Question

1. Name the gas in each of the following
2. State one observation for each of the following

1. Excess ammonium hydroxide is added to lead nitrate solution
2. Bromine vapours are passed into a solution of ethyne in carbon tetrachloride
3. A zinc granule is added to copper sulphate solution
4. Zinc nitrate crystals are strongly heated
5. Sodium hydroxide solution is added to ferric chloride solution at first a little and then in excess

Answer 1

1. When ammonium hydroxide is added to lead nitrate solution, a white precipitate is formed.

• The white precipitate is lead hydroxide [Pb (OH)₂].
• When excess ammonium hydroxide is added, white precipitate does not dissolves.

$\mathrm{Pb}{\left({\mathrm{NO}}_3\right)}_2+2{\mathrm{NH}}_4\mathrm{OH}\to \underset{\mathrm{white}\mathrm{precipitate}}{\mathrm{Pb}{\left(\mathrm{OH}\right)}_2}+2{\mathrm{NH}}_4{\mathrm{NO}}_3$

ii. When Bromine vapours are passed into a solution of ethyne in carbon tetrachloride, the reddish brown colour of Bromine decolourizes

• Ethyne is an unsaturated compound containing triple bond
• When Bromine vapours added, addition reaction takes place
• Triple bond breaks and bromine is added
• $\mathrm{CH}\equiv \mathrm{CH}+2{\mathrm{Br}}_2\to {\mathrm{CHBr}}_2-{\mathrm{CHBr}}_2$

iii. $\mathrm{Zn}+{\mathrm{CuSO}}_4\to {\mathrm{ZnSO}}_4+\mathrm{Cu}$

• Zinc is a very reactive metal compared to copper.
• Zinc can displace copper from copper sulphate solution.
• Hence zinc sulphate and copper gets deposited

iv) $\mathrm{Zn}{\left({\mathrm{NO}}_3\right)}_2\to \underset{\mathrm{white}\mathrm{precipitate}}{\mathrm{ZnO}}+\underset{\mathrm{brown}\mathrm{fumes}}{{\mathrm{NO}}_2}$

• When zinc nitrate is heated, brown fumes are evolved and a white precipitate is formed at the test tube.
• The white precipitate formed is zinc oxide ((ZnO)
• Brown fumes are due to the formation of Nitrogen dioxide

v) When ferric chloride react with sodium hydroxide is a reddish brown precipitate is formed

• This reddish brown precipitate is due to the formation of Fe(OH)₃ .
• ${\mathrm{FeCl}}_3+3{\mathrm{NH}}_4\mathrm{OH}\to \underset{\mathrm{reddish}\mathrm{brown}\mathrm{precipitate}}{\mathrm{Fe}{\left(\mathrm{OH}\right)}_3+}3{\mathrm{NH}}_4\mathrm{Cl}$
• When excess reagent is added, no reaction takes place.