Question

lines provided. A few drops of NaCkeg) solution is deep yellow are added to a solution containing Fe3. The resulting to a colour Predict the effect of adding NaF㈣ to this deep solution,The win color. Equilibrium #1 +6 F() Feagan pale yellow Fe"(aa) pale yellow colourless Equilibrium #2 4 Cl(a) deep yellow Write your answer in detail, and clearly explain: (1 mark) a) why the solution is initially deep yellow b) how each equilibrium will shift when NaF is added (and why) (2 marks) c) what changes in concentration will occur (for FeFe, Cr, and FeCls ions) and a brief explanation for each (3 marks) what you will observe (changes that you can see, feel, etc.) (1 mark)

Answer 1

Given,

Equilibrium#1 Fe3+ (pale yellow) + 6F- <==> FeF6^3- (colorless)

Equilibrium#2 Fe3+ (pale yellow) + 6Cl- <==> FeF4^- (deep yellow)

when NaF was added to the solution,

a) Initial Fe3+ can undergo d-d transition and hence is pale yellow is color. It has 3d^5 valence electron configuration and has unpaired electrons which can undergo d-d transition readily.

b) When NaF is added to equilibrium#1, it would shift the equilibrium to the right handside, so more of FeF6^3- would be formed.

When NaF is added to equilibrium#2, Fe3+ present in solution will react with added F- to form FeF6^3-. the reaction will shift to the right handside. This would continue until all of F- is consumed in the reaction and equilibrium is reestablished again.

c) according to explanation in b), FeF6^3- will increase, Fe3+ would decrease and Cl- would remain the same, FeCl4^- would remain the same since we are only reacting F- with free Fe3+.

d) we would see color of solution in equilibrium#2 changing to pale color after addition of NaF due to formation of more FeF6^3-.