When solutions containing Fe3+(aq) ions and SCN-(aq) ions are mixed together, th

Question

When solutions containing Fe3+(aq) ions and SCN-(aq) ions are mixed together, the following equilibrium is established:

Fe3+(aq) + SCN-(aq) <-> FeSCN2+(aq)

At equilibrium at some temperature, in 3.0 liters of total solution, there are 0.653 moles of FeSCN2+(aq), 0.0385 moles of Fe3+(aq), and 0.0465 moles of SCN-(aq).

a) Calculate the value of the equilibrium constant, K, for the reaction at this temperature.

b) An inquiring student pours another liter of water into the beaker holding the solution described above. She notices that the number of moles of Fe3+ and SCN- are then seen to increase, and the number of moles of FeSCN2+ decreases. Explain this observation.

Explanation / Answer

A)
[Fe3+]=0.0385/3=0.0128 M
[scn-]=0.0465/3=0.0155 M
[FeSCN+]=0.653/3=0.2177 M
K=[FeSCN+]/[Fe3+][scn-]
   =0.2177/(0.0128*0.0155)=1097.28 M^-1
B)
When 4 litres of water is there.
[Fe3+]=0.0385/4=0.009625 M
[scn-]=0.0465/4=0.011625 M
[FeSCN+]=0.653/4=0.16325 M
Q=[FeSCN+]/[Fe3+][scn-]
   =0.16325/(0.009625*0.011625)=1184.19 M^-1
since Q>K
Reaction will shift in reverse direction.
Thus increasing mole of Fe3+ and SCN- and decreasing that of FeSCN2+

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