Why does the rate of a gaseous reaction increase when the pressure is increased at a constant temperature?
▶️ Answer/Explanation
Ans: D
When the pressure of a gas is increased at constant temperature, the volume decreases (Boyle’s Law: \( pV = \text{constant} \)). This increases the number of gas particles per unit volume, leading to more frequent collisions between reactant molecules.
Since reaction rates depend on collision frequency, the rate increases. Options A, B, and C are incorrect because:
- A: Activation energy is unaffected by pressure changes (only temperature affects it).
- B: Higher pressure reduces space, not increases it.
- C: Particle speed depends on temperature, not pressure.
The table shows physical properties of four substances, W, X, Y and Z.
What are the identities of W, X, Y and Z? 
▶️ Answer/Explanation
Ans: C
Substance W has a high melting point and conducts electricity when molten, indicating it is an ionic compound (e.g., NaCl). X has a very high melting point and conducts electricity, suggesting a giant metallic structure (e.g., Fe). Y has a low melting point and does not conduct electricity, indicating a simple molecular structure (e.g., CO2). Z has a very high melting point but does not conduct electricity, consistent with a giant covalent structure (e.g., SiO2). Thus, the correct option is C.
Gas Q decomposes slowly at room temperature.
Q(g) → X(g) + Z(g)
The Boltzmann distribution curve for gas Q at room temperature is shown.
Which change occurs when a catalyst is added to gas Q?
▶️ Answer/Explanation
Ans: B
A catalyst lowers the activation energy (Ea) of a reaction, allowing more particles to have sufficient energy to react. This does not shift the peak of the Boltzmann distribution (A is incorrect) or increase the kinetic energy of unreacted particles (C is incorrect). While the activation energy decreases, the vertical dotted line (representing Ea) moves left, not right (D is incorrect). Thus, the correct change is that more particles now have enough energy to decompose, making B the correct answer.
Which statement is correct?
▶️ Answer/Explanation
Ans: D
A: Incorrect. Atomic radius decreases across Period 3 (left to right). Silicon (\(Si\)) has a smaller radius than aluminium (\(Al\)).
B: Incorrect. Chlorine (\(Cl_2\)) is a diatomic gas with a low boiling point, while silicon (\(Si\)) is a covalent solid with a very high boiling point.
C: Incorrect. Sulfur (\(S\)) has a slightly lower first ionisation energy than phosphorus (\(P\)) due to electron-electron repulsion in its \(3p^4\) configuration.
D: Correct. Magnesium (\(Mg\)) has greater electrical conductivity than sodium (\(Na\)) because it contributes two delocalized electrons per atom (vs. one for \(Na\)), enhancing metallic bonding.