A Data Booklet may be used.
Source: Learners' Alley, The Periodic Table slides
Source: Learners' Alley, The Periodic Table slides
Source: Learners' Alley, The Periodic Table slides
Source: Learners' Alley, The Periodic Table slides
Source: Learners' Alley, The Periodic Table slides
Source: Learners' Alley, The Periodic Table slides
Element J is in Period 3 of the Periodic Table. The following three statements are properties of element J or its compounds.
What is the identity of element J?
Source: IJC 2018 Prelim P1 Q29, via EJC 2022 The Periodic Table Topical Revision, Q4
CommonWhat is the correct order of the pH of the resulting solutions when the oxides below are added to water, from lowest pH to highest pH?
Source: HCI 2019 Prelim P1 Q27, via EJC 2022 The Periodic Table Topical Revision, Q9
| oxide | melting point / °C | electrical conductivity when molten | reaction with H2O | resulting pH |
|---|---|---|---|---|
| W | 2850 | good | very little | 8 |
| X | 1720 | poor | none | 7 |
| Y | 1280 | good | exothermic | 14 |
| Z | 580 | poor | exothermic | 1 |
Source: A Level 2007 and 2013 P3 (adapted), via EJC 2022 JC1 The Periodic Table Tutorial, Q1(a)
CommonThe pH values of 1.0 mol dm−3 solutions of three Period 3 chlorides are NaCl 7.0, MgCl2 6.5 and AlCl3 3.0.
Source: A Level June 1997 (adapted), via EJC 2022 JC1 The Periodic Table Tutorial, Q4(a)(i) and (b)
The same amounts of MgCO3 and BaCO3 were heated separately, and the gas given off was bubbled through a solution X. The time taken for a white precipitate to be observed in X was 3 minutes for MgCO3 and 9 minutes for BaCO3.
Source: NJC 2011 P3 Q1(d) (modified), via EJC 2022 JC1 The Periodic Table Tutorial, Q6
ChallengeExplain each of the following observations in terms of structure and bonding.
Source: RI(JC) H2 Chemistry Tutorial 22 (Group II), Q1(b) and (c)
Mg2+ < Na+ < F− < O2−. All have 10 electrons (isoelectronic).[1]
Nuclear charge decreases from Mg (12) to O (8), so the electrons are less strongly attracted and the radius increases.[1]
More vigorously: Ba is a stronger reducing agent, so it loses its valence electrons more readily.[1]
Ba + 2H2O → Ba(OH)2 + H2[1]
Na loses its valence electron, so Na+ has one less principal quantum shell.[1]
SiCl4 + 2H2O → SiO2 + 4HCl[1]
pH 2[1]
Ba2+ is larger, with lower charge density[1]
so it polarises the CO32− anion less: C–O bonds are weakened less.[1]
No: Br2 is a weaker oxidising agent than Cl2.[1]
Baluminium[1]
Based on the information provided, the only possible Period 3 element for J is Al.
AlCl3 + 3NaOH → Al(OH)3 + 3NaCl, then Al(OH)3 + NaOH → Na[Al(OH)4]
Al is a solid at room temperature. The oxide of Al, Al2O3, is amphoteric and is soluble in hydrochloric acid: Al2O3 + 6HCl → 2AlCl3 + 3H2O
ASO3, SiO2, MgO, Na2O[1]
Ionic oxides tend to be basic in nature, hence higher pH. Covalent oxides tend to be acidic oxides, hence lower pH. SiO2 is insoluble in water, hence neutral pH.
W has a giant ionic lattice structure. A large amount of energy is needed to overcome the strong electrostatic forces of attraction between oppositely charged ions in the lattice, hence the very high melting point. Mobile ions in molten W allow good electrical conductivity. W has very little reaction with water to form a basic solution, hence W is MgO: MgO(s) + H2O(l) → Mg(OH)2(aq)[1]
X has a giant covalent structure. A large amount of energy is needed to overcome the strong covalent bonds in the covalent lattice, hence the high melting point. The absence of mobile charge carriers in molten X results in poor electrical conductivity. X does not react with water, so it is SiO2.[1]
Similar to W, Y has a giant ionic lattice structure. Y reacts with water exothermically to form a strongly basic solution, hence Y is Na2O: Na2O(s) + H2O(l) → 2NaOH(aq)[1]
Z has a simple covalent structure. A small amount of energy is needed to overcome the weak intermolecular instantaneous dipole–induced dipole attraction, hence the relatively low melting point. The absence of mobile charge carriers in molten Z results in poor electrical conductivity. Z reacts with water exothermically to form a highly acidic solution, hence Z is P4O10: P4O10(s) + 6H2O(l) → 4H3PO4(aq)[1]
From the Data Booklet, charge density of Na+ = 10.095 = 10.5, of Mg2+ = 20.065 = 30.8, of Al3+ = 30.050 = 60.0
NaCl undergoes only hydration, as Na+ has the lowest charge density. Hence, the resulting solution is pH 7. NaCl(s) + aq → Na+(aq) + Cl−(aq)[1]
MgCl2 undergoes hydration and slight hydrolysis due to the relatively high charge density of the Mg2+ ion, which polarises the electron cloud of its surrounding water molecules, weakening the O–H bond, which results in the release of H+. Hence, the resulting solution is slightly acidic, pH 6.5. MgCl2(s) + 6H2O(l) → [Mg(H2O)6]2+(aq) + 2Cl−(aq), [Mg(H2O)6]2+(aq) ⇌ [Mg(H2O)5(OH)]+(aq) + H+(aq)[1]
AlCl3 undergoes hydration and extensive hydrolysis due to the very high charge density of the Al3+ ion, which polarises the electron cloud of its surrounding water molecules, weakening the O–H bond, which results in the release of H+. Hence, the resulting solution is acidic, pH 3. AlCl3(s) + 6H2O(l) → [Al(H2O)6]3+(aq) + 3Cl−(aq), [Al(H2O)6]3+(aq) ⇌ [Al(H2O)5(OH)]2+(aq) + H+(aq)[1]
SiCl4 reacts vigorously with water to form a white solid and white fumes of HCl, and the pH of the resulting solution is 2. SiCl4 + 2H2O → SiO2 + 4HCl[1]
PCl5 reacts vigorously with water to form white fumes of HCl, and the pH of the resulting solution is 2. PCl5 + 4H2O → H3PO4 + 5HCl[1]
X is Ca(OH)2(aq). (Do not accept "limewater".)[1]
Mg2+ has a smaller cationic radius, hence a greater charge density (and polarising power) than Ba2+. The electron cloud of CO32− in MgCO3 is polarised to a greater extent, and hence the C–O bond is weakened more.[1]
Less energy is required to break the C–O bond in MgCO3, and hence a shorter time (3 minutes) is required for CO2 to be produced.[1]
NoteThe CO2(g) produced forms a white precipitate of CaCO3(s) in Ca(OH)2(aq).
BeCl2 is covalent and exists as simple molecules, with weak instantaneous dipole–induced dipole interactions between its molecules.[1]
MgCl2 is ionic, with strong ionic bonds between its ions.[1]
More energy is needed to overcome the strong ionic bonds between Mg2+ and Cl− ions than the weak interactions between BeCl2 molecules, so the boiling point of MgCl2 is much higher.[1]
BeCl2 is covalent because too much energy is required to remove two electrons from its valence 2s subshell. Hence BeCl2 is much more volatile than MgCl2.[1]
Covalent oxides are generally acidic (e.g. SO2) while ionic oxides are basic (e.g. Na2O).[1]
BeO has ionic bonding with substantial covalent character, so its oxide is amphoteric and it can react with both alkalis and acids.[1]
The oxides of the other alkaline earth metals are predominantly ionic, so they are basic, reacting with acids but not bases. O2− + H2O → 2OH−[1]