iGCSE Chemistry (0620) Theory (Core):2.5 Simple molecules and covalent bonds: Exam Style Questions Paper 3- New Syllabus
Question
This question is about ionic compounds and simple molecular compounds.
(b) Aqueous magnesium chloride is prepared by adding excess magnesium oxide powder to dilute hydrochloric acid.
Describe how to prepare a sample of pure, dry magnesium chloride crystals after the reaction is complete.
In your answer describe how to:
- remove the excess magnesium oxide from the reaction mixture
- crystalline the magnesium chloride
- dry the crystals.
(c) Fig. 5.1 shows the apparatus for the electrolysis of molten cobalt(II) bromide using inert electrodes.
(i) Label Fig. 5.1 to show the:
- anode
- electrolyte.
(ii) Name the products formed at the positive and negative electrodes.
(d) Table 5.1 shows some properties of five compounds, A, B, C, D and E.
State which two of the compounds, A, B, C, D and E, are simple molecules.
Give two reasons for your answer.
Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):
• Topic 7.1 — The characteristic properties of acids and bases (Part (a))
• Topic 7.3 — Preparation of salts (Part (b))
• Topic 4.1 — Electrolysis (Parts (c)(i), (c)(ii))
• Topic 2.5 — Simple molecules and covalent bonds (Part (d))
▶️ Answer/Explanation
(a)
For the correct answer:
copper nitrate + carbon dioxide + water
Nitric acid is a strong acid, and copper(II) carbonate is an insoluble metal carbonate. The general reaction is: acid + metal carbonate → salt + water + carbon dioxide. The salt formed is copper(II) nitrate, $\mathrm{Cu(NO_3)_2}$, which is a blue solution. Bubbles of carbon dioxide gas are seen, and water is produced.
(b)
For the correct answer:
remove excess: filtration / filter (off the excess magnesium oxide).
crystallise: evaporate to point of crystallisation OR evaporate until saturated solution is formed OR heat until crystals just appear.
dry: dry with (filter) paper.
The reaction produces aqueous magnesium chloride. Since the magnesium oxide was added in excess, some unreacted solid remains. Filtration separates this insoluble solid from the soluble salt solution (filtrate). The filtrate is heated to evaporate some water until the solution is saturated (point of crystallisation, when crystals just start to form on cooling). The saturated solution is left to cool slowly, allowing larger, purer crystals to form. Finally, the crystals are removed and gently dried by pressing them between pieces of dry filter paper to absorb remaining moisture.
(c)(i)
For the correct answer:
anode labelled on the right hand electrode (1). electrolyte labelled (1).
In an electrolytic cell, the anode is the positive electrode (connected to the positive terminal of the power supply). The electrolyte is the substance that is electrolysed, in this case, the molten cobalt(II) bromide ($\mathrm{CoBr_2}$), which contains mobile ions.
(c)(ii)
For the correct answer:
positive electrode: bromine.
negative electrode: cobalt.
During electrolysis, the positive metal ion (cation) is attracted to the cathode (negative electrode) where it gains electrons (reduction). Here, cobalt ions ($\mathrm{Co}^{2+}$) are reduced to cobalt metal. The negative non-metal ion (anion) is attracted to the anode (positive electrode) where it loses electrons (oxidation). Bromide ions ($\mathrm{Br}^-$) are oxidised to bromine gas ($\mathrm{Br_2}$).
(d)
For the correct answer:
compounds: C and E (1).
reason 1: low melting point.
reason 2: does not conduct electricity (when molten).
Simple molecular compounds consist of small, discrete molecules held together by weak intermolecular forces. These weak forces require little energy to overcome, resulting in characteristically low melting and boiling points (compounds C and E melt at $-182^{\circ}\mathrm{C}$ and $-101^{\circ}\mathrm{C}$). Because there are no free ions or delocalised electrons even when molten (the molecules remain as neutral entities), simple molecular compounds are electrical insulators in all states.