Exercise practice: The d- and f-Block Elements
The d- and f-Block Elements18 min readLesson 4 of 6
4.1 How the exercise set is arranged
The chapter has 30 exercises, numbered 4.1 to 4.30. They fall into four groups. Electronic configuration, metal character and general properties cover 4.1 to 4.8. Trends in size, ionisation enthalpy, oxidation states and standard potentials cover 4.9 to 4.17. Colour, magnetic moment and the d4 problems cover 4.18 to 4.25. Preparation and reactions of permanganate and dichromate, alloys, interstitial compounds and the lanthanoids and actinoids cover 4.26 to 4.30. The worked items below show the patterns that repeat.
4.2 Configurations and general properties
Exercises 4.1 to 4.4 ask for the electronic configuration of a transition element or its ion. Write the noble gas core, then fill 4s before 3d, and remember the two anomalies: chromium is 3d5 4s1 and copper is 3d10 4s1. Exercises 4.5 to 4.8 ask why transition metals are hard, why they have high melting points and why they form alloys. The answers rest on the participation of both ns and (n-1)d electrons in metallic bonding and on the similar sizes of the atoms.
4.3 Trends and standard potentials
Exercises 4.9 to 4.13 compare atomic and ionic radii. The radius falls across a series because nuclear charge grows while d electrons shield poorly, and the lanthanoid contraction makes the second and third series nearly equal in size. Exercises 4.14 to 4.17 work with standard potentials. Exercise 4.17 gives the values for the M2+/M and M3+/M2+ couples and asks about stability and ease of oxidation. A high positive M3+/M2+ value means the +3 state is easily reduced, so Mn3+ is a strong oxidiser, while the low Cr3+/Cr2+ value means Cr2+ is a strong reducer. Iron(III) is stable in acid because its reduction potential is moderate.
4.4 Colour and magnetic moment
Exercise 4.18 asks which of Ti3+, V3+, Cu+, Sc3+, Mn2+, Fe3+ and Co2+ are coloured. Write the d configuration of each: Ti3+ is d1, V3+ is d2, Mn2+ is d5, Fe3+ is d5 and Co2+ is d7, so all are coloured by d-d transitions. Cu+ is d10 and Sc3+ is d0, so both are colourless. Exercise 4.24 asks for the number of unpaired electrons in Mn3+, Cr3+, V3+ and Ti3+ and for the most stable ion in water. The configurations are d4, d3, d2 and d1, giving four, three, two and one unpaired electrons. Cr3+ is the most stable in water because its d3 configuration is half filled within the t2g set.
4.5 The two oxidising agents
Exercise 4.26 asks for the steps in preparing potassium dichromate from chromite ore and potassium permanganate from pyrolusite. For dichromate, chromite is fused with sodium carbonate and air to give sodium chromate, which is acidified to dichromate and then crystallised as the potassium salt. For permanganate, manganese dioxide is fused with potassium hydroxide and air to give potassium manganate, which is then oxidised to permanganate in acid or by chlorine. The ionic equations for the reactions of acidified permanganate with iron(II), oxalate and iodide should be written with the electrons balanced.
Exam
For every colour question, write the d configuration first. A d0 or d10 ion cannot have a d-d transition, and that single line usually carries the reason mark.
4.6 Two worked numericals
Exercise 4.18 in full. Decide the colour of each ion from its d count. Ti3+ is d1, V3+ is d2, Mn2+ is d5, Fe3+ is d5 and Co2+ is d7, so all have partly filled d subshells and are coloured. Cu+ is d10 and Sc3+ is d0, so both are colourless. The answer is complete only when the configuration of each ion is shown.
Exercise 4.24 in full. Mn3+ is d4 with four unpaired electrons, Cr3+ is d3 with three, V3+ is d2 with two and Ti3+ is d1 with one. The moments follow from the spin only formula. Among them Cr3+ is the most stable in aqueous solution because its d3 configuration fills the lower t2g set, so it has no electron to lose easily and no vacancy that strongly attracts a ligand.
4.7 Common mistakes in this chapter
Configurations are often written with 3d filled before 4s, which is right for the ground state of the ion but wrong for the atom. In potential-based questions, a high positive M3+/M2+ value means the +3 ion is easily reduced, so the metal in the +2 state is stable; the sign is easy to flip. For lanthanoid questions, do not say that all lanthanoids are radioactive; that statement belongs to the actinoids.
Exercise 4.17 in full. The table gives M2+/M and M3+/M2+ values. For Fe the M3+/M2+ value is +0.77 V, for Cr it is -0.41 V and for Mn it is +1.57 V. The high manganese value means Mn3+ is strongly oxidising and is easily reduced to Mn2+, so the +3 state of manganese is not stable in acid. The low chromium value means Cr2+ is easily oxidised to Cr3+, so Cr2+ is a reducing agent and Cr3+ is stable. Iron(III) is stable because its value lies between the two, so Fe3+ can exist in acid solution without being strongly oxidising or strongly reducing. The same table answers the ease of oxidation of the metals: a more negative M2+/M value means the metal is more easily oxidised.
A final tip for this chapter: reason questions are marked for the cause, not for the observation. Write the observation in one line, then give the configuration, trend or energy argument that produces it, and finish with the consequence. That three part structure covers the answer space in three or five mark questions.