To calculate the number of electrons transferred in a redox reaction, follow these steps:

Step 1: Identify the Oxidation States

Determine the oxidation states of elements in the reactants and products.

Step 2: Find the Change in Oxidation States

For each atom, check how many electrons are lost (oxidation) or gained (reduction).

Step 3: Balance the Electron Transfer

Ensure that the total electrons lost = total electrons gained by multiplying coefficients as needed.

Example 1: Magnesium and Oxygen Reaction

2Mg+O2→2MgO2Mg + O_2 \rightarrow 2MgO

1. Oxidation States:
   • Mg (reactant) = 0
   • Mg (product, in MgO) = +2
   • O₂ (reactant) = 0
   • O (in MgO) = -2
2. Electrons Transferred:
   • Each Mg atom loses 2 electrons: Mg0→Mg2++2e−Mg^0 \rightarrow Mg^{2+} + 2e^-
   • Each O atom gains 2 electrons: O2+4e−→2O2−O_2 + 4e^- \rightarrow 2O^{2-}
3. Total Electrons Transferred:
   • 2 Mg atoms lose 4 electrons (since each loses 2).
   • O₂ molecule gains 4 electrons.
   • Total electrons transferred = 4.

Example 2: Iron and Copper(II) Sulfate Reaction

Fe+CuSO4→FeSO4+CuFe + CuSO_4 \rightarrow FeSO_4 + Cu

1. Oxidation States:
   • Fe (reactant) = 0
   • Fe (product, in FeSO₄) = +2
   • Cu (reactant, in CuSO₄) = +2
   • Cu (product) = 0
2. Electrons Transferred:
   • Iron loses 2 electrons: Fe→Fe2++2e−Fe \rightarrow Fe^{2+} + 2e^-
   • Copper gains 2 electrons: Cu2++2e−→CuCu^{2+} + 2e^- \rightarrow Cu
3. Total Electrons Transferred:
   • 2 electrons (Fe to Cu²⁺).

General Formula for Electron Transfer

For a reaction:

A+Bn→Am+BA + B^n \rightarrow A^m + B

The number of electrons transferred is:

∣n−m∣|n – m|

Where:

• n = Oxidation state of B in reactant.
• m = Oxidation state of A in product.

Key Takeaways

• Identify oxidation states before and after reaction.
• Count the change in electrons for oxidized and reduced species.
• Balance the reaction to ensure electrons lost = electrons gained.