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Describe the construction and working of the standard hydrogenelectrode (S.H.E.) . |
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Answer» Solution :(1) Construction: (a) The standardhydrogen electrode (S.H.E) consistsof a glass tube at the end of whicha pieceof platinisedplatinumfoil is attachedas SHOWN in the given figure. Aroundthis platethere is an outerjacket of glass which has a sideinlet throughwhich pure and DRY hydrogengas  is bubbledat one atmospherepressure. The innertube is filled with a littlemercuryand a copperwire isdippedinto it for an electrical contact. (b) The wholeassemblyis kept immersed in a solution containing hydrogen ions `(H^(+))` of unit activity. (c) Thiselectrodeis arbitrarily assignedzero potential . (2) Representation of S.H.E. : `H^(+) (1M)|H_(2)(g,1 ATM)|Pt` (3) Working : If the electrodeacts asanode then, Oxidation: `1/2H_(2(g)) hArr H_((aq))^(+) + e^(-) E^(0) = 0.00 V` If the electrodeacts ascathodethen, Reduction: `H_((aq))^(+) + e^(-)hArr 1/2 H_(2^((g))) E^(0) = 0.00 V` `H_(2)` gasin CONTACTWITH `H_((aq))^(+)`ions attainsan equilibriumesta-blishing a potential. APPLICATIONS of SHE :A reversiblegalvanic cell with the experimental electrode , `Zn^(2+) (1M) |Zn_((ag))^(2+)(1 M)` and SHEcan be developedas follows : `Zn|Zn_((aq))^(2+) (1M)||H_((aq))^(+) (1M)|H_(2)(g,1atm) |Pt` Then, `E_(cell) = E_(SHE) - E_(M^(n+))//M` `= 0 - E_(M^(n+)//M)` ` = - E_(M^(n+)//M)` Thus the potential can be directlyobtained. (5) Disadvantages (Drawbacks or Difficulties) : (1) It is difficultto constructand handleSHE. (2) Pure anddry `H_(2)` gascannot beobtained. (3) Pressureof `H_(2)` gas cannot be maintainedexactlyat 1atmosphere. (4) The activemass or concentrationof `H^(+)` from `HCl` cannot be maintainedexactlyunity . |
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