Chemistry 2024 Jamb Examination

When a metal reacts with an acid, hydrogen gas is liberated and a salt is produced.
The general reaction is: Metal + Acid → Salt + Hydrogen gas
For example: Zn + 2HCl → ZnCl₂ + H₂↑
Thus, the product formed when a metal reacts with an acid to liberate gas is a salt.

In the electrolysis of concentrated sodium chloride using a mercury cathode and graphite anode:
At the anode (graphite), chloride ions are oxidised to chlorine gas:
\( 2Cl^- \rightarrow Cl_2 + 2e^- \)
At the cathode (mercury), sodium ions are reduced to form sodium amalgam:
\( Na^+ + e^- \rightarrow Na(Hg) \)
Thus, the products at the anode and cathode are chlorine and sodium, respectively.

The rate of a chemical reaction is influenced by factors such as temperature, surface area, concentration, and catalysts.
Volume does not directly affect the rate of a reaction because it does not alter the frequency of collisions between reacting particles unless concentration changes.
Therefore, volume is not a factor that affects reaction rate.

In the contact process for the manufacture of sulphuric acid, sulphur(IV) oxide is oxidised to sulphur(VI) oxide in the presence of vanadium(V) oxide (V\(_2\)O\(_5\)) catalyst:
\( 2SO_2 + O_2 \xrightarrow{V_2O_5} 2SO_3 \)
Vanadium(V) oxide is used because it is more efficient and less expensive than platinum.

An acid anhydride reacts with water to form an acid. Elements that form acidic oxides are typically non-metals.
Sulphur combines with oxygen to form sulphur dioxide (SO\(_2\)), which dissolves in water to produce sulphurous acid (H\(_2\)SO\(_3\)):
\( SO_2 + H_2O \rightarrow H_2SO_3 \)
Thus, sulphur forms an acid anhydride of the form XO\(_2\).

Rusting occurs when iron reacts with oxygen and moisture in the air, forming hydrated iron(III) oxide. The reaction is
: 4Fe + 3O₂ + 6H₂O → 4Fe(OH)₃ → 2Fe₂O₃·3H₂O
This hydrated iron(III) oxide gives rust its reddish-brown colour and flaky appearance.

Lime water (calcium hydroxide solution) turns milky when carbon dioxide gas is passed through it due to the formation of calcium trioxocarbonate(IV):
\( Ca(OH)_2 + CO_2 \rightarrow CaCO_3 + H_2O \)
The gas is therefore carbon dioxide, which originates from trioxocarbonate(IV) compounds.

For any principal quantum number \( n \), the total number of possible electrons in that shell is given by \( 2n^2 \).
Substituting \( n = 3 \):\[ 2n^2 = 2(3)^2 = 2 \times 9 = 18 \]Hence, when \( n = 3 \), the maximum number of electrons in the shell is 18.

A reaction between a strong acid and a strong base results in complete neutralisation, forming a neutral (normal) salt and water.
For example:
\( HCl + NaOH \rightarrow NaCl + H_2O \)
The salt produced is called a normal salt because there is no excess hydrogen or hydroxide ion in the product.

Carbon(IV) oxide (CO\(_2\)) has a linear shape because the two double bonds between carbon and oxygen are arranged symmetrically, resulting in a bond angle of 180°.
There are no lone pairs on the central carbon atom to cause deviation, hence the molecule remains linear.