Oxidation and Reduction: MDCAT Chemistry notes
Oxidation and Reduction for MDCAT: electron-transfer definitions, oxidation number rules with worked values, and anode and cathode in electrolytic cells.
What oxidation and reduction mean
A redox reaction is one in which electrons are transferred. Two processes always happen together:
- Oxidation is loss of electrons, and the oxidation number increases. Older definitions: addition of oxygen or removal of hydrogen.
- Reduction is gain of electrons, and the oxidation number decreases. Older definitions: removal of oxygen or addition of hydrogen.
Mnemonic: OIL RIG (Oxidation Is Loss, Reduction Is Gain). When magnesium forms $\mathrm{MgCl_2}$, $\mathrm{Mg \to Mg^{2+} + 2e^-}$ is oxidation. The substance that is oxidized is the reducing agent. The substance that is reduced is the oxidizing agent.
Rules for assigning oxidation numbers
- An element in the free (uncombined) state is 0, e.g. Mg, $\mathrm{O_2}$, $\mathrm{Cl_2}$.
- A monatomic ion has an oxidation number equal to its charge.
- Group I metals are always +1 and group II metals +2 in their compounds.
- Hydrogen is +1. In metal hydrides such as NaH it is −1.
- Oxygen is usually −2. Exceptions: peroxides ($\mathrm{Na_2O_2}$, $\mathrm{H_2O_2}$) −1, superoxides ($\mathrm{KO_2}$) $-\tfrac12$, and $\mathrm{OF_2}$ +2.
- Halogens are usually −1. Fluorine is always −1.
- The oxidation numbers in a neutral compound add up to zero. In an ion they add up to the charge on the ion.
The group number tells you the number of valence electrons, so it points to an element's usual (maximum) oxidation state.
Worked oxidation numbers
| Species | Working | Result |
|---|---|---|
| $\mathrm{KMnO_4}$ | $1 + x - 8 = 0$ | Mn = +7 |
| $\mathrm{K_2Cr_2O_7}$ | $2 + 2x - 14 = 0$ | Cr = +6 |
| $\mathrm{HClO_2}$ | $1 + x - 4 = 0$ | Cl = +3 |
| $\mathrm{S_2O_3^{2-}}$ | $2x - 6 = -2$ | S = +2 |
| $\mathrm{Na_2CO_3}$ | $2 + x - 6 = 0$ | C = +4 |
| $\mathrm{CrO_4^{2-}}$ | $x - 8 = -2$ | Cr = +6 |
| $\mathrm{CH_3OH}$ | $x + 4 - 2 = 0$ | C = −2 |
| $\mathrm{HCOOH}$ | $x + 2 - 4 = 0$ | C = +2 |
| $\mathrm{KO_2}$ | $1 + 2x = 0$ | O = $-\tfrac12$ (fractional) |
So $\mathrm{CH_3OH \to HCOOH}$ is an oxidation of carbon from −2 to +2, a loss of four electrons.
Electrolytic cells
An electrolytic cell uses a direct current (DC) source to drive a non-spontaneous reaction. AC is not used because it would keep swapping the electrodes.
- Anode: the positive electrode. Oxidation happens here and anions move towards it.
- Cathode: the negative electrode. Reduction happens here. Cations move towards it, gain electrons and are discharged.
- In the external wire, electrons flow from anode to cathode. Inside the electrolyte, the current is carried by positive and negative ions, not by free electrons.
Whatever the type of cell, oxidation is always at the anode and reduction at the cathode. Only the signs change: in a galvanic cell the anode is negative.
Electrorefining of copper: impure copper is made the anode and a thin sheet of pure copper the cathode, with $\mathrm{CuSO_4}$ solution as electrolyte. Copper dissolves from the anode and pure copper is deposited on the cathode.
Common MDCAT traps
- Oxygen is not always −2. $\mathrm{KO_2}$ gives $-\tfrac12$, $\mathrm{Na_2O_2}$ gives −1 and $\mathrm{OF_2}$ gives +2.
- Thiosulphate sulphur is +2 on average, not +4 or +6.
- In electrolysis the anode is positive. Do not carry over the galvanic-cell sign.
- Impure metal goes at the anode in refining, not the cathode.
- Current in an electrolyte is carried by both kinds of ion.
Quick revision
- Oxidation: loss of electrons, rise in oxidation number.
- Free element: oxidation number zero.
- Mn in $\mathrm{KMnO_4}$ is +7 and Cr in $\mathrm{K_2Cr_2O_7}$ is +6.
- Halogens commonly show −1.
- Electrolytic cell: DC supply, positive anode, reduction at the cathode.