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Iron (III) thiocyanate [Fe(SCN)_3] dissolves readily in water to give a red solution. The red color of the solution deepens when________is added. (i) oxalic acid (H_2C_2O_4) (ii) sodium thiocyanate (NaSCN) (iii) iron (III) nitrate [Fe(NO_3)_3] (iv) mercuric chloride (HgCl_2) |
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Answer» (i), (ii), (iii) `[Fe(SCN)_3]rarr FeSCN^(2+)+2SCN^(-)` The equilibrium between undissociated `FeSCN^(2+)` and the `Fe^(3+)` and `SCN^(-)` ions is given by `{:(FeSCN^(2+)(AQ.),hArr,Fe^(3+)(aq.),+,SCN^(-)(aq.)),(Red,,"Pale-yellow",,"Colorless"):}` When we add some SODIUM thiocyanate `(NaSCN)` to the solution, the STRESS applied to the equilibrium system is an increase in the concentration of `SCN^(-)` (form the dissociation of `NaSCN`).To offest this stress, some `Fe^(3+)` ions react with the added `SCN^(-)` ions, and the equilibrium shifts form right to left. `FeSCN^(2+)(aq.)]larr Fe^(3+)(aq.)+SCN^(-)(aq.)` CONSEQUENTLY, the red colour of the solution deepens. Simillarly, if we add iron (III) nitrate `[Fe(NO_(3))_(3)]` to the original solution, the red color would also deepen because the additional `Fe^(3+)` ions `["form Fe(NO_(3))_(3)]` will shift the equilibrium form right to left. Note that both `Na^(+)` and `NO_(3)^(-)` are colorless spectator ions.When we add some oxialic acid `(H_(2)C_(2)O_(4))` to the original solultion, it ionizes in water to form the cxalate ion, `C_(2)O_(4)^(2-)`, which binds strongly to the `Fe^(3+)` ions. The formation of the stable yellow complex ion `Fe(C_(2)O_(4))_(3)^(3-)` removes free dissociate (to relieve the concentration stress of removed `Fe^(3+)` ions) to replenish the `Fe^(3+)` ions and the equilibrium shifts form left to right. `FeSCN^(2+)(aq.)]rarr Fe^(3+)(aq.)+SCN^(-)(aq.)` The intensity of red color decreases (due to decrease of concentration of `FeCN^(2+)`) and the red solultion turns yellow due to the FOMATION of `Fe(C_(2)O_(4))_(3)^(3-)` ion.The addition of `HgCI_(2)` (aq.) also decreases red solor because the `Hg^(2+)` ions react with `SCN^(-)` ions to form stable complex ion `[Hg(SCN)_(4)]^(2-)`. The removal of free `SCN^(-)` (aq.) shifts the equilibrium form left to right to replenish `SCN^(-)` ions.This experiment demostrates that at equilibrium, all reactants and products are present in the reacting system. Second, increasing the equilibrium to the products `(Fe^(3+)` or `SCN^(-))` shift the equilibrium to the left, and decreasing the concentration of the products `(Fe^(3+) or SCN^(-))` shift the equilibrium to the right. These results are just as predicted by Le Chatelier's principle. Note that oxialic acid is sometimes used to remive bathtub ring that consists of rust or `Fe_(2)O_(3)`. |
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