The D-and F-Block ElementsClass 12 Chemistry Case Study Questions

Potassium dichromate, $\text{K}_2\text{Cr}_2\text{O}_7$, is a staple in chemistry labs, known for its vibrant orange color and powerful oxidizing properties. It is prepared from chromite ore ($\text{FeCr}_2\text{O}_4$). The process involves roasting the ore with sodium carbonate in the presence of air to form sodium chromate ($\text{Na}_2\text{CrO}_4$), which is yellow. The yellow solution is then filtered and acidified with sulfuric acid. This acidification causes a distinct color change to orange as the chromate ions are converted into dichromate ions.\n\nA chemist observes that the equilibrium between the yellow chromate ion ($\text{CrO}_4^{2-}$) and the orange dichromate ion ($\text{Cr}_2\text{O}_7^{2-}$) is highly sensitive to the pH of the solution. In acidic solutions, the dichromate ion predominates, making it a strong oxidizing agent used in volumetric analysis, for instance, in the titration of iron(II) salts. In this reaction, the orange solution turns green.

Modern cars are equipped with a catalytic converter, a device that plays a crucial role in reducing air pollution. This device uses a ceramic honeycomb structure coated with a combination of d-block elements, typically platinum (Pt), palladium (Pd), and rhodium (Rh). As hot exhaust gases from the engine pass through the converter, these metals act as catalysts, facilitating chemical reactions that convert toxic pollutants into less harmful substances.

The transition metals are particularly effective due to their ability to adsorb reactant molecules onto their surface and provide a pathway with lower activation energy. For instance, rhodium catalyzes the reduction of nitrogen oxides ($\text{NO}_x$) to harmless nitrogen gas ($\text{N}_2$), while platinum and palladium catalyze the oxidation of unburnt hydrocarbons and carbon monoxide (CO) to carbon dioxide ($\text{CO}_2$) and water. This efficiency is linked to the characteristic properties of transition metals, including their variable oxidation states and the presence of vacant d-orbitals.

A geologist sent a sample of zircon ore to a chemistry lab for analysis. The chief chemist, Dr. Anya Sharma, reported that the ore contained a significant amount of zirconium (Zr). However, her report also noted the presence of a small but persistent amount of hafnium (Hf), which was proving exceptionally difficult and expensive to separate from the zirconium.

Dr. Sharma explained to the geologist, 'The problem lies deep within the periodic table. Hafnium follows the lanthanoid series. The 14 elements before it, from cerium to lutetium, involve the filling of the $4f$ orbitals. This filling leads to a steady decrease in atomic size across the series, a phenomenon we call the lanthanoid contraction.' She concluded, 'The cumulative effect of this contraction almost perfectly cancels out the expected increase in size from one period to the next. As a result, zirconium (from the 4d series) and hafnium (from the 5d series) end up with nearly identical atomic radii and, consequently, very similar chemical properties, making them chemical twins.'