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A freshly cut piece of sodium metal appe...

A freshly cut piece of sodium metal appear shiny but its metallic iustre soon gets tarnished when exposed to air. Give reason.

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When a freshly cut piece of metallic sodium is exposed to moist air, it readily reacts with oxygen to form sodium monoxide `(Na_2O)`. The resultant sodium monoxide and also the metal itself readily react with the moisture of air to form sodium hydroxide (NaOH). In the subsequent step, both `Na_2O` and NaOH combine with `CO_2` of air to form sodium carbonate `(Na_2CO_3)`. Thus, a coating of sodium carbonate is formed on the surface of the metal and as result of this the metallic lustre is tarnished.
`4Na + O_2 to 2Na_2O , Na_2O + H_2O to 2NaOH`
`2Na + 2H_2O to 2NaOH + H_2 uarr, Na_2O + CO_2 to Na_2CO_3`
`2NaOH +CO_2 to Na_2CO_3+ H_2O`
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Knowledge Check

  • A metal gets coated with a green basic carbonate when exposed to moist air. The metal is -

    A
    Co
    B
    Ni
    C
    Cu
    D
    Mn

  • Alkali metals on combustion in excess of air form different type of oxides depending on the size of metal cations e.g . lithium forms mainly the oxide Li_2O , sodium forms peroxide and some super oxides NaO_2 and potassium, Rubidium and caesium form the superoxides, MO_2 . The stability of peroxide and superoxide increases, as the size of the metal cation increases because of the stabilisation of larger anion by larger cations through lattice energy effect. These oxides are easily hydrolysed by water. The oxides and the peroxides are colourless when pure, but the superoxides are yellow or orange in colour. Peroxide and superoxide of sodium and potassium are widely used as oxidising agent and as air purifier in space capsules, submarines and breathing mask. Which of the following compound is used in the emergency breathing apparatus ?

    A
    Potassium superoxide `KO_2`
    B
    Soda lime (NaOH + CaO)
    C
    Sodium monoxide `(Na_2O)`
    D
    Hydrated calcium sulphate `(CaSO_4 . H_2O)`

  • Alkali metals on combustion in excess of air form different type of oxides depending on the size of metal cations e.g . lithium forms mainly the oxide Li_2O , sodium forms peroxide and some super oxides NaO_2 and potassium, Rubidium and caesium form the superoxides, MO_2 . The stability of peroxide and superoxide increases, as the size of the metal cation increases because of the stabilisation of larger anion by larger cations through lattice energy effect. These oxides are easily hydrolysed by water. The oxides and the peroxides are colourless when pure, but the superoxides are yellow or orange in colour. Peroxide and superoxide of sodium and potassium are widely used as oxidising agent and as air purifier in space capsules, submarines and breathing mask. When sodium reacts with excess of oxygen at 350^@ C , the oxidation number of oxygen per atom changes from:

    A
    0 to `-1`
    B
    0 to `-1/2`
    C
    `-1` to `-2`
    D
    `+1` to `-1`

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