(4) endothermic reaction 3. Iron(IID) ion (Fe) reacts with thiocyanate ion (SCN)

Question

(4) endothermic reaction 3. Iron(IID) ion (Fe) reacts with thiocyanate ion (SCN) to form a red complex ion with the formula IFeSCN)2. The net ionic equation describing this reaction is given in Equation 10 Fe (aq. It. yellow) +SCN (aq, colorless)FeSCN (aq, red) Eq. 10) A student studying this equilibrium begins with an equilibrium mixture that is light pink. (1) what change will the student observe when a solution containing Fe-ion is added to this mixture? (2) Briefly explain how your answer to (1) is consistent with Le Châtelier's principle (3) Silver ion (Ag) reacts with SCN ion to form silver thiocyanate (AgSCN). What change will the student observe when a solution containing Ag ion is added to the mixture? (4) Briefly explain how your answer to (3) is consistent with Le Châtelier's principl 56

Explanation / Answer

(4) Endothermic reaction : Reactions in which heat is absorbed by the system. dH is +ve.

3. For the reaction,

Fe3+ + SCN- --> [FeSCN]2+ (red complex)

If started with equilibrium reaction,

(1) When solution containing Fe3+ is added, the color of the solution would turn brighter. This is because more SCN- would react with added Fe3+ in solution to form the complex until all of added Fe3+ is consumed and equilibrium is resestablished. This is according to LeChatellier's principle.

(2) According to LeChatellier's principle when the concentration of one of the species in an equilibrium reaction is changed, the equilibrium is destroyed. The system would try to move in the direction such that it can nullify the induced change and try to regain equilibrium state again. therefore, in (1) after extra Fe3+ is added, the reaction shifts to right hand side and more of complex is formed.

(3) When Ag+ is added to the system, some of SCN- in solution would now react with Ag+ and forms AgSCN. Thereby reducing the concentration of SCN- on left handside of the equilibrium reaction. So the reaction will move towards reactant end, such that more of complex [FeSCN]2+ would dissolve to generate SCN- until equilibrium is reestablished.

(4) As explained above, the change in concentration of SCN- on reactant end disturbs equilibrium. Therefore, more SCN- is needed to form from the complex to reattain equilibrium state.

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