# How to Balance Redox Reactions by Half Reaction Method - Tutorial

## How to Balance Redox Reactions by Half Reaction Method - Tutorial with Definition, Equations, Example

#### Definition

Redox Reaction is a chemical reaction in which oxidation and reduction occurs simultaneously and the substance which gains electrons is termed as oxidizing agent.
Refer the following table which gives you oxidation numbers.

#### Positive Ions

Valency 1 Valency 2 Valency 3
Lithium Li+ Magnesium Mg2+ Aluminium Al3+
Sodium Na+ Calcium Ca2+ Iron III Fe3+
Potassium K+ Strontium Sr2+ Chromium Cr3+
Silver Ag+ Barium Ba2+
Hydronium H3O+ Copper II Cu2+
Ammonium NH4+ Zinc Zn2+
Copper I Cu+ Manganese II Mn2+
Mercury I Hg+ Iron II Fe2+
Tin II Sn2+

#### Negative Ions

Valency 1 Valency 2 Valency 3
Fluoride F- Oxide O2- Phosphate PO43-
Chloride Cl- Sulphide S2-
Bromide Br- Carbonate CO32-
Iodide I- Sulphate SO42-
Hydroxide OH- Sulphite SO32-
Nitrate NO3- Dichromate Cr2O72-
Bicarbonate HCO3- Chromate CrO42-
Bisulphate HSO3- Oxalate C2O42-
Nitrite NO2- Thiosulphate S2O32-
Chlorate ClO3- Tetrathionate S4O62-
Permanganate MnO4- Monohydrogen Phosphate HPO42-
Hypochlorite OCl-
Dihydrogen Phosphate H2PO4-

#### Example: 1

##### Balance the given redox reaction:

H2+ + O22- -> H2O

Considering the equation above, we have 2 hydrogen (H) with the total charge +1[Refer the charges of the elements in the above table] and 2 oxygen (O) with the total charge -2 on the L.H.S and 2 hydrogen (H) with total charge +2 and only 1 oxygen (O) with the total charge -2 on the R.H.S. Hence with the unequal number of oxygen molecule charges, the chemical equation is said to be unbalanced.

##### How to balance an unbalanced chemical equation?

Let us learn here how to balance the above unbalanced equation using half reaction method with step by step procedure.

#### Given,

H+12 + O-22 -> H+12 O-2

To balance the unbalanced oxygen molecule charges, we add 2 in front of the product on R.H.S.
We get,

H+12 + O-22 -> (2) H+12 O-2

Now, we have 2 Oxygen's with the total charge -2 but 4 Hydrogen's with the total charge +4 on the R.H.S.
In order to balance the same, we add an another 2 in front of Hydrogen on the L.H.S

(2) H+12 + O-22 -> (2) H+12 O-2

Now, the equation is balanced with 4 Hydrogen's (H) with total charge +4 and 4 Oxygen's with total charge -4 on both sides.

#### Example: 2

Cr+3 + Cl-1 = Cr+3 + Cl-12

Considering the equation above, we have 1 chromium (Cr) with the total charge +3[Refer the charges of the elements in the above table] and 1 chloride (Cl) with the total charge -1 on the L.H.S and 1 chromium (Cr) with total charge +3 and only 2 chloride(Cl) with the total charge -1 on the R.H.S. Hence with the unequal number of oxygen molecule charges the chemical equation is said to be unbalanced.

##### How to balance an unbalanced chemical equation?

Let us learn here how to balance the above unbalanced equation with step by step procedure.

#### Given:

Cr+3 + Cl-1 = Cr+3 + Cl-12

To balance the unbalanced chloride molecule charges, we add 2 in front of the chloride on L.H.S.
We get,

Cr+3 + (2)Cl-1 = Cr+3 + Cl-12

Now, the equation is balanced with 2 Chloride's (Cl) with total charge -2 and 3 Chromium's with total charge +3 on both sides.
This is how the redox equations are balanced.