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2019 O Level Chem P1 Q7 — Ionic Bonding / Properties of Ionic Compounds Solution

Ionic Bonding / Properties of Ionic Compounds 2019 O Level Pure Chemistry · Paper 1 · Q7

Question

Both calcium chloride and potassium chloride are ionic compounds. Which statements about these compounds are correct? 1 Potassium chloride has a higher melting point than calcium chloride because the potassium ion has a greater charge than the calcium ion. 2 In the calcium chloride lattice, the ratio of calcium ions to chloride ions is half that of the ratio of potassium ions to chloride ions in the potassium chloride lattice. 3 Molten calcium chloride has a greater electrical conductivity than molten potassium chloride because calcium chloride contributes more electrons to the delocalised 'sea of electrons' than does potassium.

A1 and 2
B1 and 3
C2 only
D3 only

Correct answer: C

Video Solution

Explanation

Answer: C (statement 2 only). Both calcium chloride and potassium chloride are ionic compounds, but the reasoning in two of the statements is flawed.

Statement 1: wrong. Ca²⁺ has a higher charge (+2) and a smaller radius than K⁺, so it has the higher charge density and stronger ionic attraction. Calcium chloride therefore has the higher melting point, not potassium chloride.

Statement 2: correct. In CaCl₂ the ratio of cations to chloride ions is 1:2, that is 0.5, while in KCl it is 1:1, that is 1. Since 0.5 is half of 1, the statement is accurate.

Statement 3: wrong. Ionic compounds do not contain a 'sea of electrons'. Molten ionic salts conduct because their ions are free to move, not because of delocalised electrons. The sea-of-electrons model belongs to metallic bonding. Only statement 2 stands.

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