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In my book it is given

Reducing property increases down the group in gaseous or molten state . The increasing order is Li , Na , K , Rb ,Cs

In according to me reducing property is based on ionisation property . As Cs has highest ionisation property so it can easily loose elctron (oxidation potential)

But in aqueous solution incresing order is given as Na , Cs ,Rb , k ,Li

For this what could be the reason .

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Reducing property in aqueous phase depends upon (1) enthalpy of sublimation (2)enthalpy of ionisation (3)enthalpy of hydration. First two processes are endothermic while last one is exothermic. For lithium enthalpy of hydration is very high because of its small size hence Li acts as stronge reducing agent in aqueous phase. For Cs enthalpy of hydration is very small hence overall process is less exothermic .

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  • $\begingroup$ Then why Na shows different trend in enthalpy of hydration $\endgroup$ – user123733 Jan 2 '17 at 6:42
  • $\begingroup$ It is because of high ionisation enthalpy of Na compared to K,Rb and Cs. $\endgroup$ – narendra kumar Jan 2 '17 at 6:45

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