (+)
HALOGENS (-)

Au Cl
using the (RCC) Reverse Criss Cross Method (which is only applicable to IONIC Compounds)

1. Write Subscripts
Au (1+) Cl (1-)

After undergoing RCC
the Oxidation State of Au (GOLD)= +1
written as : O.S. Au = + 1

the Oxidation State of Cl (CHLORINE)= -1
written as : O.S. Cl = - 1

(Au is the symbol for GOLD which is a Metal)
(Cl is the symbol for CHLORINE which is a Halogen)

Anther Method which works for ALL COMPOUNDS is the ALGEBRA METHOD

Rules in Oxidation Numbers/Oxidation State:
1. The Oxidation State (O.S.) for a monoatomic ion is equal to the ionic charge.

2. The O.S. of Hydrogen or a compound is always +1, except in metal hydrides such as NaH ; where it is -1.

The formation of binary compounds by hydrogen with other elements, are called hydrides. Hydrogen combines with most of the metals and non-metals (except noble gases).

3. The O.S. of an atom in uncombined form is ZERO (0).

4. The O.S. of oxygen in a compound is -2 except in peroxides such as hydrogen peroxide H2O2 where it is -1.

A peroxide is a compound containing an oxygen-oxygen single bond. The simplest stable peroxide is hydrogen peroxide

5. For any neutral compound, the sum of the O.S. of all the atoms must be equal to ZERO (0). (without charge)

6. For a polyatomic ion, the sum of the O.S. must be equal to the charge of the ion.

7. In compounds, the O.S. of many elements corresponds to the element's position in the Periodic Table.

a. GROUP 1A elements, O.S. = +1
b. GROUP 2A elements, O.S. = +2
c. ALUMINUM, 0.S. = +3
d. HALOGENS, O.S. = -1

RECALL:
1. NaF
O.S. Na = +1
O.S. F = -1

2. N2
O.S. N = 0

3. HI
O.S. H = +1
O.S. I = -1

4. NH4 Cl (Ammonium Chloride)
O.S. Cl = -1
O.S. H = +1
O.S. N = ?

O.S. N + [+1*4] + [-1*1] = 0
O.S. N + 4 - 1 = 0
O.S. N + 3 = 0

O.S. N = -3

5. H2 PO4-
There is a charge in this example, so instead of equating the sum to ZERO (0); equate it to the charge which -1 in this compound.

O.S. H = +1
O.S. O = -2
O.S. P = ?

O.S. P + (+1*2) + (-2*4) = -1
O.S. P + 2 - 8 = -1
O.S. P - 6 = -1

O.S. P = 5

Sample Problem 1

What is oxidized and what is reduced in this reaction? Identify the reducing agent and oxidizing agent.

2Na (s) + Cl2 (g) ==> 2NaCl

Na (SODIUM) belongs to GROUP 1A so,
O.S. Na = +1

Cl (Chlorine) belongs to Halogens so,
O.S. Cl = -1

Oxidation means losing electrons; basing the solution on LEDS (Lewis Electron Dot Structure):


We see here that the single sodium electron transfers to the seven (7) electrons of chlorine (satisfying the octet rule).
Na (SODIUM) loses a valence electron. Cl (CHLORINE) gains a valence electron.

The number of valence electrons is the number of electrons in the outermost shell of the atom's structure.

The oxidized element is Sodium (Na) and the reduced element is Chlorine (Cl).
The oxidizing agent is Chlorine (Cl) while the reducing agent is Sodium (Na).