3. The concentration of FeNCS 2+ in each of the Standard Solutions at equilibriu

Question

3. The concentration of FeNCS 2+ in each of the Standard Solutions at equilibrium is taken to be the same as the initial concentration of KSCN in the solution. Explain the rational behind equating the initial concentration of KSCN to that of [FeNCS2+]

In addition, the ultimate goal is to determine the equilibrium constant of the iron (III) /thiocynate syste. How does the preparation and use of the Standard Solution in Part 1 contribute to this larger goal? (What data will Part 1 give you that will help you in the overall analysis?

Explanation / Answer

The reaction in consideration here is,

Fe^3+(aq) + SCN^-(aq) <===> Fe(SCN)^2+(aq)

So, as we can see above 1 mole of SCN^- or KSCN would result in the formation of 1 mole of Fe(SCN)^2+. Thus, the concentration of Fe(SCN^2+ = KSCN in solution.

Typical reaction of this type consists of a large excess of Fe^3+ in solution to form the complex.

Now, Keq for the reaction,

Keq = [Fe(SCN)^2+]eq/[Fe^3+]eq.[SCN^-]eq

To get solutions with known [FeSCN^2+] standard solutions are prepared by mixing carefully measured volumes of solutions of Fe3+ using Fe(NO3)3 stock solution and SCN^– using KSCN standard stock solution of known concentrations. The key to getting a known concentration of FeSCN^2+ in each of these solutions is that the initial concentration of Fe^3+ is much greater than the initial concentration of SCN^– ion. When the Fe^3+ concentration is in large excess, the equilibrium will shift (according to LeChatelier’s Principle) to the product side until virtually all the SCN^– is converted to FeSCN^2+. Thus the equilibrium concentration of FeSCN^2+ in a standard solution will be virtually the same as the initial concentration of SCN^– in the solution. This initial value, [SCN^–] initial, which equals to [FeSCN^2+] in the mixed standard solution, can be calculated from the volume and molarity of the SCN^– of standard stock solution (mol SCN^– = Molarity x Volume) and the final total volume (in L) of the mixed standard solution.

This is then used for the Keq calculation as follows,

the concentration of [Fe^3+] remaining would be,

[Fe^3+]initial - [Fe(SCN)^2+]eq = [Fe3+]eq.

Note, for every Fe(SCN)^2+ formed one Fe^3+ is used.

similarly concentration of [SCN^-] at equilibrium would be,

[SCN^-]eq = [SCN^-]initial - [Fe(SCN)^2+]eq

For every [Fe(SCN)^2+] formed 1 [SCN^-] is used.

These values are then fed into the Keq equation,

Keq = [Fe(SCN)^2+]eq/[Fe^3+]eq.[SCN^-]eq

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