What is the balanced chemical equation for rust formation on iron?

• A. Fe + O2 -> FeO
• B. 2Fe + O2 -> 2FeO
• C. 4Fe + 3O2 -> 2Fe2O3
• D. Fe + 3O2 -> Fe2O3

Answer: (C)

Rust forms when iron reacts with oxygen to make iron(III) oxide, especially in the presence of water. To balance the equation, you match the number of iron and oxygen atoms on both sides and use the smallest whole-number coefficients. The product must be iron(III) oxide, Fe2O3. Using 4 iron atoms and 3 O2 molecules, you get 2 Fe2O3 as the product: 4Fe + 3O2 -> 2Fe2O3. This balances iron (4 on the left, 4 in Fe2O3) and oxygen (3 O2 gives 6 O on the left; 2 Fe2O3 has 6 O on the right). The oxidation state also makes sense: iron goes from 0 to +3, and oxygen from 0 in O2 to -2 in oxide. Other options either form iron(II) oxide (FeO), which isn’t rust, or are unbalanced in atoms. Real rust is actually hydrated, often written as Fe2O3·nH2O, but the stoichiometric ratio to illustrate rust is 4Fe + 3O2 -> 2Fe2O3.