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Concentrated H_2 SO_4 is added to the test tubes containing (a) to (e). {:("Test tube",,"Compounds"),(a.,,"Cane sugar"),(b.,,"sodium bromide"),(c.,,"Copper turnings"),(d.,,"Sulphur powder"),(e.,,"Potassium chloride"):} Identify in which of the above test tubes, the following change will be observed on heating. Also give the chemical equations involved. (i) Formation of black substance. (ii) Evolution of brown gas. (iii) Evolution of colourless gas. (iv) Formation of brown substance, which on dilution becomes blue. (v) Disappearance of yellow powder along with the evolution of a colourless gas. |
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Answer» Solution :(i) Conc. `H_2 SO_4` + Cane sugar `overset (Delta) rarr` Black substance Conc. `H_2 SO_4` acts a dehydrating AGENT, hence it removes water from cane sugar on heating and forms a black substance KNOWN as sugar charcoal. `C_12 H_22 O_11 overset (Conc. H_2 SO_4)rarr 12 C + 11 H_2 O` Thus, formation of black substance occurs in test + tube `(a)`. (II) Conc. `H_2 SO_4` + sodium bromide `overset (Delta) rarr` Brown gas On heating `NaBr` along with conc. `H_2 SO_4`, brown gas `Br_2` is evolved. Initially `Hbr` is produced, as `HBr` is a moderate reducing agent, it reduces `H_2 SO_4` to `SO_2` and itself is oxidised to bromine gas, `Br_2`. `{:(2NaBr+H_(2)SO_(4) overset(Delta)(rarr)Na_(2)SO_(4)+2HBr),(2HBr +H_(2)SO_(4) overset(Delta)(rarr)Br_(2)+SO_(2)+2H_(2)O),(ulbar(2NaBr +H_(2)SO_(4) rarr Na_(2)SO_(4) +Br_(2) +SO_(2)+2H_(2)O)):}` (iii) Potassium chloride + conc. `H_2 SO_4 overset(Delta) rarr` colourless gas `KCl` on heating with conc. `H_2 SO_4`, liberates `HCl` gas which is colourless. `2KCL + H_2 SO_4(conc.) overset(Delta) rarr K_2 SO_4 + 2 HCl` since `HCl` is not a reducing agent, it does not reduce `H_2 SO_4` to `SO_2`. Thus evolution of colourless gas occurs in test tube `( e)`. (iv) `Cu` turnings + `conc . overset (Delta) rarr` Brown substance `overset (H_2 O) underset ("Blue soln") rarr`. `Cu + 2 H_2 SO_4 overset (Delta) underset ("Anhydrous") CuSO_4 + SO_4 + H_2 O` `CuSO_4 + 5H_2 O overset (Delta) rarr overset ("copper sulphate") underset ("Blue") CuSO_4. 5 H_2 O` On heating, `Cu` turnings with `conc. H_2 SO_4`, a brownish substance, i.e., a mixture of `Cu` turnings and white anhydrous copper sulphate is formed along with evolution of pungent smelling `SO_2` gas. On dilution, the solution TURNS blue due to the formation of hydrated copper sulphate which is soluble in water. Hence, formation of brown substance, which on dilution becomes blue occurs in test tube `(c)`. (v) Sulphur powder + conc. `H_(2)SO_(4) overset(Delta)(rarr)` `{:([H_(2)SO_(4) overset(Delta)(rarr) H_(2)O +SO_(2) +[O]] xx 16),(S_(8)+16[O] rarr 8SO_(2)),(ulbar(underset("Yellow powder")(S_(8)+16H_(2)SO_(4))underset("Colourless pungent smelling gas")(rarr 24SO_(2))+16 H_(2)O)):}` (v) On heating sulphur powder, which is yellowish in colour with conc. `H_2 SO_4`, pungent smelling colourless, `SO_2` gas is produced, this yellow power disappears and the above reaction will occur in test tube (d). |
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