Chemistry 2023 Jamb Examination

Ionisation energy refers to the amount of energy required to remove an electron from a gaseous atom. Across a period, the nuclear charge increases as more protons are added to the nucleus, while the shielding effect remains relatively constant.
This results in a stronger attraction between the nucleus and the outermost electrons, making it harder to remove an electron.
Hence, ionisation energy increases progressively from left to right across a period.

In an ionic bond, one atom donates one or more electrons to another atom, leading to the formation of positively and negatively charged ions.
These oppositely charged ions are then held together by strong electrostatic forces of attraction.
This type of bonding usually occurs between metals and non-metals — for example, in sodium chloride (NaCl), where sodium donates an electron to chlorine.

Straight-chain alkanes are those in which all carbon atoms are connected in a continuous linear arrangement without branching or rings.
Pentane (C₅H₁₂) is a straight-chain alkane, while isobutane and 2-methylpropane are branched isomers of butane, and cyclopentane is a cyclic compound.
Therefore, pentane is the correct example of a straight-chain alkane.

Diluting an acid means adding more water, which decreases the concentration of hydrogen ions (H⁺) in the solution.
Since pH is inversely related to hydrogen ion concentration (\( \text{pH} = -\log[H^+] \)), reducing [H⁺] leads to an increase in pH.
Therefore, the pH of an acidic solution increases upon dilution, though the solution remains acidic.

In the electrolysis of aqueous sodium chloride using inert electrodes, water is reduced at the cathode, producing hydrogen gas, while chloride ions are oxidised at the anode to produce chlorine gas.
Sodium ions and hydroxide ions remain in solution, forming sodium hydroxide (NaOH).
Hence, hydrogen gas forms at the cathode and chlorine gas at the anode.

The rate of a chemical reaction depends on factors such as the concentration of reactants, temperature, surface area, and the presence of a catalyst.
The molecular weight of the products, however, has no influence on how fast the reaction proceeds — it is a property of the substances formed, not of the process itself.
Therefore, the molecular weight of products does not affect reaction rate.

Esters are organic compounds formed by the reaction between carboxylic acids and alcohols.
They are known for their pleasant fruity and floral odours and are widely used in flavourings and perfumes.
For instance, isoamyl acetate gives banana its characteristic smell.
Thus, esters are responsible for the aroma of most fruits.

The molar mass of water is determined by summing the atomic masses of its constituent atoms.
Hydrogen (H) has an atomic mass of approximately 1 g/mol, and oxygen (O) has a mass of about 16 g/mol.
Thus, \( 2(1) + 16 = 18 \; \text{g/mol} \).
Therefore, the molar mass of water is 18 g/mol.

Rust is primarily composed of hydrated iron(III) oxide, which has the empirical formula Fe₂O₃·xH₂O.
The essential oxide component of rust is Fe₂O₃ (iron(III) oxide), formed when iron reacts with oxygen and water over time.
It gives rust its reddish-brown colour and flaky texture.
Hence, the formula representing rust is Fe₂O₃.

Alkenes are hydrocarbons that contain at least one double bond between carbon atoms.
Each double bond consists of one sigma (σ) bond and one pi (π) bond.
A simple six-carbon alkene, such as hex-1-ene or hex-2-ene, contains only one double bond, and thus one pi bond.
However, a polyene such as hexa-1,3,5-triene would contain three double bonds and therefore three pi bonds.
Hence, an alkene with six carbon atoms typically contains three pi bonds if it has alternating double bonds (triene form).