Identify redox reactions by changes in oxidation number using: (a) the oxidation number of elements in their uncombined state is zero (b) the oxidation number of a monatomic ion is the same as the charge on the ion (c) the sum of the oxidation number

Chemical Reactions – Redox ⚡️

What is a Redox Reaction?

A redox (reduction‑oxidation) reaction is a chemical change where electrons move from one atom or ion to another.

Think of it as a friendly game of electron trading – one player (the oxidised species) loses an electron, while another (the reduced species) gains it.

Key Rules to Spot Redox Reactions

  • (a) The oxidation number of an element in its uncombined state is zero.

    Example: \$O_2\$ has \$O\$ with oxidation number \$0\$.

  • (b) The oxidation number of a monatomic ion equals its charge.

    Example: \$Na^+\$ → \$Na\$ has oxidation number \$+1\$.

  • (c) The sum of the oxidation numbers in a neutral compound is zero.

    Example: In \$H_2O\$, \$2(+1) + (-2) = 0\$.

  • (d) The sum of the oxidation numbers in an ion equals the ion’s charge.

    Example: In \$SO_4^{2-}\$, \$S(+6) + 4(-2) = -2\$.

How to Find Oxidation Numbers

  1. Assign \$0\$ to all elements in their elemental form.

  2. Give the charge of a monatomic ion to its oxidation number.

  3. Use the rules (c) and (d) to solve for unknowns.

  4. Check that the total sums to \$0\$ for compounds or to the ion charge for ions.

Examples of Redox Reactions

ReactionOxidation Numbers (Before)Oxidation Numbers (After)Oxidised / Reduced
\$Fe^{2+} + Cu^+ \rightarrow Fe^{3+} + Cu\$\$Fe^{2+}(+2), Cu^+(+1)\$\$Fe^{3+}(+3), Cu(0)\$\$Fe\$ oxidised, \$Cu\$ reduced
\$2Na + Cl_2 \rightarrow 2NaCl\$\$Na(0), Cl_2(0)\$\$Na^+(+1), Cl^-(−1)\$\$Na\$ oxidised, \$Cl\$ reduced

Exam Tips 📚

Tip 1: Always write the oxidation numbers of all atoms before and after the reaction.

Tip 2: Look for changes in oxidation numbers – that’s the key to redox.

Tip 3: Remember the four rules (a–d) as a quick checklist.

Tip 4: Use the analogy of “electron trading” to keep the concept clear.