Question

A. Give one word or a phrase for the following. [2 marks]
(i) The energy released when an electron is added to a neutral gaseous isolated atom to form a negatively charged ion.
(ii) The amount of any substance that contains $6.023\times {10}^{23}$ particles.

B. How will you distinguish between ammonium hydroxide and sodium hydroxide using copper sulphate solution? [2 Marks]

C. Give a reason for each of the following. [3 marks]
(i) Ionic compounds have a high melting point.
(ii) The blue colour of aqueous copper sulphate solution fades when it is electrolysed using platinum electrodes.
(iii) Conductivity of dilute hydrochloric acid is greater than that of acetic acid.

D. Arrange the following as per the the instruction given in the brackets: [3 marks]
(i) 50 cc of ${\text{SO}}_2$, 100 cc of ${\text{N}}_2$, 150 cc of ${\text{O}}_2$ (In increasing order of number of molecules at STP)
(ii) $K^{+},{\mathrm{Al}}^{3+},{\mathrm{Cu}}^{2+}$(In the increasing order of discharge at the cathode)
(iii) Na, K, Li (In the increasing order of atomic size)

Answer 1

A. (i) Electron affinity [1 mark]
(ii) Mole [1 mark]

B. Ammonium hydroxide when added to copper sulphate drop-wise forms a pale blue precipitate of copper hydroxide which dissolves in excess of ammonium hydroxide to form a deep blue solution.
[1 mark]
Sodium hydroxide when added to copper sulphate drop-wise forms a pale blue precipiate which is insoluble in excess sodium hydroxide. [1 mark]

C. (i) Ionic compounds have high melting points because there is a strong electrostatic force of attraction between the oppositely charged ions and hence a large amount of energy is required to break the strong bonding force between ions. [1 mark]

(ii) During the electrolysis of copper sulphate using platinum electrodes copper ions from copper sulphate solution get reduced on the platinum electrode. $C{u}^{2+}\left(aq\right)$ + $2e^{-}$ → Cu (s).
The blue colour of copper ions fades due to a decrease in $C{u}^{2+}$ ions and finally, the solution becomes colourless as soon as $C{u}^{2+}$ ions are exhausted. [1 mark]

(iii) Acetic acid is weak electrolyte which dissociates partially whereas HCl is strong electrolyte which dissociates completely. Therefore, conductivity of dilute hydrochloric acid is greater than acetic acid. [1 mark]

D. (i) At STP, 22.4 L or 22400 cc of any gas contain Avogadro number of molecules.

Here, the volumes of the gases are in the order of ${\text{SO}}_2$ < ${\text{N}}_2$ < ${\text{O}}_2$. So the required arrangement of molecules in the increasing order is ${\text{SO}}_2$ < ${\text{N}}_2$ < ${\text{O}}_2$. [1 mark]

(ii) In the electrochemical series of metals, the lower a metal is placed in the series, the easier is for it to get discharged at the cathode.
Among $K^{+},{\mathrm{Al}}^{3+},{\mathrm{Cu}}^{2+}$, ${\mathrm{Cu}}^{2+}$
lies below ${\mathrm{Al}}^{3+}$ which in turn lies below $K^{+}$ in the electrochemical series.
So the required order is ${\mathrm{Cu}}^{2+}<{\mathrm{Al}}^{3+}<K^{+}$. [1 mark]

(iii) We know that atomic size increases from right to left and from top to bottom in the periodic table. So the required correct order of atomic radius is Li < Na < K. [1 mark]