Dioxygen

Dioxygen ($O_2$) is the elemental, molecular form in which oxygen is normally encountered, and it is central both to laboratory chemistry and to industrial gas production.

Preparation

Several laboratory routes exist:

1. Heating oxygen-containing salts such as chlorates, nitrates and permanganates:

   $2KClO_3 \xrightarrow[MnO_2]{\text{Heat}} 2KCl + 3O_2$

2. Thermal decomposition of oxides of metals that lie low in the electrochemical series, and of certain higher oxides of some metals:

   $2Ag_2O(s) \rightarrow 4Ag(s) + O_2(g)$

   $2Pb_3O_4(s) \rightarrow 6PbO(s) + O_2(g)$

   $2HgO(s) \rightarrow 2Hg(l) + O_2(g)$

   $2PbO_2(s) \rightarrow 2PbO(s) + O_2(g)$

3. Catalytic decomposition of hydrogen peroxide. Finely divided metals or manganese dioxide readily catalyse the breakdown of $H_2O_2$ into water and dioxygen:

   $2H_2O_2(aq) \rightarrow 2H_2O(l) + O_2(g)$

   On the large scale, dioxygen is instead produced from water or from air. Electrolysis of water liberates hydrogen gas at the cathode while oxygen collects at the anode. On the industrial scale, oxygen is pulled out of ordinary air by first scrubbing out carbon dioxide and water vapour, then liquefying the remaining gases and fractionally distilling them to separate dinitrogen from dioxygen.

Properties

Dioxygen is a colourless, odourless gas. Its solubility in water is modest — about 3.08 cm³ per 100 cm³ of water at 293 K — yet this small amount is exactly what sustains marine and aquatic life. It liquefies at 90 K and freezes at 55 K. Naturally occurring oxygen consists of three stable isotopes: $^{16}O$, $^{17}O$ and $^{18}O$. A particularly notable feature of the $O_2$ molecule is that it is paramagnetic, despite having an even number of electrons — a property explained by molecular orbital theory (covered in Class XI Chemistry).

Dioxygen reacts directly with nearly every metal and non-metal, the principal exceptions being a few noble metals (such as Au and Pt) and the noble gases. These combinations are usually strongly exothermic, which helps the reaction sustain itself once started, though some external heating is typically needed to initiate it — a consequence of the high bond dissociation enthalpy of the oxygen–oxygen double bond, $493.4\ kJ\ mol^{-1}$.

Representative reactions with metals, non-metals and compounds include:

Some oxidations proceed only with the help of a catalyst: …