Answer the questions.[Che-3M]

Question 13 Marks

A zinc plate was kept in a glass container having copper sulphate solution. On examining it was found that the blue colour of the solution is fading slowly. After a few days when the zinc plate was taken out of the solution, a number of small holes were noticed in it. State the reason and give chemical equation of the reaction involved.

Answer

Zinc is more reactive than copper. Hence, when a zinc plate is kept in a solution of copper sulphate, it slowly displaces copper from the solution and blue colour of the solution keeps fading away. Because of zinc going into solution as zinc sulphate, a number of holes are seen in the zinc plate. The reaction is$
\underset{\text { Blue }}{\mathrm{CuSO}_{4}(a q)}+\mathrm{Zn}(s) \rightarrow \underset{\text { Colourless }}{\mathrm{ZnSO}_{4}(a q)}+\mathrm{Cu}(s)
$

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Question 23 Marks

What happens when dilute hydrochloric acid is added to iron filings?

Answer

Hydrogen gas and Iron chloride are produced.
$2 \mathrm{HCl}(\mathrm{aq})+\mathrm{Fe}(\mathrm{s}) \rightarrow \mathrm{FeCl}_{2}(\mathrm{aq})+\mathrm{H}_{2}(\mathrm{~g})$
This is a redox reaction
$\mathrm{Fe}(0)-2 \mathrm{e}^{-} \rightarrow \mathrm{Fe}(\mathrm{II})$ oxidation loss of electrons
$2 \mathrm{H}^{+}+2 \mathrm{e}^{-} \rightarrow \mathrm{H}_{2}$ reduction: gain of electrons
So it is certainly a chemical reaction: bonds are broken and made.
HCl is not a sufficiently strong oxidizing agent to produce $\mathrm{FeCl}_{3}$ (need Cl ₂ ).

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Question 33 Marks

i. Why are the chips packets puffed when we buy them from market?
ii. Paint is applied on articles made up of iron, why?

Answer

i. Cooking oil gets oxidized when comes in contact with air and gives a bad smell or turns "rancid". The oxygen present in the chips packet is replaced by flushing nitrogen in the packet, this in turn puffs up the packet.
ii. Iron easily reacts with atmospheric oxygen in presence of slight moisture to give iron oxide. In this process of 'corrosion', the outer layer of iron is oxidized and hence sacrificed leading to the shaping of the iron article. To avoid this oxidation, the article is coated with paint so as to make a barrier between the article's surface and atmosphere.

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Science Answer the questions.[Che-3M]

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