HKDSE Chemistry Question Review (2024-05-20)

Question:

[MCQ] Which of the following statements about Chlorine is incorrect?

A. It is naturally occurring at seas.

B. It can react with water to form a weak acid.

C. It's electronegativity is comparative with Oxygen.

D. It's oxidation number can reach +8.

Hint:

Solution:

The answer is D.

A. Thunder will electrolyze NaCl (aq) from the sea to hydrogen gas and chlorine gas.

B. Cl2 (aq) + H2O (l) <----> HCl (aq) + HClO (aq - a weak acid)

C. Electronegativity decreases down the group but increases across the group from left to right.

D. The maximum outermost electrons that can be shared by Cl is 7, so the oxidation number cannot exceed +7.

Note the peroxide group -O-O-.

Upon radiation, the peroxide group will break down by homolytic fissure such that no electron transfer will be done, i.e. it forms free radicals.

So clearly Cl2O8 has 2 oxygen atoms from the peroxide group which exhibit zero oxidation number. As shown by the lewis structure above, the total bond order from the Cl atom can never exceed 7.