Many aqueous solutions of complex ions display brilliant colors that depend on the identities of the metal ion and ligand(s). Some ligands bind selectively to certain metal ions and produce a complex ion with characteristic colors. These distinctive complex ions serve as qualitative indicators of the presence of particular metal ions. For example, \(\mathrm{Fe}^{3+}\) is identified by the rapid formation of the intensely colored pentaaquathiocyanatoiron(III) complex ion, \(\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{5} \mathrm{SCN}\right]^{2+}\), when thiocyanate, \(\mathrm{SCN}^{-}\), is added to a solution containing hexaaquairon(III), \(\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}\), according to the balanced chemical equation shown here:
\(\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}(a q)+\mathrm{SCN}^{-}(a q) \rightleftharpoons\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{5} \mathrm{SCN}\right]^{2+}\)
pale violet colorless intensely colored
Examine the absorption spectrum of an aqueous solution of \(\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{5} \mathrm{SCN}\right]^{2+}\) shown here and answer the questions.
a. Based on the spectrum, what is the color of an \(\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{5} \mathrm{SCN}\right]^{2+}\) solution?
b. Calculate the crystal field splitting energy,\(\Delta\), of \(\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{5} \mathrm{SCN}\right]^{2+}\) in kJ/mol.
c. The hexaaquairon(III) complex ion, \(\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}\), produces a pale violet aqueous solution. Is the crystal field splitting energy, \(\Delta\), of \(\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}\) smaller or larger than the \(\Delta\) of \(\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{5} \mathrm{SCN}\right]^{2+}\)?
d. On the basis of your answers to parts b and c, compare the crystal field strengths of water and thiocyanate ligands.
e. The complex ion hexacyanoferrate(III), \(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{3-}\), is red in aqueous solution. What can you conclude about the relative crystal field splitting energies of \(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{3-}\) and \(\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{5} \mathrm{SCN}\right]^{2+}\)?
Text Transcription:
Fe^3+
[Fe(H_2O)_5SCN]^2+
SCN^-
[Fe(H_2O)_6]^3+
[Fe (H_2O)_6]^3+(aq) SCN^-(aq)rightleftharpoons[Fe(H_2O)_6]^3+
[Fe(CN)_6]^3-
delta
Chapter 2: 9/6/2016 Cation and anions Monatomic and polyatomic ions Ionic compounds - Recognizing them and - building a balanced molecular formulas give the exact number of atoms of each element in a compound empirical formulas give the lowest whole number ration of atoms of each element in a compound structural formulas show the order in which atoms are bonded (the connectivity) Oxidation Numbers: the atomic charge of a cation or anion in an ionic compound is also called its oxidation number for a molecular compound the atoms don’t have charges like in ionic compounds 1. pure elements are assigned zero for their oxidation number 2. ions have the oxidation number of their common charge anion- ide cation- ate 3. common oxidation numbers of H, F, and O o Hydrogen (H): i. +1 when bonded to nonmetal ii. -1 when bonded to a metal o Fluorine (F) is always -1 o Oxygen (O): iii. -2 in almost all compounds iv. -1 in peroxides v. Exception: +2 in OF2 4. Compounds and polyatomic ions o in a neutral compound, the sum of the oxidation numbers of all elements in the formula must equal zero o in polyatomic ion the sum of the oxidation numbers of all elements in the formula must equal the net charge on the ion o in general the more electronegative Acids and Bases - hydrogen cation (H+) also called the proton - hydroxide anion (OH-) Acid: substance that produces H+ in
