Question

# Select the CORRECT order:

A
Thermal Stability : LiF > NaF > KF > RbF > CsF
B
Bond Angle : BF3 < BCl3 < BBr3 < BI3
C
Rate of hydrolysis : SiF4 > SiCl4 > SiBr4 > SiI4
D
Acidic Nature : Mn2O7 > MnO2 > MnO

Solution

## The correct options are A Thermal Stability : LiF > NaF > KF > RbF > CsF C Rate of hydrolysis : SiF4 > SiCl4 > SiBr4 > SiI4 D Acidic Nature : Mn2O7 > MnO2 > MnO (A) Higher the lattice energy, more stable will be the lattice therefore more difficult it is to break and hence higher will be its thermal stability. In case of LiF, both Li+ and F− are of small size hence lattice formed due to them is most stable as compared to other alkali metal fluorides, hence the order follows : LiF > NaF > KF > RbF > CsF (B) In case of sp2 hybridisation, bond angle is always 120∘ if no lone pair is present. Here we have been given BF3 , BCl3 , BBr3 , BI3, all of them have same sp2 hybridisation and none of them has lone pair hence bond angle will be same in all of them hence order becomes : BF3 = BCl3 = BBr3 = BI3 which is not given in option. Hence, (B) is incorrect. (C) Order of rate of hydrolysis will be same as order of Lewis acidic strength in case of silicon tetra halides SiX4. This is because the effect of π back bonding is less dominant over − I effect. Hence, order is : SiF4 > SiCl4 > SiBr4 > SiI4  (D) Acidic strength of oxides is proportional to oxidation state on central atom. Here oxidation state on Mn is as follows : Mn2O7 = +7 MnO2 = +4 MnO= +2 Hence, order of acidic strength is : Mn2O7 > MnO2 > MnO

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