Manufacture of sodium hydroxide, chlorine and hydrogen;Advantages of mercury cathode cell and disadvantages of mercury cathode cell
Answer Text: Manufacture of sodium hydroxide, chlorine and hydrogen- Sodium hydroxide is prepared by the electrolysis of brine, for which 3 methods are available- The method depends o the type of electrolytic cell.- These cells are:- The mercury cell- The diaphragm cell- The membrane cella) The mercury cell:- Components- The electrolyte is concentrated sodium chloride- The anodes are made of graphite or titanium, which are placed above the cathode.- The cathode consists of mercury, which flows along the bottom of the cell.Chemical reactionsAnode- Both chloride and hydroxide ions are attracted.- Chloride ions are preferentially discharged due to their high concentration- The chloride ions undergo oxidation to form green – yellow chlorine gas.EquationAt the cathode (flowing mercury)- #Na^(+)# (aq) migrate to the cathode- Sodium ions are preferentially discharged.- They undergo reduction to form sodium solid.Equation- the discharged sodium atoms combine with mercury to form sodium diagramEquation- The sodium amalgam is then passed into another reactor containing water.- The amalgam reacts with water forming hydrogen and sodium hydroxide.The mercury is regenerated and it is recycled into the main cell.- Main product: Sodium hydroxide- By products: Sodium and chlorine.Advantages of mercury cathode cell:- The resultant sodium hydroxide is very pure; as it has no contamination from sodium chloride.- It is highly concentrated; i.e. about 50%.Disadvantages- Some of the mercury said its way into the environment leading to mercury pollution; a common case of brain damage in humans.- At the operating temperatures (#70^0C – 80^0C#), mercury vapours escape into the atmosphere and cause irritation and destruction of lungs tissues.- Its operation requires highly skilled man power.
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