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Explain effect of concentration on equilibrium by suitable experiments. underset"Yellow"(Fe_((aq))^(3+))+ underset"Colourless"(SCbarN_((aq))) hArr underset"Deep red"([Fe(SCN)]_((aq))^(2+)) Explain effect by added, (i) Oxalic acid (H_2C_2O_4) (ii) HgCl_2 and (iii) Potassium thiocynate (KSCN) in equilibrium reaction. |
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Answer» Solution :Experiment (Step-1): A reddish colour appears on adding two drops of 0.002 M potassium thiocynate solution to 1 mL of 0.2 M IRON(III) nitrate solution due to the formation of `[Fe(SCN)]^(2+)`. The intensity of the red colour becomes constant on attaining equilibrium. `underset"Yellow"(Fe_((AQ))^(3+)) + underset"Colourless"(SCbarN_((aq))) hArr underset"Deep red"([Fe(SCN)]_((aq))^(2+))` and At equilibrium `K_c=([Fe(SCN)_((aq))^(2+)])/([Fe_((aq))^(3+)]+[SCbarN_((aq))])` This equilibrium can be shifted in either FORWARD or reverse directions depending on our choice of adding a reactant or a product. Experiment (Step-2): Adding of oxalic acid : Oxalic acid `[H_2C_2O_4]` reacts with `Fe^(3+)` ions to form the stable complex ion `[Fe(C_2O_4)_3]^(3-)` , thus decreasing the concentration of free `Fe^(3+)` `Fe^(3+) + 3C_2O_4^(2-) to [Fe(C_2O_4)]^(3-)` Such `Fe^(3+)` ion is USED and there is disturbance in equilibrium, `[Fe^(3+)]`decreases. In ACCORDANCE with the Le-Chatelier.s principle, the concentration stress of removed `Fe^(3+)` is relieved by dissociation of `[Fe(SCN)]^(2+)` to replenish the `Fe^(3+)` ions. Because the concentration of `[Fe(SCN)]^(2+)` decreases, the intensity of red colour decreases. Experiment (Step-3) : `HgCl_2` is added in equilibrium of Step-1 : Addition of `HgCl_2` also decreases red colour because `Hg^(2+)` reacts with `(SCN^-)` ions to form stable complex ion `[Hg(SCN)_4]^(2-)` and `HgCl_(2(aq)) hArr Hg_((aq))^(2+) + 2Cl_((aq))^(-)` `Hg_((aq))^(2+) + 4SCN^(-) to [Hg(SCN)_4]_((aq))^(2-)` Thus by addition of `HgCl_2SCN^-` in equilibrium decreases so the amount of product decreases. So the reaction moves in forward reaction and intensity of red colour decreases. Experiment (Step-4): If KSCN is added to solution than `[SCN^-]` increases so it consume in reaction and reaction proceed in reverse direction. `Fe_((aq))^(3+) +SCN_((aq))^(-) to [Fe(SCN)]^(2-)` and `SCN^-`decreases and new equilibrium is established. At new `[Fe(SCN)]^(2-)` equilibrium concentration of red colour is more. |
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