IB DP Chemistry Topic 13.2 Coloured complexes HL Paper 2

Question

(i) Deduce, giving a reason, which complex ion [Cr(CN6) ]3 or [Cr(OH6) ]3 absorbs higher energy light. Use section 15 of the data booklet.  [1]

(ii) [Cr(OH6) ]3- forms a green solution. Estimate a wavelength of light absorbed by this complex, using section 17 of the data booklet. [1]

Answer/Explanation

Ans

iii

[Cr(CN)6] 3− AND CN− /ligand causes larger splitting «in d-orbitals compared to OH»

OR [Cr(CN)6]3− AND CN− /ligand associated with a higher Δ/«crystal field» splitting energy/energy difference «in the spectrochemical series compared to OH »

 Accept “[Cr(CN)6] 3− AND «CN− » strong field ligand”.

iv any value or range between 647 and 700 nm 

Question

Brass is a copper containing alloy with many uses. An analysis is carried out to determine the percentage of copper present in three identical samples of brass. The reactions involved in this analysis are shown below.

\[\begin{array}{*{20}{l}} {{\text{Step 1: Cu(s)}} + {\text{2HN}}{{\text{O}}_3}{\text{(aq)}} + {\text{2}}{{\text{H}}^ + }{\text{(aq)}} \to {\text{C}}{{\text{u}}^{2 + }}{\text{(aq)}} + {\text{2N}}{{\text{O}}_2}{\text{(g)}} + {\text{2}}{{\text{H}}_2}{\text{O(l)}}} \\ {{\text{Step 2: 4}}{{\text{I}}^ – }{\text{(aq)}} + {\text{2C}}{{\text{u}}^{2 + }}{\text{(aq)}} \to {\text{2CuI(s)}} + {{\text{I}}_2}{\text{(aq)}}} \\ {{\text{Step 3: }}{{\text{I}}_2}{\text{(aq)}} + {\text{2}}{{\text{S}}_2}{\text{O}}_3^{2 – }{\text{(aq)}} \to {\text{2}}{{\text{I}}^ – }{\text{(aq)}} + {{\text{S}}_4}{\text{O}}_6^{2 – }{\text{(aq)}}} \end{array}\]

In step 1 the copper reacts to form a blue solution.

State the full electronic configuration of \({\text{C}}{{\text{u}}^{2 + }}\).[1]

c.i.

Explain why the copper solution is coloured.[2]

c.ii.
Answer/Explanation

Markscheme

\({\text{1}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{p}}^{\text{6}}}{\text{3}}{{\text{s}}^{\text{2}}}{\text{3}}{{\text{p}}^{\text{6}}}{\text{3}}{{\text{d}}^{\text{9}}}\);

Do not allow [Ar]3d9.

c.i.

d orbitals are split;

(3d) electrons move between orbitals and absorb light/energy / complementary colour is transmitted when energy absorbed by d electrons moving / unpaired d electrons move between the different orbitals;

Accept levels instead of orbitals.

c.ii.

Examiners report

Several errors were seen in the electron configuration, the commonest of which was to give that of elemental copper.

c.i.

Few attempts at the explanation of colour referred to the splitting of the d orbitals and electron transitions, and in several instances candidates referred to emission instead of absorption. This proved to be the most difficult part of question 1.

c.ii.

Question

Iron has three main naturally occurring isotopes which can be investigated using a mass spectrometer.

State the full electronic configurations of a Cu atom and a \({\text{C}}{{\text{u}}^ + }\) ion.

Cu:

\({\text{C}}{{\text{u}}^ + }\):[2]

d.

Explain the origin of colour in transition metal complexes and use your explanation to suggest why copper(II) sulfate, CuSO4(aq), is blue, but zinc sulfate, ZnSO4(aq), is colourless.[4]

e.

\({\text{C}}{{\text{u}}^{2 + }}{\text{(aq)}}\) reacts with ammonia to form the complex ion \({{\text{[Cu(N}}{{\text{H}}_{\text{3}}}{{\text{)}}_{\text{4}}}{\text{]}}^{2 + }}\). Explain this reaction in terms of an acid-base theory, and outline how the bond is formed between \({\text{C}}{{\text{u}}^{2 + }}\) and \({\text{N}}{{\text{H}}_{\text{3}}}\).[3]

f.
Answer/Explanation

Markscheme

Cu:

\({\text{1}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{p}}^{\text{6}}}{\text{3}}{{\text{s}}^{\text{2}}}{\text{3}}{{\text{p}}^{\text{6}}}{\text{3}}{{\text{d}}^{{\text{10}}}}{\text{4}}{{\text{s}}^{\text{1}}}\);

\({\text{C}}{{\text{u}}^ + }\):

\({\text{1}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{p}}^{\text{6}}}{\text{3}}{{\text{s}}^{\text{2}}}{\text{3}}{{\text{p}}^{\text{6}}}{\text{3}}{{\text{d}}^{{\text{10}}}}\);

Ignore relative order of 3d and 4s.

Penalize only once if noble gas core is given.

d.

d orbitals are split (into two sets of different energies);

frequencies of (visible) light absorbed by electrons moving from lower to higher d levels;

colour due to remaining frequencies/complementary colour transmitted;

\({\text{C}}{{\text{u}}^{2 + }}\) has unpaired electrons/partially filled d sub-level;

\({\text{Z}}{{\text{n}}^{2 + }}\) has filled d sub-shell;

electronic transitions/d-d transitions possible for \({\text{C}}{{\text{u}}^{2 + }}\) / no electronic/d-d transitions possible for \({\text{Z}}{{\text{n}}^{2 + }}\);

Allow wavelength as well as frequency.

e.

\({\text{N}}{{\text{H}}_{\text{3}}}\): Lewis base / \({\text{C}}{{\text{u}}^{2 + }}\): Lewis acid;

each \({\text{N}}{{\text{H}}_{\text{3}}}\)/ligand donates an electron pair (to \({\text{C}}{{\text{u}}^{2 + }}\));

\({\text{N}}{{\text{H}}_{\text{3}}}\) replace \({{\text{H}}_2}{\text{O}}\) ligands around \({\text{C}}{{\text{u}}^{2 + }}\) ion/around central ion;

forming coordinate (covalent)/dative covalent bond;

f.

Examiners report

Many candidates identified the electronic configuration of Cu as an exception but the 3d electron was often removed in forming the ion instead of the 4s.

d.

Precision of language proved to be an issue in (e) with some candidates referring to Cu and Zn and not their ions and some students explained the colour as a result of “reflection” or “emission”.

e.

In (f), many candidates mentioned proton donors and proton acceptors and made no reference to Lewis theory.

f.
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