Answer the questions.[Che-3M]

Question 13 Marks

A cleaned aluminium foil was placed in an aqueous solution of zinc sulphate. When the aluminium foil was taken out of the zinc sulphate solution after 15 minutes, its surface was found to be coated with a silvery grey deposit. From the given observation, what can be concluded?

Answer

Aluminium is more reactive than zinc hence it displaces zinc from zinc sulphate solution and forms silvery white zinc metal. The reaction is as follows:
$\underset{\text { Zinc sulphate }}{3 \mathrm{ZnSO}_{4}(a q)}+\underset{\text { Alu min ium }}{2 \mathrm{Al}(s)} \rightarrow \underset{\text { Alu min ium sulphate }}{\mathrm{Al}_{2}\left(\mathrm{SO}_{4}\right)_{3}(a q)}+3 \mathrm{Zn}(s)$

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Question 23 Marks

On heating blue coloured powder of copper (II) nitrate in a boiling tube, copper oxide (black), oxygen gas and a brown gas X is formed.
i. Write a balanced chemical equation of the reaction.
ii. Identify the brown gas X evolved.
iii. Identify the type of reaction.
iv. What could be the pH range of the aqueous solution of the gas X ?

Answer

i. $\underset{\text { Copper (II) nitrate }}{2 \mathrm{Cu}\left(\mathrm{NO}_{3}\right)_{2}(\mathrm{~s})} \xrightarrow{\text { Heat }} \underset{\text { (Black) }}{2 \mathrm{Cu}} \mathrm{O}(\mathrm{s})+\underset{\text { (Brown) }(\mathrm{X})}{4 \mathrm{NO}_{2}(\mathrm{~g})}+\underset{\text { Oxygen }}{\mathrm{O}_{2}(\mathrm{~g})}$
ii. Nitrogen dioxide is the brown gas(X).
iii. Thermal decomposition reaction
iv. $\mathrm{pH}<7$ because $\mathrm{NO}_{2}$ dissolves in water to form acidic solution ( pH lies below 7).

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Question 33 Marks

Silver chloride kept in a china dish turns grey in sunlight.
a. Write the colour of silver chloride when it was kept in the china dish.
b. Name the type of chemical reaction taking place and write the chemical equation for the reaction.
c. State one use of the reaction. Name one more chemical which can be used for the same purpose.

Answer

a. White colour
b. The chemical reaction is as:
\[
2 \mathrm{AgCl}(\mathrm{~s}) \longrightarrow 2 \mathrm{Ag}(\mathrm{~s})+\mathrm{Cl}_{2}(\mathrm{~g})
\]
This is a decomposition chemical reaction.
c. One major application of decomposition reactions is in the extraction of metals from their ores. For example, zinc can be obtained from calamine by subjecting it to a decomposition reaction. In a similar manner, sodium can be obtained from sodium chloride ( NaCl ).

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