2.1.5 Redox
Definitions
| Term | Definition |
|---|---|
| Redox reactions | A reaction involving reduction and oxidation. |
| Oxidising agent | A reagent that accepts / takes in electrons / oxidises another species / is reduced. |
| Reducing agent | A reagent that donates / gives out electrons / reduces another species / is oxidised. |
| Oxidation number | A measure of the number of electrons that an atom uses to bond with atoms of another element. Oxidation numbers are derived from a set of rules. |
Oxidation number
Oxidation number (oxidation state) rules
- Elements
- Always 0
- Any bonding is to atoms of the same element in pure elements
- Compound and ions
- Each atom in a compound has an oxidation number
- Sign is placed before the number
- Sum of oxidation numbers in a compound / ion = total charge
Fixed oxidation numbers
| Combined element | Oxidation number |
|---|---|
| O (normally) | -2 |
| H (normally) | +1 |
| F | -1 |
| Group 1 | +1 |
| Group 2 | +2 |
| Group 3 | +3 |
Oxidation number for special cases
| Combined element | Oxidation number |
|---|---|
| H in metal hydrides (e.g. \(NaH\), \(CaH_2\)) | -1 |
| O in peroxide ions (\(O_2^{2-}\)) (e.g. \(H_2O_2\)) | -1 |
| O bonded to F (e.g. \(F_2O\)) | +2 |
Roman numerals in chemical names
- Show oxidation number without sign
- Nitrate = assume to be \(NO_3^-\)
- Sulfate = assume to be \(SO_4^{2-}\)
- e.g. chlorate(I) = \(ClO^-\)
Redox reactions
How to identify redox reactions
- Identifying oxidation
- Gain of oxygen
- Loss of hydrogen
- Loss of electrons
- Increase in oxidation number
- Identifying reduction
- Loss of oxygen
- Gain of hydrogen
- Gain of electrons
- Decrease in oxidation number
- Oxidation and reduction always happen together
Redox reaction of acids
- Metal + acid \(\rightarrow\) salt + hydrogen
- Metal oxidised (oxidation number increases from 0 to …)
- Hydrogen in acid reduced (oxidation number decreases from +1 to 0)
- (Iron is normally \(Fe^{2+}\) in redox reactions)