9701 · 6.1
Redox processes: electron transfer and changes in oxidation number (oxidation state)
Redox reactions are all about the transfer of electrons from one chemical species to another. We use a system called oxidation numbers to keep track of this electron movement.
Need to know
What you need to know
- **Elements:** The oxidation number of an atom in an uncombined element is 0 (e.g., $\text{Fe}$, $\text{Cl}_2$, $\text{S}_8$).
- **Simple Ions:** The oxidation number of an atom in a monatomic ion is equal to the charge on the ion (e.g., $\text{Na}^+$ is +1, $\text{O}^{2-}$ is -2).
- **Compounds & Ions:** The sum of oxidation numbers in a neutral compound is 0. The sum in a polyatomic ion equals the ion's charge.
- **Specific Elements (Hierarchy):**
- 1. Group 1 metals are +1, Group 2 metals are +2 in their compounds.
- 2. Fluorine is always -1 in its compounds.
- 3. Hydrogen is usually +1 (except in metal hydrides like $\text{NaH}$, where it is -1).
- 4. Oxygen is usually -2 (except in peroxides like $\text{H}_2\text{O}_2$ where it's -1, and with fluorine like $\text{OF}_2$ where it's +2).
Explanation
The Electron Exchange
- Oxidation is electron loss; reduction is electron gain (OIL RIG).
- Oxidation number rules: element = 0; ion = charge; O usually −2, H usually +1.
- Redox reactions involve simultaneous oxidation and reduction.
- Half-equations balance atoms and charge; combine for full ionic equation.