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# Give reasons for the following :(i) Silver bromide is used in photography.    (ii) Most transition metal compounds are coloured.(iii) Zinc and not copper is used for the recovery of metallic silver from complex [Ag(CN)2]-. Explain.                (iv) The colour of mercurous chloride, Hg2CI2, changes from white to black when treated with ammonia.                      (v) The species [CuCI4]2- exists while [CuI4]2- does not      (vi) CrO3 is an acid anhydride.

6 years ago
(i) It is because silver bromide, being sensitive to light, reduces into metallic silver grains when light fall on it….
(ii) The transition metals form coloured compounds and coloured complexes. They have vacant d-orbitals. Electrons take up energy from the visible region and move to higher energy levels. The visible colour of the substance is the complementary colour of the absorbed light.
[NOTE : The colour is due to d-d transitions]
(iii) Zinc is cheaper and stronger reducing agent than copper and zinc is volatile
(iv) Mercurous chloride changes from white to black when treated with ammonia due to the formation of finely divided mercury.
(v) Cu2+ is reduced to Cu+ by I-, hence cupric iodide is converted into cuprous iodide so [CuI4]2- does not exist, CI- cannot effect this change and thus [CuCI4]2- exists.
(vi) CrO3 is acid anhydride of H2CrO4 (Chromic acid) [Anhydride are formed by loss of water from acid]
H2CrO4 $\overset{Dehydration}{\rightarrow}$CrO3 + H2O
In H2Cr2O4 , Cr is present in +6 oxidation state.