Electrochemistry - Result Question 91
####39. An aqueous solution of $NaCl$ on electrolysis gives $H _2(g), Cl _2(g)$ and $NaOH$ according to the reaction.
$$ 2 Cl^{-}(a q)+2 H _2 O \rightleftharpoons 2 OH^{-}(a q)+H _2(g)+Cl _2(g) $$
A direct current of $25 A$ with a current efficiency of $62 %$ is passed through $20 L$ of $NaCl$ solution ( $20 %$ by weight). Write down the reactions taking place at the anode and cathode. How long will it take to produce $1 kg$ of $Cl _2$ ? What will be the molarity of the solution with respect to hydroxide ion? (Assume no loss due to evaporation)
(1992, 3M)
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Solution:
- At anode
$$ 2 Cl^{-} \longrightarrow Cl _2+2 e^{-} $$
At cathode $2 H _2 O+2 e^{-} \longrightarrow H _2+2 OH^{-}$
$1 kg Cl _2=\frac{1000}{35.5}$ equivalent of $Cl _2=28.17$ equivalent
$\Rightarrow$ Theoretical electricity requirement $=28.17 F$
$\because \quad$ Efficiency is only $62 %$
$\therefore$ Electricity requirement (experimental)
$$ \begin{array}{rlrl} = & \frac{28.17 \times 100}{62} F & =45.44 F \ \Rightarrow & & 45.44 \times 96500 & =25 t \text { (in second) } \ \Rightarrow & & t & =48.72 h \end{array} $$
Also, gram equivalent of $HO^{-}$produced $=28.17$
$\Rightarrow$ Molarity of $HO^{-}=\frac{28.17}{20}=1.4085 M$
