Chemical equation and reaction 5
Samuel had a silver coin which turned black. He kept the coin in a bowl lined with aluminium foil. Then he filled the bowl with water and boiled it. After sometime, he found that the coin has become new. Its blackness disappeared. How did it happen?
The blackness of silver coin is due to the formation of silver sulphide on its surface due to its exposure to air. On boiling, the aluminium foil reacts with the layer of silver sulphide and displaces silver from silver sulphide to form aluminium sulphide and silver. This makes the coin shiny.
3Ag2S + 2Al → 6Ag + Al2S3
During electrolysis of water a few drops of sulphuric acid is added into water. Why?
Pure water is a bad conductor of electricity. By adding a few drops of sulphuric acid, it becomes a good conductor of electricity.
Justify with the help of an example that displacement reaction is also a redox reaction.
Consider the following displacement reaction in which Fe displaces Cu from CuSO4 to form FeSO4:
Fe(s)Iron+CuSO4(aq)Coppersulphate(Blue)→FeSO4(aq)Iron(II)sulphate(Green)+Cu(s)Copper(Reddishbrown)
In the above reaction, Fe is gaining oxygen. Hence, Fe is oxidised. CuSO4 is loosing oxygen. Hence, it is reduced. So, it is a redox reaction.
Compound ‘A’ when dissolved in water gives compound ‘B’ which is used in whitewashing. Compound ‘B’ reacts with CO2 to form a white precipitate of compound ‘C’. Identify compounds ‘A’, ‘B’ and ‘C’. Also write the equations involved.
'A'−→−−Exothermicreaction+H2O'B'Usedforwhitewashing+CO2→'C'Whiteppt
Slaked lime [(Ca(OH)2] is used for whitewashing. It is obtained when quicklime, CaO reacts with water.
CaOQuicklime+H2O→Ca(OH)2Calciumhydroxide(Slakedlime)+Heat
So ‘A’ is CaO and ‘B’ is Ca(OH)2.
Ca(OH)2(aq)+CO2(g)→CaCO3(s)Calciumcarbonate(Whiteppt.)+H2O(l)
‘C’ is CaCO3.
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.
Write a balanced chemical equation of the reaction.
Identify the brown gas X evolved.
Identify the type of reaction.
What could be the pH range of aqueous solution of the gas X?
[NCERT Exemplar]
2Cu(NO3)2(s) Heat →Heat 2CuO(s) + O2(g) + 4NO2(g)
The brown gas X evolved is nitrogen dioxide (NO2).
This is a decomposition reaction.
Nitrogen dioxide dissolves in water to form acidic solution because it is an oxide of non-metal. Therefore, pH of this solution is less than 7.
On adding a drop of barium chloride solution to an aqueous solution of sodium sulphite, white precipitate is obtained.
Write a balanced chemical equation of the reaction involved.
What other name can be given to this precipitation reaction?
On adding dilute hydrochloric acid to the reaction mixture, white precipitate disappears. Why?
[NCERT Exemplar]
Na2 SO3 (aq) + BaCl2 (aq) → BaSO3 (s) + 2NaCl(aq)
Sodium sulphite Barium sulphite Barium sulphite Sodium chloride
(White precipitate)
This reaction is also known as double displacement reaction.
BaSO3 is a salt of a weak acid (H2SO3), therefore dilute acid such as HCl decomposes barium sulphite to produce sulphur dioxide gas which has the smell of burning sulphur.
BaSO3(s) + 2HCl(aq) → BaCl2(aq) + H2O(l) + SO2(g)
White ppt.
BaCl2 is soluble in water, hence white precipitate disappears.
A brown substance ‘X’ on heating in air forms a substance ‘Y’. When hydrogen gas is passed over heated ‘Y’, it again changes back into ‘X’.
Name the substance ‘X’ and ‘Y’.
Name the type of chemical reactions occurring during both the changes.
Write the chemical equations of the reactions.
[CCE 2016]
X—Cu ; Y—CuO.
When copper is heated in air, oxidation takes place. When hydrogen gas is passed over heated copper oxide, reduction takes place.
2Cu + O2 →Δ 2CuO (Oxidation)
CuO + H2 →Δ Cu + H2O (Reduction)
