p Block Elements
13.0 Oxygen
13.0 Oxygen
Preparation: Oxygen can be obtained in the laboratory by following methods.
(i) From oxide:
(a) By thermal decomposition of the oxide of metals which are in the lower part of electrochemical series.
\[\begin{gathered} 2HgO\xrightarrow{{450^\circ C}}2Hg + {O_2} \hspace{1em} \\ 2Ag{O_3}\xrightarrow{{350^\circ C}}4Ag + {O_2} \hspace{1em} \\ \end{gathered} \]
(b) By the thermal decomposition of higher oxides.
\[\begin{gathered} 3Mn{O_2}\xrightarrow{{Heat}}M{n_3}{O_4} + {O_2} \hspace{1em} \\ 2P{b_3}{O_4}\xrightarrow{{Heat}}2BaO + {O_2} \hspace{1em} \\ \end{gathered} \]
(c) By the action of conc. ${H_2}S{O_4}\;or\;Mn{O_2}$.
\[3Mn{O_2} + 2{H_2}S{O_4} \to 2MnS{O_4} + 2{H_2}O + {O_2}\]
(d) By the action of water on sodium peroxide (ozone).
\[3N{a_2}{O_2} + 2{H_2}O \to 4NaOH + {O_2}\]
(ii) From salts:
(a) Alkali nitrates on heating evolve oxygen.
\[\begin{gathered} 3Na{O_3} \to 3Na{O_2} + {O_2} \hspace{1em} \\ 3KN{O_3} \to 2KN{O_2} + {O_2} \hspace{1em} \\ \end{gathered} \]
(b) ${K_2}C{r_2}{O_7},KMn{O_4}$ and $KCl{O_3}$ decomposes at high temperature evolving oxygen.
\[\begin{gathered} 4{K_2}C{r_2}{O_7}\xrightarrow{{400^\circ C}}4{K_2}Cr{O_4} + 2C{r_2}{O_3} + 3{O_2} \hspace{1em} \\ 4KMn{O_4}\xrightarrow{{250^\circ C}}{K_2}Mn{O_4} + Mn{O_2} + {O_2} \hspace{1em} \\ 2KCl{O_3}\xrightarrow{{400^\circ C}}2KCl + 3{O_2} \hspace{1em} \\ \end{gathered} \]
(c) By heating ${K_2}C{r_2}{O_7},KMn{O_4}$ with conc. ${H_2}S{O_4}$.
\[\begin{gathered} 4KMn{O_4} + 6{H_2}S{O_4} \to 2{K_2}S{O_4} + 4MnS{O_2} + 6{H_2}O + 5{O_2} \hspace{1em} \\ 6{K_2}C{r_2}{O_7} + 8{H_2}{S_4} \to 2{K_2}S{O_4} + 2C{r_2}{(S{O_4})_3} + 8{H_2}O + 3{O_2} \hspace{1em} \\ \end{gathered} \]
(d) By adding $H_2O_2$ to acidified $KMn{O_4}$ solution.
\[\begin{gathered} \frac{\begin{gathered} 2KMn{O_4} + 3{H_2}S{O_4}(dil.) \to {K_2}S{O_4} + MnS{O_4} + 3{H_2}O + 5[O] \hspace{1em} \\ [{H_2}{O_2} + O \to {H_2}O + {O_2}] \times 5 \hspace{1em} \\ \end{gathered} }{{2KMn{O_4} + 3{H_2}S{O_4} + 5{H_2}{O_2} \to {K_2}S{O_4} + 2MnS{O_4} + 8{H_2}O + 5{O_2}}} \hspace{1em} \\ \hspace{1em} \\ 4KCl{O_3} \to 3KCl{O_4} + KCl \hspace{1em} \\ KCl{O_4}\xrightarrow{{650^\circ C}}KCl + 2{O_2} \hspace{1em} \\ \end{gathered} \]
Properties:
- Oxygen is a colorless, odorless and tasteless gas.
- It is little heavier than air and is slightly soluble in water, this small amount of dissolved oxygen in water supports the respiration of fish and other aquatic animals.
- It is soluble in alkaline pyrogallol.
- It exhibits allotropy. Its allotropic modification is ozone $(O_3)$.
- There are $3$ isotopes of oxygen with mass numbers $16$, $17$ and $18$ i.e., $({O^{16}},\ {O^{17}}\;and\;{O^{18}})$ in which ${O^{16}}$ is the main one with an abundance of nearly $99.8 \%$.
- Oxygen is not a combustible gas, but it is a strong supporter of combustion.
- It is a very active element.
- Oxygen combines nearly with all other element excepts inert gas, noble metal and halogens.
- The binary compounds of oxygen with other elements are called oxides. However, the compounds of oxygen and fluorine $(OF_2,O_2F_2)$ are not called as oxides but oxygen fluorides as fluorine is more electronegative than oxygen.
Structure:
According to the M.O.T. structure of $O_2$ molecule is as follows.
\[KK\sigma {(2S)^2}{\sigma ^*}{(2S)^2}\sigma {(2{P_x})^2}\pi {(2{P_z})^2}{\pi ^*}{(2{P_y})^1}{\pi ^*}{(2{P_z})^1}\]
Bond order in this case is $\frac{1}{2}(8 - 4) = 2$
Oxides may be prepared by burning of elements such as $C,P,S,Na,K,Mg,etc$ in air or oxygen atmosphere.
\[\begin{gathered} C + {O_2} \to C{O_2} \hspace{1em} \\ S + {O_2} \to S{O_2} \hspace{1em} \\ 4Na + {O_2} \to 2N{a_2}O \hspace{1em} \\ 4K + {O_2} \to 2{K_2}O \hspace{1em} \\ 2Mg + {O_2} \to 2MgO \hspace{1em} \\ \end{gathered} \]
Peroxide: The oxides when diluted acids from hydrogen peroxide. These oxides are constrained as derivatives of $H_2O_2$ and contain peroxo linkage in their molecule.
Example:
\[\begin{gathered} N{a_2}O,{K_2}{O_2},Ba{O_2}\,etc. \hspace{1em} \\ N{a_2}O + {H_2}S{O_4}(dil.) \to N{a_2}S{O_4} + {H_2}{O_2} \hspace{1em} \\ Ba{O_2} + {H_2}S{O_4}(dil.) \to BaS{O_4} + {H_2}{O_2} \hspace{1em} \\ \end{gathered} \]
$Na_2O_2$ can be written as $Na - O - O - Na$. The linkage $-O-O-$ is called peroxo linkage.
Super-oxides contains $O_2^ - $ ion. The super-oxides known are $KO_2,RbO_2$ and $CsO_2$. These react with water to give hydrogen peroxide and oxygen.
\[2K{O_2} + 2{H_2}O \to 2KOH + {H_2}{O_2} + {O_2}\]
