Chemistry 2014 Jamb Examination

When ethyne is passed through a hot tube containing a nickel catalyst, a polymerisation and cyclisation process occurs. Ethyne molecules combine to form benzene through a three-step reaction involving intermediate compounds such as but-1,3-diyne and hex-1,3,5-triyne. The nickel catalyst facilitates the cyclisation and hydrogen rearrangement, converting linear carbon chains into an aromatic ring structure. This reaction demonstrates how simple alkyne molecules can form stable aromatic hydrocarbons under catalytic conditions.

Ethyne (acetylene) is highly unstable under pressure and can explosively decompose into carbon and hydrogen. To safely store it, it is dissolved in propanone (acetone), which acts as a stabilising solvent. The dissolution reduces the risk of decomposition by dispersing the molecules evenly, preventing the build-up of pressure or localised heat. Acetone is ideal because it can dissolve large amounts of ethyne and is chemically inert under storage conditions.

The conversion of starch to ethanol occurs through fermentation. During this biochemical process, starch is first hydrolysed into simple sugars like glucose using enzymes such as amylase. Then, yeast (Saccharomyces cerevisiae) converts the glucose into ethanol and carbon dioxide under anaerobic conditions. The overall reaction can be represented as:\[\text{C}_6\text{H}_{12}\text{O}_6 \rightarrow 2\text{C}_2\text{H}_5\text{OH} + 2\text{CO}_2\]This process is widely used in the production of alcoholic beverages and biofuels.

Fluorine is the most electronegative and reactive of all the elements. Its high reactivity causes it to form compounds readily with nearly all elements, including noble gases under certain conditions. As a result, it cannot exist freely in nature but occurs in combined forms such as fluorite (CaF₂) and cryolite (Na₃AlF₆). Its extreme oxidising power leads it to react even with water, liberating oxygen and forming hydrofluoric acid.

During the electrolysis of fused sodium chloride (Downs process), chlorine gas is produced at the anode and sodium metal at the cathode. Platinum is chosen as the anode material because it is chemically inert and resistant to attack by chlorine gas, which is highly reactive. Using platinum prevents the formation of unwanted side products or electrode corrosion, ensuring a stable and efficient process.

In the electrolysis of molten calcium chloride, calcium ions (Ca²⁺) are reduced at the cathode to form calcium metal. Iron is used as the cathode material because it conducts electricity efficiently and is resistant to attack by molten calcium. At the same time, chlorine gas is released at the graphite anode. Using iron ensures durability and minimises contamination of the calcium produced.

Calcium ions impart a characteristic brick-red colour to a non-luminous flame during a flame test. This occurs because the heat excites the electrons in the calcium atoms, and as they return to lower energy levels, they emit light of a specific wavelength corresponding to the brick-red colour. Flame tests are used to identify metal ions based on their unique emission spectra.

When sodium hydroxide solution is added to a solution containing calcium ions, a white precipitate of calcium hydroxide forms, which remains insoluble in excess NaOH. The reaction is represented as:\[\text{Ca}^{2+} + 2\text{OH}^- \rightarrow \text{Ca(OH)}_2 (s)\]Other cations like aluminium and zinc produce white precipitates that dissolve in excess alkali due to amphoteric behaviour, while calcium hydroxide remains insoluble, confirming the presence of Ca²⁺ ions.

A large equilibrium constant (K) indicates that the reaction strongly favours the formation of products at equilibrium. The equilibrium constant is related to the standard Gibbs free energy change \(\Delta G^\circ\) by the equation:\[\Delta G^\circ = -RT \ln K\]When \(K\) is very large, \(\ln K\) is positive and \(\Delta G^\circ\) becomes highly negative, meaning the reaction proceeds almost completely toward the products, achieving maximum yield.

The rate of a chemical reaction is defined as the change in concentration of a reactant or product per unit time. It provides a quantitative measure of how fast a reaction proceeds. Mathematically, the rate is expressed as:\[\text{Rate} = -\dfrac{d[\text{Reactant}]}{dt} = \dfrac{d[\text{Product}]}{dt}\]This concept is fundamental in kinetics and helps determine the mechanism and order of a reaction.