Chemical Bonding — EOY Practice (Answer Key)

(a) protons: 16; valence electrons: 6 [2]

(b)(i) Lewis diagram of calcium sulfide ([Ca]2+ and [S]2- with the transferred electrons shown as a dot-and-cross); "Not penalized although not recommended: Brackets for calcium/no brackets for sulfide; not pairing of 'dot' with 'cross' for transferred electrons" [1]

(b)(ii) "No. Calcium sulfide has a giant ionic crystal lattice [1] at room temperature. Its ions are held in fixed positions/are not mobile [1]. Note: Penalty for any contradictory terms e.g. use of 'atoms'/'molecules' interchangeably with 'ions'." [2]

Source: Nygh 2017, P2 Q2

(d) "TiCl₄ is likely to be simple or discrete molecules / have a simple molecular structure. [1] Its low melting and boiling points show that it has weak intermolecular forces of attraction [1] that require little energy to overcome. Its lack of electrical conductivity in any state shows that there are no mobile charged particles available [1]. Accept: Simple covalent structure. Reject: Simple covalent bond (bonding is different from structure); Any contradiction in answers (e.g. it has a simple molecular structure with weak electrostatic forces of attraction between ions)." [3]

Source: Nygh 2017, P2 Q7

(b) "Gray arsenic [1]; The presence of delocalized valence electrons [1]" [2]

(c)(i) Lewis diagram of arsine: As in the centre with a lone pair of electrons and three As–H single bonds shown as dot-and-cross ("Not accepted without lone pair of electrons; ionic formulations not accepted.") [1]

(c)(ii) "Arsine is a simple covalent compound with a low boiling point, hence it has weak intermolecular forces between the molecules [1] and little energy is needed to overcome these forces of attraction. [1] Arsine is insoluble in water due to an inability to form strong interactions/intermolecular forces with water molecules. Accept reference to weak interactions/weak intermolecular forces [1] (Also accept reference to inability to hydrogen bonding). Not accepted arsine molecules are non-polar." [3]. (d) 3Cu₂⁺ + 2AsO₃^3- → Cu₃(AsO₃)₂ [1]. (e) "Number of moles of As₂O₃ = 8870 g ÷ 197.8 g mol-1 = 44.84 mol; Number of moles of As = 89.69 mol [1]; Mass of As = 6.717 kg; % yield = 5.33 kg ÷ 6.717 kg × 100% = 79.4% [1]" [2]

Source: Nygh 2017, P2 Q9

(c)(i) [1] for wholly correct diagram. Note: all electron symbols should be “dots” – no crosses (unlike dot-and-cross diagram)

(c)(ii) Number of sigma-bonds = 3; Number of pi-bonds = 1. [1] for both correct answers. No half-marks

(c)(iii) Trigonal planar. [1] The central atom has 0 lone pairs and 3 bonding pairs/ 3 electron groups. [1]

(c)(iv) (Phosgene contains three polar bonds that are not symmetrically arranged around the central C atom hence) there is a net dipole moment. [1] Reject: movement instead of moment; comparison of electronegativity between O & Cl; “O is the most electronegative atom in the molecule”; any mention of specific polar bonds without looking at overall molecule. Total [8]

Source: Nygh 2019, P2 Q2

(a)(i) The greater the number of bonds between two atoms, the larger the bond energy. [1]

(a)(ii) Any set containing 2 or more bonds e.g. C-C, C=C and C≡C; N-N and N=N; etc (Atoms involved must be same across all bonds, i.e. cannot compare H-H and H-C) [1]

(b)(i) The bond length decreases with an increased number of bonds between the same types of atoms. [1]

(b)(ii) The greater the number of shared electrons (or bonds) [1], the stronger the (electrostatic forces of) attraction OR stronger covalent bonds (between the nucleus of each atom and the shared electrons) [1] (Must show correct cause and effect - cause: number of bonds increase; effect: bond length decreases)

(c) Across a period, effective nuclear charge increases OR force of attraction between electrons and nucleus increases [1], shielding remains constant [1] and bond length decreases. [1] Total [8]

Source: Nygh 2019, P2 Q7

(b)(i) covalent and ionic bonding

(b)(ii) Dot-and-cross diagram of hydrogen chloride: H and Cl share one pair of electrons (one electron contributed by H, one by Cl); Cl has 7 valence electrons in total, i.e. the shared pair plus three lone pairs drawn as crosses, giving Cl a full shell. Mark scheme: 1 mark for correct number of shared electrons; 1 mark for correct number of atoms and valence electrons for chlorine; 1 mark deducted if Lewis structure is drawn

Source: Nygh 2020, P2 Q1

(a)(i) A covalent bond is the electrostatic forces of attraction between the shared/bonding (pair of) electrons and the (positive) nuclei (of the two atoms)

(a)(ii) HI is able to conduct electricity in the aqueous state, as it is able to ionise/dissociate in water to form mobile ions. HI(aq) → H+(aq) + I–(aq) (accept no state symbols given)

(b) In solid state, the oppositely charged ions are held in fixed positions/ there are no free-moving / mobile ions to conduct electricity. In liquid state, the electrostatic forces of attraction between ions are weakened, Hence, there are free-moving / mobile ions to conduct electricity

(c) Strong covalent bonds between atoms require a lot of energy to break

(d) From the properties, aluminium chloride is covalently bonded, but it is expected to have ionic bonds (because the compound is formed between a metal and a non-metal). Accept answers with the same idea, e.g. aluminium chloride forms a covalent bond between a metal and a non-metal although covalent bonding usually occurs between non-metals

Source: Nygh 2020, P2 Q6

(b) Boiling point increases down the group 17 halogens. [1] as number of electrons increases/Mr increases/molecular size increases/electron cloud size increases from C_l2 to B_r2 to I₂. [1] More energy required to overcome stronger intermolecular forces. [1]

Source: Nygh 2021, P2 Q1

2 [Al]3+ 3[O]2− (dot-and-cross diagram: two aluminium ions Al₃⁺ shown with an empty outer shell, and three oxide ions O₂⁻ each shown with a full outer shell of 8 electrons drawn as dots and crosses; the electrons transferred from the two Al atoms are shown as dots/crosses on the oxide ions).

Source: Nygh 2021, P2 Q3

Silicon has a giant molecular/covalent structure; Larger amount of energy needed to overcome strong covalent bonds between silicon atoms. Chlorine has a simple molecular/covalent structure; Smaller amount of energy needed to overcome weak intermolecular forces (between its molecules). [1] each

Source: Nygh 2021, P2 Q8

(a)(ii) Strong electrostatic forces of attraction between metallic cations and ‘sea’ of delocalised electrons. [1] well-labelled diagram [1]

(b) Potassium loses 1 valence electron to form potassium ion. Calcium loses 2 valence electrons to form calcium ion. OR Potassium has 1 valence electron / Calcium has 2 valence electrons. OR Potassium has 1 less valence electron than calcium. [1] Calcium contributes more delocalised / free-moving / mobile electrons per atom and has higher electrical conductivity. [1]

(c) Each carbon atom uses 3 out of 4 of its valence electrons for bonding / forms bonds with 3 other carbon atoms. [1] Covalent bonds / giant covalent/molecular structure / shares electrons [1]. one valence electron (not used in bonding) that is delocalised / free-moving / mobile and is able to conduct electricity. [1]

Source: Nygh 2021, P2 Q8

(a) Simple covalent/molecular structure [1]; No mobile ions or delocalised electrons to conduct electricity [1]

(b) Low boiling point [1]; Weak intermolecular forces between molecules require little energy to overcome [1]

(c) Borospherene has more electrons / larger Mr / larger molecular size than methane, [1] so it has stronger intermolecular forces that require more energy to overcome [1]

(d) Boron has only 3 valence electrons so it is not expected to form more than 3 bonds / not expected to form 4 or 5 bonds. [1]

Source: Nygh 2022, P2 Q3

(a)(i) Transition metals have a giant metallic structure. [1] The strong electrostatic forces of attraction between the metallic cations and the sea of delocalised electrons require a lot of energy to overcome. [1]

(c) Dot-and-cross diagram of zinc chloride: the Zn₂⁺ ion is shown with an empty outer shell, and the two Cl^- ions are each shown with a full outer shell of 8 electrons drawn as dots and crosses (two electrons transferred from the zinc atom shown as crosses on the chloride ions). [1]

Source: Nygh 2022, P2 Q8

Lewis structure O=C=O with two lone pairs on each O atom and two double bonds to the central C; shape: linear; bond angle: 180o (two bond pairs and no lone pair on the central carbon lead to a linear molecule).

Source: Acsi 2019, P2 Q2

The two simple covalent products are CO₂ and H₂O. CO₂ molecules have only weak London (dispersion) forces between them, so CO₂ is a gas at r.t.p.; H₂O molecules have much stronger hydrogen bonds between them, so more energy is needed to separate them and water is a liquid at r.t.p.

Source: Acsi 2019, P2 Q4

(a) Magnesium has a higher melting point than sodium because each Mg atom contributes 2 delocalised electrons and forms Mg₂⁺ ions of higher charge, so the electrostatic metallic bonding is stronger and more energy is needed to overcome it. (b)(i) The more efficiently (closely) the atoms are packed, the higher the melting point, because the atoms are held by stronger metallic bonds. (b)(ii) The packing/closely-packed rigid lattice does not contribute, because on melting the rigid lattice is broken down, so packing no longer affects the strength of the metallic bonds. (c) Any one of: good electrical conductivity / good thermal conductivity / malleable / ductile / high density / shiny (lustrous)

Source: Acsi 2019, P2 Q8

(a) Electronegativity is the ability/tendency of an atom to attract a shared pair of electrons towards itself in a covalent bond. (b)(i) From PH₃ to SbH₃ the boiling point increases down the group: the molecules have more electrons and a larger surface area, so London dispersion forces between molecules become stronger and more energy is needed to overcome them. (b)(ii) NH₃ is an exception because it forms hydrogen bonds between its molecules (N–H···N), giving it an unexpectedly high boiling point for such a small molecule

(c) Lewis structure: N with one lone pair and three single N–H bonds; bond angle about 107°; shape trigonal pyramidal

Source: Acsi 2020, P2 Q6

(a) Graphite has a layered (giant covalent) structure in which the layers of carbon atoms are held together only by weak forces of attraction (London dispersion forces); these allow the layers to slide over one another, so a higher graphite content lets the lead slide across the paper more easily. (b) Graphite has delocalised electrons that are free to move, so pencil ‘lead’ conducts electricity and can act as an inert electrode

(c) High melting point — a carbon nanotube is a giant covalent (macromolecular) structure in which each carbon atom is held by strong covalent bonds, so a large amount of energy is needed to break them

(d)(i) β-Sn (white tin) — it has a metallic close-packed lattice whose layers can slide over one another

(d)(ii) e.g. soldering / tin-plating (protective coating) of other metals

Source: Acsi 2020, P2 Q7

Mark scheme answer is a dot-and-cross diagram of the nitrogen molecule (N₂) showing a triple bond - three shared pairs of electrons between the two nitrogen atoms - with the remaining valence electrons shown as lone pairs on each nitrogen atom. (The answer file prints this label with no accompanying text.)

Source: Rvhs 2023, P2 Q3

(b) Both 1st and 2nd series of coins are made of alloys. In an alloy, the different size of atoms disrupts the orderly arrangement of atoms in pure metals. Hence, the layer of atoms cannot slide easily in an alloy (brass vs copper). Hence coins made from mixture of metals are stronger / harder

(d)(i) A lot of energy is required to overcome the strong electrostatic attraction between metal cations and (sea of) delocalised / mobile valence electrons. Hence it does not have the ability to expand upon heating

(d)(ii) Diamond has a giant covalent structure while poly(tetrafluoroethene) has a macromolecular structure. More energy is required to overcome the stronger covalent bonds between the carbon atoms than the weaker intermolecular forces of attraction between the poly(tetrafluoroethene) molecules. Hence, diamond has a lower CTE than poly(tetrafluoroethene)

(d)(iii) Hard; Non-conductor of electricity; Insoluble in all solvents / water; High melting & boiling points

Source: Rvhs 2023, P2 Q5