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Inorganic Chemistry |
Formation of oxides and hydroxides |
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(i) These are most reactive metals and have strong affinity for O2 quickly tranish in air due to the formation of a film of their oxides on the surface. These are, therefore, kept under kerosene or paraffin oil to protect them from air,
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| M + O2 —> M2O ———> M2O2
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| (ii) When burnt air (O2), lithium forms lithium oxide (Li2O) sodium forms sodium peroxide (Na2O2) and other alkali metals form super oxide (Mo2 i.e. KO2,RbO2 or CsO2) |
2Li + 1/2 O2 —> Li2O
;
2Na + O2 —> Na2O2
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| Lithium oxide |
K + O2 —> KO2 Potassium super oxide
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| |
Li |
Na |
K |
Rb |
Cs |
Fr |
| Specific heat (Cal/g) |
0.941 |
0.293 |
0.17 |
0.08 |
0.049 |
- |
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| The reactivity of alkali metals towards oxygen to form different oxides is due to strong positive field around each alkali metal cation. Li+ being smallest, possesses strong positive field and thus combines with small anion O2– to form stable Li2O compound. The Na+ and K+ being relatively larger thus exert less strong positive field around them and thus reacts with larger oxygen anion i.e, O2–2 and O21–
to form stable oxides. |
| The monoxide, peroxides and superoxides have O2 and O2–2 and O21– ions respectively. The structures of each are, |
The O2–1 ion has a three electron covalent bond and has one electron unpaired. It is therefore superoxides are paramagnetic and coloured KO2 is light yellow and paramagnetic substance. |
| (iii) The oxides of alkali metals and metal itself give strongly alkaline solution in water with evolution of heat |
M + H2O —> MOH + 1/2 H2; ΔH = –ve
Li2O + H2O —> 2LiOH; ΔH = –ve
Na2O2 + 2H2O —> 2NaOH + H2O2(s); ΔH = –ve |
| 2KO2 + 2H2O —> 2KOH + H2O2(s) + O2(g); ΔH = –ve
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The peroxides and superoxides act as strong oxidising agents due to formation of H2O2 |
| (iv) The reactivity of alkali metals towards air and water increases from Li to Cs that is why lithium decomposes H2O very slowly at 25oC whereas Na does so vigorously, K reacts producing a flame and Rb, Cs do so explosively. |
| M + H2O —> MOH + 1/2 H2
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(v) The basic character of oxides and hydroxides of alkali metals increases from Li to Cs. This is due to the increase in ionic character of alkali metal hydroxides down the group which leads to complete dissociation and leads to increase in concentration of OH– ions. |
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