Activity . Take three iron nails and clean them by rubbing with sand paper. Take two test tubes marked as (A) and (B). In each test tube, take about mL copper sulphate solution.
Tie two iron nails with a thread and immerse them carefully in the copper sulphate solution in test tube B for about minutes [Fig. . (a)]. Keep one iron nail aside for comparison.
After minutes, take out the iron nails from the copper sulphate solution. Compare the intensity of the blue colour of copper sulphate solutions in test tubes (A) and (B) [Fig. . (b)].
Also, compare the colour of the iron nails dipped in the copper sulphate solution with the one kept aside [Fig. . (b)]. Take about g barium hydroxide in a test tube.
Add g of ammonium chloride and mix with the help of a glass rod. Touch the bottom of the test tube with your palm. What do you feel? Is this an exothermic or endothermic reaction?
Carry out the following Activity Silver bromide also behaves in the same way. 2AgBr(s) Sunlight 2Ag(s) + Br (g) ( . ) The above reactions are used in black and white photography. What form of energy is causing these decomposition reactions?
We have seen that the decomposition reactions require energy either in the form of heat, light or electricity for breaking down the reactants. Reactions in which energy is absorbed are known as endothermic reactions. Why does the iron nail become brownish in colour and the blue colour of copper sulphate solution fades? The following chemical reaction takes place in this Activity– Fe(s) + CuSO (aq) → FeSO (aq) + Cu(s) ( .
) (Copper sulphate) (Iron sulphate) In this reaction, iron has displaced or removed another element, copper, from copper sulphate solution. This reaction is known as displacement reaction. Other examples of displacement reactions are Zn(s) + CuSO (aq) → ZnSO (aq) + Cu(s) ( . ) (Copper sulphate) (Zinc sulphate) Pb(s) + CuCl (aq) → PbCl (aq) + Cu(s) ( .
) (Copper chloride) (Lead chloride) Zinc and lead are more reactive elements than copper. They displace copper from its compounds.