CIE iGCSE Co-Ordinated Science C2.4 Ions and ionic bonds Exam Style Questions Paper 3
Question
Most-appropriate topic codes (Cambridge IGCSE Coordinated Sciences 0654):
• Topic C2.4 — Ions and ionic bonds (Part (a)(i))
• Topic C2.4 — Ions and ionic bonds (Part (a)(ii))
• Topic C8.3 — Group VII properties (Part (b))
• Topic C7.1 — Characteristic properties of acids and bases (Part (c))
▶️ Answer/Explanation
(a)(i) eight electrons added to outer shell; – sign shown
A chlorine atom has 7 electrons in its outer shell. It gains one electron to achieve a stable octet, becoming a chloride ion (Cl⁻) with a single negative charge.
(a)(ii) generally soluble in water; good electrical conductivity; high melting point
Ionic compounds have high melting points due to strong electrostatic attractions. They conduct electricity when dissolved in water (but not when solid) because ions are free to move. They are generally soluble in water.
(b) (pale) yellow-green; gas
Chlorine is a pale yellow-green gas at room temperature and pressure. It is a diatomic molecule (Cl₂) with a distinctive colour.
(c) hydrochloric acid; sodium hydroxide
Neutralisation of hydrochloric acid with sodium hydroxide produces sodium chloride and water. The acid provides chloride ions, and the alkali provides sodium ions.
Question
Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654):
• Topic C2.5 — Simple molecules and covalent bonds (Part (a)(i))
• Topic C8.3 — Group VII properties (Part (a)(ii))
• Topic C5.1 — Exothermic and endothermic reactions (Part (b)(i))
• Topic C3.1 — Formulas (Part (b)(ii))
• Topic C2.4 — Ions and ionic bonds (Part (c))
• Topic C4.1 — Electrolysis (Part (d))
▶️ Answer/Explanation
(a)(i) Covalent (bonding)
Chlorine atoms share a pair of electrons to form a single covalent bond. Each chlorine atom has seven electrons in its outer shell and needs one more to achieve a stable octet. By sharing one pair of electrons, both atoms achieve a full outer shell.
(a)(ii) At₂
Astatine is in Group VII (the halogens) and exists as diatomic molecules, similar to chlorine, bromine and iodine. The formula for a molecule of astatine is At₂.
(b)(i) (Thermal) energy is released
An exothermic reaction releases thermal energy to the surroundings. The orange flame and the melting of sodium indicate that heat is being produced, which is evidence that the reaction is exothermic.
(b)(ii) \(2\text{Na(s)} + \text{Cl}_2\text{(g)} \rightarrow 2\text{NaCl(s)}\)
The balanced equation requires 2 sodium atoms to react with 1 chlorine molecule, producing 2 formula units of sodium chloride. The state symbols are: (s) for solid sodium and sodium chloride, and (g) for chlorine gas.
(c) Sodium ion: electronic configuration 2,8 with charge +1; Chloride ion: electronic configuration 2,8,8 with charge -1
A sodium atom (2,8,1) loses its single outer-shell electron to achieve the stable configuration of neon (2,8), forming a sodium ion with a +1 charge. A chlorine atom (2,8,7) gains one electron to achieve the stable configuration of argon (2,8,8), forming a chloride ion with a -1 charge. The dot-and-cross diagrams should show the sodium ion with no dots in the outer shell (2,8) and a +1 charge, and the chloride ion with a full outer shell (2,8,8) and a -1 charge.
(d)(i) positive electrode: anode; negative electrode: cathode
In electrolysis, the positive electrode is called the anode and the negative electrode is called the cathode. At the anode, negatively charged ions lose electrons (oxidation); at the cathode, positively charged ions gain electrons (reduction).
(d)(ii) gas at positive electrode: chlorine; gas at negative electrode: hydrogen
In the electrolysis of concentrated aqueous sodium chloride:
• At the anode (positive electrode), chloride ions (\( \text{Cl}^- \)) are discharged, losing electrons to form chlorine gas: \(2\text{Cl}^- \rightarrow \text{Cl}_2 + 2e^-\).
• At the cathode (negative electrode), hydrogen ions from the water are preferentially discharged over sodium ions, gaining electrons to form hydrogen gas: \(2\text{H}^+ + 2e^- \rightarrow \text{H}_2\).