How do you count for the following observation?

When concentrated sulphuric is added to an inorganic mixture containing chloride, we get colourless pungent-smelling gas $\mathrm{HCl}$ but if the mixture contains bromide then we get the red vapours of bromine. Why?

Identify the substance oxidized, reduced, oxidizing agent, and reducing agent in the following reaction:$\mathrm{HCHO}\left(\mathrm{l}\right) +2{\mathrm{Cu}}^{2+} \left(\mathrm{aq}\right)+ 5{\mathrm{OH}}^{-}\left(\mathrm{aq}\right) \to {\mathrm{Cu}}_2\mathrm{O} \left(\mathrm{s}\right)+ {\mathrm{HCOO}}^{-}\left(\mathrm{aq}\right) + 3{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$

Identify the substance oxidized, reduced, oxidizing agent, and reducing agent in the following reaction.

${\mathrm{N}}_2{\mathrm{H}}_4\left(\mathrm{l}\right) + {\mathrm{H}}_2{\mathrm{O}}_2\left(\mathrm{l}\right) \to {\mathrm{N}}_2\left(\mathrm{g}\right)+ 4{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$

Identified the substance oxidized, reduced, oxidizing agents, and reducing agent for the following reaction:

$\mathrm{Pb}\left(\mathrm{s}\right) + {\mathrm{PbO}}_2\left(\mathrm{s}\right)+ 2{\mathrm{H}}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right) \to 2{\mathrm{PbSO}}_4\left(\mathrm{s}\right) +2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$