JAMB Chemistry 2016 Past Questions & Answers

Polarization is a defect that occurs in simple electrochemical cells, such as the Leclanché cell. It is caused by the accumulation of hydrogen gas bubbles on the surface of the positive electrode (cathode). This layer of hydrogen gas increases the internal resistance of the cell and sets up a back e.m.f., which opposes the main current and reduces the cell's efficiency.

Common mistake: Students may mistakenly choose an option like Tetraoxosulphate (IV) acid, confusing the electrolyte used in some cells with the hydrogen gas that is actually produced at the electrode to cause polarization.

Corrosion, such as the rusting of iron, is a chemical process where a metal is destroyed by reacting with its environment. For this reaction to happen, two substances are absolutely essential: water (H₂O) and oxygen (O₂). The metal is oxidized by oxygen in the presence of water, which acts as a medium for the reaction, leading to the formation of the metal oxide, like rust.

Common mistake: Students may mistakenly choose option C because they know that carbon (IV) oxide in water forms a weak acid that can speed up corrosion, but it is only an accelerator and not a necessary condition for the process to start.

Enzymes are biological catalysts that are highly specific to the substance they act on. Starch is a complex carbohydrate, which is a natural polymer or macromolecule. The enzyme amylase is specifically responsible for the hydrolysis (chemical breakdown using water) of starch into smaller sugar units like dextrin and maltose.

Common mistake: Students often confuse the various enzymes involved in carbohydrate metabolism and may incorrectly choose Zymase, which is the enzyme complex in yeast that ferments glucose into ethanol.

In the laboratory, chlorine gas is produced mixed with water vapour, so it needs to be dried. A good drying agent must remove water without reacting with the gas itself. Since chlorine is an acidic gas, an acidic drying agent like concentrated tetraoxosulphate (VI) acid (H₂SO₄) is used because it will not react with the chlorine. Basic drying agents like calcium oxide or sodium hydroxide cannot be used because they would react with the acidic chlorine gas.

Common mistake: Students often mistakenly choose anhydrous calcium chloride because it is a common drying agent, but they forget that it can react with chlorine gas.

Nylon is a synthetic polymer, which is a large molecule made from repeating smaller units called monomers. The common type, Nylon-6,6, is formed through the condensation polymerization of two different monomers: hexanedioic acid (also known as adipic acid) and hexane-1,6-diamine. Since hexanedioic acid is listed as an option, it is the correct answer.

Common mistake: Students often confuse the monomers of different synthetic polymers, for instance, they might mistakenly choose Benzene-1,4-dicarboxylic acid, which is a monomer for Terylene (Dacron), not Nylon.

Column chromatography is a physical method used to separate components from a mixture. The separation is achieved because each component interacts differently with the stationary phase (the solid in the column) and the mobile phase (the solvent). This difference in interaction causes the components to travel down the column at different speeds, leading to their separation.

Common mistake: Students may incorrectly choose option A because they think a reaction must occur, but chromatography is a physical separation process, not a chemical one.

The correct answer is sublimation because ammonium chloride is a substance that sublimes, meaning it changes directly from a solid to a gas when heated. Common salt (sodium chloride) does not sublime. By heating the mixture, the ammonium chloride turns into a gas, leaving the solid common salt behind, which effectively separates them.

Common mistake: Students might mistakenly choose evaporation, but this method is used to separate a soluble solid from a liquid, and it would not work here since both salts are soluble in water.

Catenation is the specific term for the ability of an element's atoms to link with one another through covalent bonds to form long chains or rings. Carbon exhibits this property to a greater extent than any other element, which is the reason for the vast number of organic compounds. This ability is the foundation for forming the carbon backbones found in all hydrocarbons and their derivatives.

Common mistake: Students might mistakenly choose 'Carbonation' because it contains the word 'carbon', but this term actually refers to the process of dissolving carbon dioxide in a liquid, such as in making soft drinks.

The Bosch process is the correct name for the industrial method used to produce large quantities of hydrogen gas. This process starts with water gas (a mixture of carbon monoxide and hydrogen) and reacts it with steam over a catalyst. This reaction, called the water-gas shift reaction, converts the carbon monoxide to carbon dioxide, increasing the yield of hydrogen gas.

Common mistake: Students often confuse the Bosch process with the Haber process, but the Haber process is used for the industrial production of ammonia, not hydrogen.

The relative atomic mass is the average mass of an element's isotopes, weighted by their natural abundance. To find it, you multiply the mass of each isotope by its percentage abundance and then add the results together. For lithium, the calculation is $(90/100 × 7)$ + $(10/100 × 6)$, which simplifies to $(0.9 × 7)$ + $(0.1 × 6)$. This gives 6.3 + 0.6, which equals 6.9.

Common mistake: Students might mistakenly swap the percentages, calculating (10% of 7) + (90% of 6), which gives 6.1, leading them to incorrectly choose option A.