7.11 Dioxygen
Preparation
NEETprep Audio Note:
(i) By heating oxygen containing salts such as chlorates, nitrates and permanganates.
(ii) By the thermal decomposition of the oxides of metals low in the electrochemical series and higher oxides of some metals.
2Ag2O(s) → 4Ag(s) + O2(g); 2Pb3O4(s) → 6PbO(s) + O2(g)
2HgO(s) → 2Hg(l) + O2(g) ; 2PbO2(s) → 2PbO(s) + O2(g)
(iii) Hydrogen peroxide is readily decomposed into water and dioxygen by catalysts such as finely divided metals and manganese dioxide.
2H2O2(aq) → 2H2O(1) + O2(g)
On large scale it can be prepared from water or air. Electrolysis of water leads to the release of hydrogen at the cathode and oxygen at the anode.
Industrially, dioxygen is obtained from air by first removing carbon dioxide and water vapour and then, the remaining gases are liquefied and fractionally distilled to give dinitrogen and dioxygen.
Properties
NEETprep Audio Note:
Dioxygen directly reacts with nearly all metals and non-metals except some metals ( e.g., Au, Pt) and some noble gases. Its combination with other elements is often strongly exothermic which helps in sustaining the reaction. However, to initiate the reaction, some external heating is required as bond dissociation enthalpy of oxgyen-oxygen double bond is high (493.4 kJ mol–1).
Some of the reactions of dioxygen with metals, non-metals and other compounds are given below:
2ZnS + 3O2 → 2ZnO + 2SO2
Some compounds are catalytically oxidised. For example,
7.16 Which of the following does not react with oxygen directly?
Zn, Ti, Pt, Fe
7.17 Complete the following reactions:
(i) C2H4 + O2 →
(ii) 4Al + 3 O2 →