Ncert Exercises & Solutions

Account for the following observations.

1. AlCl₃ is a Lewis acid.

2. Though fluorine is more electronegative than chlorine yet BF₃ is a weaker Lewis acid than CI₃.

3. PbO₂ is a stronger oxidising agent than SnO₂.

4. The +1 oxidation state of thallium is more stable than its +3 state.

(a) In AlCl₃, Al has only six electrons in its valence shell. It is an electron deficient species. Therefore, it acts as a Lewis acid (electron acceptor)

(b) In BF₃ boron has a vacant 2p-orbital and fluorine has one 2p-completely filled unutilised orbital. Both of these orbitals belong to same energy level therefore, they can overlap effectively and form

pπ - pπ

bond. This type of bond formation is known as back bonding.

While back bonding is not possible in BCl₃ because there is no effective overlapping between the 2p-orbital of boron and 3p-orbital of chlorine. Therefore, electron deficiency of B is higher in BCl₃ than that of BF₃. That is why BF₃ is a weaker Lewis acid than BCl₃.

(c) In PbO₂ and SnO₂, both lead and tin are present in +4 oxidation state. But due to stronger inert pair effect, Pb²⁺ ions is more stable than Sn²⁺ ion. In other words, Pb⁴⁺ ions i.e., PbO₄ is more easily reduced to Pb²⁺ ions than Sn⁴⁺ ions reduced Sn²⁺ ions. Thus, PbO₂ acts as a stronger oxidising agent than SnO₂

(d) Tl⁺ is more stable than Tl³⁺ because of inert pair effect.

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