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CBSE Class 12 Chemistry Solved Question Paper 2018

Short Answer Type

21.

A first-order reaction is 50% completed in 40 minutes at 300 K and in 20 minutes at 320 K. Calculate the activation energy of the reaction.

(Given: log 2 = 0.3010, log 4 = 0.6021, R = 8.314 JK^–1 mol^–1)

22.

What happens when

A freshly prepared precipitate of Fe(OH)₃ is shaken with a small amount of FeCl₃ solution?

23.

What happens when

Persistent dialysis of a colloidal solution is carried out?

24.

What happens when

An emulsion is centrifuged?

25.

For the reaction

$2 A g C l (s) + H_{2} (g) (1 a t m) \overset{}{\to} 2 A g (s) + 2 H^{+} (0.1 M) + 2 C l^{-} (0.1 M), ∆ G^{o} = - 43600 J a t 25^{o} C$

Calculate the e.m.f. of the cell.
[log 10^–n = – n]

26.

Define fuel cell and write its two advantages.

27.
Give reasons:
H₃PO₃undergoes disproportionation reaction but H₃PO₄ does not.

H₃PO₃undergoes disproportionation reaction but H₃PO₄ does not. This is because in H₃PO₃, P is in +3 oxidation state which can get oxidised as well as reduced. Thus, H₃PO₃gives disproportionation, reaction as:

$H_{3} P O_{3} \to H_{3} P O_{4} + P H_{3}$

On the other hand, in H₃PO₄, P is in its maximum oxidation state of +5, which can duly be reduced but not oxidised further. So, H₃PO₄ does not show disproportionation.