2025 JC2 Prelims H2 Chem Paper 2 QP and Ans_TJC

2025 JC2 Prelims H2 P2 (Review) 1 In 1932, the American chemist Linus Pauling developed the most common scale of relative electronegativity (EN) values for the elements. The Pauling EN values of elements can be used to predict the chemical properties of compounds. The EN values of four Period 3 elements are given in Table 1.1. Table 1.1 Element Sodium Aluminium Phosphorus Chlorine Pauling EN value 0.93 1.61 2.19 3.16 l (a) (i) Explain the difference in the Pauling electronegativity values of Na and Cl. [2] Comparing sodium and chlorine, [✓] chlorine has a higher nuclear charge due to larger number of protons, [✓] Screening effect remains approximately constant as electrons are added to the same electronic shell. [✓] As there is stronger a ttraction between the nucleus and the bonding electrons in the outer shell, chlorine has higher [✓] electronegativity, resulting in a larger EN value. 2[✓] = [1] The ionic character of a bond is directly related to the electronegativity difference (EN) between the bonded atoms. Fig. 1.1 shows the plot of percent ionic character against EN for NaCl. Fig. 1.1

(ii) Using the values in Table 1.1, calculate EN for A lCl3 and PC l5 respectively. Hence plot the percent ionic character of A lCl3 and PCl5 on Fig. 1.1. Label your points clearly. [2] EN for AlCl3 = 3.16 – 1.61 = 1.55 [✓] EN for PCl5 = 3.16 – 2.19 = 0.970 [✓] 2[✓] = [1] [1] Correct relative order for percent ionic character for the three compounds (iii) Explain the difference in bonding for NaC l and PCl5 in terms of electronegativity. [2] [1] Ionic bonds exist in NaC l due to large EN (or large difference in electronegativities) between Na and Cl. [1] Covalent bonds exist in PCl5 due to small EN (or small difference in electronegativities) between P and Cl. (b) (i) The polarity of bonds in covalent molecules is also affected by EN. SeF4 and BrF3 are two fluorine-containing molecules. State whether SeF4 or BrF3 contains covalent bonds with a higher ionic character. [1] [1] SeF4 > BrF3 (ii) Using VSEPR theory, predict and explain the shape and bond angles of SeF4. Illustrate the shape of SeF4 with an appropriate diagram. [3] There are [✓] 4 bond pairs and 1 lone pair in the valence shell of Se atom. [✓] To minimise repulsion and maximise stability, the shape of SeF4 is [✓] see-saw.

[✓] Since [✓] lone pair – bond pair repulsion > bond pair – bond pair repulsion, the [✓] bond angles are 88o and 118o (accept other bond angles < 90o and < 120o). 2[✓] = [1] [Total: 10]

2 PCl5 is a Period 3 chloride commonly used as a chlorinating agent and catalyst in making organic compounds. Industrial production of PCl5 involves the reaction of Cl2 with PCl3. Reaction (1) PCl3(g) + Cl2(g) ⇌ PCl5(g) (a) Suggest, with an explanation, how the position of equilibrium and the composition of the equilibrium mixture might change when chlorine is added