Identify the type of reactions on the basis of following observations:
(i) Black lead sulphide is heated in pressure of O₂ and turns into white lead sulphate.
(ii) On heating orange colored ammonium dichromate, chromium oxide is produced along with nitrogen and water vapour.
(iii) In a test tube filled with dilute sulphuric acid, adding small Mg ribbon piece results in brisk effervescence & evolution of hydrogen.
(iv) Addition of dilute hydrochloric acid to silver nitrate solution results in formation of white curdy precipitate.
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Naming the type of reactions :
(i) Black lead sulphide is heated in pressure of O₂ and turns into white lead sulphate.
• PbS (black) + 2O₂ → PbSO₄ (white)
This is an example of redox reaction, in which both oxidation and reduction are taking place.
• PbS combines with oxygen and gets oxidised to PbSO₄, whereas, oxygen itself gets reduced to PbSO₄.
(ii) On heating orange colored ammonium dichromate, chromium oxide is produced along with nitrogen and water vapour.
• (NH₄)₂Cr₂O₇ + ∆ → Cr₂O₃ + 2N₂ + 4H₂O
This is an example of decomposition reaction.
• Ammonium dichromate breaks down or decomposes into its smaller constituents on application of heat.
(iii) In a test tube filled with dilute sulphuric acid, adding a small Mg ribbon piece results in brisk effervescence & evolution of hydrogen.
• Mg + dil. H₂SO₄ → MgSO₄ + H₂
This is an example of displacement reaction.
• In this reaction, Mg being more reactive than H₂, displaces it from sulphuric acid vigorously and takes its place.
(iv) Addition of dilute hydrochloric acid to silver nitrate solution results in the formation of a curdy white precipitate.
• AgNO₃ + dil. HCl → AgCl (white ppt.) + HNO₃
This is an example of double-displacement precipitation reaction.
• In this reaction, both AgNO₃ and HCl displace each other from their compounds and exchange their positions.
• A white precipitate of AgCl is formed in the reaction. So, it is called a double-displacement precipitation reaction.