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Question

The reducing power of a metal depends on various factor. Suggest the factor which makes Li, the strongest reducing agent in aquous solution.
(a) Sublimation enthalpy     (b)   Lonisation enthalpy  
(c) Hydration enthalpy        (d)   Electron - gain enthalpy
 


Solution

(c)

 Standard reduction potential (ERP Is   ameasure f tendency fo an element to lose electron in aqueous solution. Higher the negative ERP greater is the ability to lose electrons. 
 ERP 
(i) enthalpy of sublimation
(ii) ionisation enthalpy
(iii) enthalpy of hydration
Thus, in aqueous medium, order of reactivity of alkali metals is Na < K Rb < Cs < Li, 
  ERP value of Li is least (-3, 04V) among all alkali imetals
The formation's of Li^{+} (aq) from Li involves following steps 
 (i) Li(s)Sublimation−−−−−−Li(g)H5=  Enthalpy of sublimation
Li(g)Li+(s)IE1= Ionisation enthalpy
(iii) Li+(g)Li+(aq)Hn  = Enthalpy of hydration
For alkali metals, enthalpies of sublimation are almost same,    IE1 value of Li is endohermic and highest and hydration is exothermic and maximum for Li+ Endothermic and highest and hydration is exotherrnic and maximum for Li+ The highly exothermic step (iii) for smallest Li+ makes it strongest reducing agent.
 

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