Question

1. figure 9 represents acids and related entities at the molecular level. the white spheres represent hydrogen atoms or ions. the coloured spheres represent the other part of the acid. which diagram best represents each of the following substances? explain why in each case. (a) hydrochloric acid (b) ethanoic acid (c) hydrogen chloride gas

Explanation:

Part (a): Hydrochloric Acid

Step 1: Recall HCl Properties

Hydrochloric acid (\( \text{HCl} \)) is a strong acid, so it fully dissociates in water: \( \text{HCl}
ightarrow \text{H}^+ + \text{Cl}^- \). The solution should have separate \( \text{H}^+ \) (white) and \( \text{Cl}^- \) (colored) ions, with no intact \( \text{HCl} \) molecules.

Step 2: Analyze Diagrams

• Diagram (iii): Shows white (\( \text{H}^+ \)) and blue (\( \text{Cl}^- \)) ions, no intact molecules. This matches full dissociation of HCl.

Part (b): Ethanoic Acid

Step 1: Recall Ethanoic Acid Properties

Ethanoic acid (\( \text{CH}_3\text{COOH} \)) is a weak acid, so it partially dissociates: \( \text{CH}_3\text{COOH}
ightleftharpoons \text{H}^+ + \text{CH}_3\text{COO}^- \). Most molecules remain intact, with few ions.

Step 2: Analyze Diagrams

• Diagram (ii): Has many purple (intact acid molecules) and some white (\( \text{H}^+ \)) ions. This matches partial dissociation of a weak acid.

Part (c): Hydrogen Chloride Gas

Step 1: Recall HCl Gas Structure

Hydrogen chloride gas consists of intact \( \text{HCl} \) molecules (each with one white \( \text{H} \) and one colored \( \text{Cl} \) atom bonded). No dissociation occurs in gas phase.

Step 2: Analyze Diagrams

• Diagram (i): Shows orange (Cl) and white (H) atoms bonded as molecules (each molecule has one white and one orange sphere), no separate ions. This matches \( \text{HCl} \) gas.

Answer:

(a) Diagram (iii) – HCl is a strong acid, fully dissociates into \( \text{H}^+ \) and \( \text{Cl}^- \) ions (no intact molecules).
(b) Diagram (ii) – Ethanoic acid is weak, partially dissociates (most molecules intact, few ions).
(c) Diagram (i) – \( \text{HCl} \) gas has intact \( \text{HCl} \) molecules (no dissociation).