6.P: Determination of Kc for a Complex Ion Formation (Pre-Lab)
- Page ID
- 127156
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A student mixes 5.0 mL of 0.00200 M \(\ce{Fe(NO3)3}\) with 5.0 mL 0.00200 M \(\ce{KSCN}\). She finds that the concentration of \(\ce{FeSCN^{2+}}\) in the equilibrium mixture is 0.000125 M. Follow these steps to determine the corresponding experimental value of \(K_{c}\) for the reaction of \(\ce{Fe^{3+}}\) and \(\ce{SCN^{-}}\) to produce this complex ion. Show your calculations for each step below and then place the appropriate value(s) in the equilibrium (or 'ICE') table near the bottom of the page.
- Step 1. Calculate the molarity of \(\ce{Fe^{3+}}\), \(\ce{SCN^{-}}\), and \(\ce{FeSCN^{2+}}\) initially present after mixing the two solutions, but prior to any reaction taking place. (\(M_{1}V_{1} = M_{2}V_{2}\))
- Step 2. Determine the expression and initial value for \(Q_{c}\). Then give the appropriate signs of the concentration changes for each species in terms of the reaction's shift, or \(x\), into the 'ICE' table.
- Step 3. Fill in the equilibrium value for the molarity of \(\ce{FeSCN^{2+}}\). From this, you can determine the value of \(x\).
- Step 4. Given the value of \(x\), determine the equilibrium molarities of \(\ce{Fe^{3+}}\) and \(\ce{SCN^{-}}\).
| \(\ce{Fe^{3+}}\) (aq) | \(+ \quad \ce{SCN^{-} (aq)}\) | \(\ce{ <=>\quad FeSCN^{2+} (aq)}\) | |
|---|---|---|---|
| I | |||
| C | |||
| E |
- Step 5. Give the correct expression for \(K_{c}\) for this equation. Then calculate the value of \(K_{c}\) for the reaction from the equilibrium concentrations. Use correct significant figures.
- Step 6. On the reverse side, complete an 'ICE' table using this same procedure, but using a different reaction stoichiometry: \(\ce{Fe^{3+} + 2 SCN^{-} <=> Fe(SCN)2^{2+}}\) (ignore that fact that this is incorrect since the reaction is not balanced with respect to charge). Assume that the equilibrium concentration of \(\ce{FeSCN^{2+}}\) is 0.0000625 M, or one-half its previous value. Remember how the reaction stoichiometry affects the expression for \(K_{c}\).