6.P: Determination of Kc for a Complex Ion Formation (Pre-Lab)
- Page ID
- 127156
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)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 \(K_{c}\).