Chemistry 2015 Jamb Examination

Straight-chain hydrocarbons in detergents allow microorganisms to break them down easily during biodegradation. Branched-chain hydrocarbons resist microbial attack, leading to environmental persistence. Therefore, using straight-chain hydrocarbons makes detergents environmentally friendly and biodegradable.

Acid rain is caused mainly by the release of sulphur dioxide (SO₂) and nitrogen oxides (NO_x) into the atmosphere. These gases react with water vapour to form sulphuric and nitric acids, which fall as acid rain. Therefore, oxides of sulphur are key contributors to acid rain formation.

Temporary hardness in water is due to calcium hydrogentrioxocarbonate (Ca(HCO₃)₂). When heated, it decomposes to form insoluble calcium trioxocarbonate (CaCO₃), which deposits on kettle surfaces as scale (furring). Hence, the presence of calcium hydrogentrioxocarbonate causes furring in kettles.

A chemical change involves the breaking and formation of chemical bonds, resulting in the creation of one or more new substances with properties different from the original ones. Physical changes, by contrast, do not form new substances but only alter the physical state. Thus, the formation of a new substance distinguishes a chemical change.

The reaction involves the decomposition of one mole of gaseous PCl₅ into two moles of gaseous products (PCl₃ and Cl₂). According to Le Chatelier’s principle, a decrease in pressure favours the side with more gas molecules to counter the change by increasing pressure. Therefore, lowering pressure shifts the equilibrium towards the right, resulting in more gaseous products being formed. Hence, the yield of PCl₃ and Cl₂ increases.

When sodium hydroxide is added to aluminium chloride (AlCl₃), a white precipitate of aluminium hydroxide, Al(OH)₃, is formed:
AlCl₃ + 3NaOH → Al(OH)₃↓ + 3NaCl
Other compounds such as NH₄Cl and CH₃COONa do not form insoluble hydroxides, while Na₂CO₃ produces soluble sodium hydroxide and carbonate ions. Therefore, AlCl₃ produces a precipitate with NaOH solution.

The atomic number 13 corresponds to aluminium. Filling electrons according to the Aufbau principle, the configuration is:
1s² → 2 electrons
2s² → 2 electrons
2p⁶ → 6 electrons
3s² → 2 electrons
3p¹ → 1 electron
Hence, the configuration is 1s²2s²2p⁶3s²3p¹.

When salt is added to ice, it lowers the freezing point of water — a phenomenon known as freezing point depression. The dissolved salt disrupts the formation of the orderly solid ice lattice, meaning ice melts at a lower temperature. This helps to prevent ice from forming on roads in very cold conditions.

In the steel-making process, oxygen is blown through molten pig iron to oxidise impurities such as carbon, sulphur, and phosphorus, forming gaseous oxides that escape or form slag. This process, known as the basic oxygen process, refines the iron into steel of desired purity.

Concentrated tetraoxosulphate(VI) acid (H₂SO₄) is a strong dehydrating agent. When it is added to glucose, it removes water molecules, leaving behind pure carbon. The reaction produces a black residue of carbon and water vapour is evolved:\[ \text{C}_6\text{H}_{12}\text{O}_6 \xrightarrow{\text{conc. }H_2SO_4} 6C + 6H_2O \]