Question

(a) Study the diagram given alongside and answer the questions that follow:
(i) Give the names of the electrodes A and B.
(ii) Which electrode is oxidizing electrode?
(b) A strip of copper is placed in four different colourless salt solutions. They are KNO₃, AgNO₃, Zn(NO₃)₂, Ca(NO₃)₂. Which one of the solutions will finally turn blue?
(c) Write the equations of the reactions which take place at the cathode and anode when acidified water is electrolysed.
Figure

Answer 1

(a) (i) Electrode A is anode and electrode B is cathode.
​ (ii) Anode is the oxidising electrode. At anode, release of electrons (i.e., oxidation) takes place.

(b) AgNO₃ solution will turn blue. Potassium, zinc and calcium are highly reactive metals and are above copper in the electrochemical series. Thus, copper cannot displace these metals and no reaction takes place when copper strip is placed in these respective metal salt solutions. But as copper is more reactive than silver (silver is placed below copper in the electrochemical series), it displaces silver from its silver nitrate solution. Thus, silver nitrate solution turns blue due to the formation of copper nitrate.
2AgNO₃ + Cu → 2Ag + Cu(NO₃)₂

(c) The equations of the reactions which takes place at the cathode and anode when acidified water is electrolysed are:
At cathode:

$$\begin{gathered} {\mathrm{H}}^{+}+{\mathrm{e}}^{-}\to \mathrm{H} \\ \mathrm{H}+\mathrm{H}\to {\mathrm{H}}_2 \end{gathered}$$

At anode:

$$\begin{gathered} 4{\mathrm{OH}}^{-}\to 4\mathrm{OH}+4{\mathrm{e}}^{-} \\ 4\mathrm{OH}\to 2{\mathrm{H}}_2\mathrm{O}+{\mathrm{O}}_2 \end{gathered}$$

Hydrogen gas is liberated at the cathode and oxygen gas is liberated the at anode.