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Question

Account for the following:
a. When NaBr is heated with conc.H2SO4,Br2 is produced but when NaCl is heated with conc.H2SO4,HCl is produced.
(b) H2S acts only as a reducing agent but SO2 acts both as a reducing agent as well as an oxidising agent.
(c) The acid strength decreases in the order : HCl>H2S>PH3.

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Solution

(a) When NaBr is heated with conc.H2SO4,HBr is first produced which being a reducing agent reduces H2SO4 to SO2 while HBr itself gets oxidised to Br2.
NaBr+H2SO4NaHSO4+HBr
2HBr+H2SO42H2O+SO2+Br2
As a result, only Br2 is produced.

Similarly, NaCl reacts with conc.H2SO4 to form HCl. Since HCl does not act as a reducing agent, it does not get oxidised to Cl2.
NaCl+H2SO4NaHSO4+HCl
HCl+H2SO4No action
As a result, only HCl is produced.

(b) The minimum oxidation number (O.N) of S is 2 while its maximum O.N is +6. In SO2, the O.N of sulphur is +4, hence, it cannot only increase it O.N by losing electrons but also reduce it O.N by gaining electrons. Thus, it acts both as a reducing agent as well as an oxidising agent.
In contrast, in H2S,S has an O.N of 2. Thus, it can only increase its O.N by losing electrons and hence acts only as a reducing agent.

(c) Greater the polarity of the HA bond, more easily the bond break and hence greater is the acid strength. As the electronegativity of atom A decreases in the order; Cl>S>P, therefore the polarity of the bond decreases in the order HCl>HS>HP and hence the acid strength decreases in the same order HCl>H2S>PH3.

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