iGCSE Chemistry (0620) Theory (Core):6.4 Redox: Exam Style Questions Paper 3- New Syllabus
Question
Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):
• Topic 2.2 — Atomic structure and the Periodic Table (Part (a)(i))
• Topic 2.3 — Isotopes (Part (a)(ii))
• Topic 10.3 — Air quality and climate (Parts (b)(i), (b)(ii))
• Topic 6.4 — Redox / balancing equations (Part (b)(iii))
• Topic 2.5 — Simple molecules and covalent bonds (Part (b)(iv))
• Topic 3.2 — Relative masses of atoms and molecules (Part (c))
▶️ Answer/Explanation
(a)(i) Sulfur has 6 electrons in its outer shell. (The group number for Groups I to VII equals the number of outer shell electrons.)
Explanation: In the Periodic Table, elements are arranged in groups based on the number of electrons in their outermost shell. Sulfur has electronic configuration \(2,8,6\), with 6 electrons in its outer shell, which places it in Group VI.
(a)(ii)
Explanation: The atomic number (subscript, 16) gives the number of protons. In a neutral atom, the number of electrons equals the number of protons (16). The mass number (superscript) is the sum of protons and neutrons. For \(\frac{33}{16}\text{S}\): neutrons = \(33 – 16 = 17\). For \(\frac{36}{16}\text{S}\): neutrons = \(36 – 16 = 20\).
(b)(i) Combustion / burning of fossil fuels (which contain sulfur compounds) / volcanic eruptions.
Explanation: Fossil fuels such as coal and petroleum often contain sulfur compounds as impurities. When these fuels undergo combustion, the sulfur is oxidised to sulfur dioxide (\(\text{SO}_2\)), which is released into the atmosphere.
(b)(ii) Acid rain.
Explanation: Sulfur dioxide reacts with water and oxygen in the atmosphere to form sulfuric acid (\(\text{H}_2\text{SO}_4\)), which falls as acid rain. This damages buildings (especially limestone), acidifies lakes and rivers (harming aquatic life), and damages forests and vegetation.
(b)(iii) \(3\,\text{S}\) and \(2\,\text{H}_2\text{O}\)
\[\text{SO}_2 + 2\text{H}_2\text{S} \rightarrow 3\text{S} + 2\text{H}_2\text{O}\]
Explanation: Balancing the equation:
Left side: 1 S (from \(\text{SO}_2\)), 2 S (from \(2\text{H}_2\text{S}\)) = 3 S atoms; 4 H atoms; 2 O atoms.
Right side must have: 3 S atoms, 4 H atoms (so \(2\text{H}_2\text{O}\)), 2 O atoms (from \(2\text{H}_2\text{O}\)). Equation balanced.
(b)(iv) covalent (bonding)
Explanation: Sulfur dioxide is a molecular compound where sulfur and oxygen are both non-metals. They share electrons to form covalent bonds, resulting in discrete \(\text{SO}_2\) molecules.
(c) Aluminium: \(2 \times 27 = 54\) (2 atoms); Sulfur: \(3 \times 32 = 96\) (3 atoms).
Relative formula mass = \(192 + 54 + 96 = 342\).
Explanation: \(\text{Al}_2(\text{SO}_4)_3\) contains: 2 Al atoms, 3 S atoms, and \(3 \times 4 = 12\) O atoms. The relative formula mass is the sum of the relative atomic masses of all atoms present:
\(\text{Al}\): \(2 \times 27 = 54\), \(\text{S}\): \(3 \times 32 = 96\), \(\text{O}\): \(12 \times 16 = 192\)
Total \(M_r = 54 + 96 + 192 = 342\).