AQA A-Level Chemistry: Bonding (7405) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

These questions cover section 3.1.3 (Bonding), 3.1.3.1 to 3.1.3.7, of the AQA AS and A-level Chemistry specification (7404/7405). Read the study guide and revision notes first. Course hub: AQA A-level Chemistry. Checklist: printable checklist. To spot weak areas fast, try the free 10-minute diagnostics.

Questions

1. (a) Predict the charges on the simple ions formed by strontium and selenium. [2] (b) Write the formulas of ammonium carbonate and aluminium sulfate. [2]

2. Ammonia reacts with boron trifluoride, BF₃, to form an addition compound.

(a) State what is meant by a co-ordinate bond. [1] (b) Draw the displayed formula of the product, using an arrow to show the co-ordinate bond. [2] (c) Explain why boron trifluoride can accept a pair of electrons. [1]

3. (a) Outline how the particles in magnesium metal are held together. [2] (b) Account for magnesium melting at a higher temperature than sodium. [3]

4. Four solids, P to S, were tested. Data are invented for this question.

Solid	Melting point / °C	Solid conducts?	Molten conducts?	Soluble in water?
P	782	No	Yes	Yes
Q	Above 2500	No	Not melted	No
R	56	No	No	No
S	930	Yes	Yes	No

State the type of structure of each solid and give one piece of evidence for each. [4]

5. (a) Draw a diagram of one layer of graphite, showing at least three joined rings. [2] (b) Graphite conducts electricity, whereas diamond is an insulator. Explain the difference. [2] (c) Explain why the layers of graphite slide over one another easily. [2]

6. (a) Explain why the temperature of a pure solid stays constant while it melts. [2] (b) State what is overcome when solid iodine sublimes and when sodium chloride melts, and why the second needs a much higher temperature. [2] (c) Explain why ice floats on liquid water. [2]

7. Boron trifluoride, BF₃, and nitrogen trifluoride, NF₃, are both molecules with three fluorine atoms. Predict the shape of each and explain why their bond angles differ. [5]

8. (a) Explain why SF₆ is octahedral, and give its bond angle. [3] (b) Xenon has 8 outer electrons. Deduce the shape of XeF₄, explaining why it is not tetrahedral. [3]

9. (a) State the shape of PCl₅ and the bond angles in it. [3] (b) The ion ICl₂⁻ has a central iodine atom with 7 outer electrons. Deduce its shape and bond angle. [2]

10. (a) State what is meant by electronegativity. [1] (b) Show that the C-Cl bond is polar. [1] (c) Explain why BF₃ has polar bonds but no permanent dipole, whereas CH₂Cl₂ has a permanent dipole. [3]

11. (a) The boiling points of HCl, HBr and HI increase in that order, but HF boils at a much higher temperature than HCl. Explain both observations. [5] (b) Ethanol, CH₃CH₂OH, and methoxymethane, CH₃OCH₃, both have relative molecular mass 46.0. Explain why ethanol has the higher boiling point. [3]

Answers

1. (a) Strontium is in Group 2, so Sr²⁺ [1]. Selenium is in Group 16, so Se²⁻ [1]. (b) Two NH₄⁺ balance one CO₃²⁻: (NH₄)₂CO₃ [1]. Two Al³⁺ (+6) balance three SO₄²⁻ (-6): Al₂(SO₄)₃ [1]. Examiner insight: Brackets round the compound ion are usually needed to show a multiple, and a formula written without them, such as Al₂SO₄₃, would not be clear.

2. (a) A pair of electrons shared between two atoms, both supplied by the same atom [1]. (b) H₃N→BF₃, drawn with N bonded to three H, B bonded to three F, and the arrow pointing from N to B [1]. All other bonds drawn as lines [1]. (c) Boron has only six electrons in its outer shell in BF₃ (three bonding pairs), so it has a vacancy for a lone pair from nitrogen [1]. Examiner insight: The direction of the arrow is the point being tested. An arrow drawn from boron to nitrogen names the wrong atom as donor.

3. (a) Positive Mg²⁺ ions arranged in a lattice [1], attracted electrostatically to the delocalised electrons around them [1]. (b) Magnesium ions have a higher charge and are smaller [1]. Each atom contributes two delocalised electrons, not one [1]. The attraction between ions and delocalised electrons is stronger, so more energy is needed to melt it [1]. Examiner insight: A comparison needs both substances addressed. Stating only that magnesium has strong bonding does not explain the difference from sodium.

4. P ionic: conducts only when molten, and dissolves in water [1]. Q macromolecular (giant covalent): melting point above 2500 °C and no conduction [1]. R molecular: low melting point, no conduction [1]. S metallic: conducts as a solid [1]. Examiner insight: Naming the structure alone is not enough. A property taken from the table is required to support each choice.

5. (a) Hexagons sharing edges [1], with each carbon joined to three others [1]. (b) Each carbon in graphite uses three outer electrons in bonding, so one per atom is delocalised and free to move between layers [1]. Diamond has all four outer electrons held in localised covalent bonds, so no electrons are free to move [1]. (c) Only weak induced dipole-dipole forces act between layers [1], so little energy is needed to move them past each other [1]. Examiner insight: Saying that the layers are “not bonded” is weaker than naming the force that acts between them.

6. (a) The energy supplied is used to overcome the attractions between particles [1] and raises their potential energy but not their mean kinetic energy, so the temperature stays constant [1]. (b) Iodine: weak intermolecular forces between I₂ molecules, not the covalent bond in I₂ [1]. Sodium chloride: strong electrostatic attraction between oppositely charged ions in the lattice, which needs much more energy [1]. (c) In ice every molecule forms hydrogen bonds with four neighbours, giving an open lattice with large gaps [1]. In liquid water some of these bonds break and molecules pack closer, so ice is less dense [1]. Examiner insight: “Bonds break” is ambiguous for molecular substances. Specify intermolecular forces or covalent bonds.

7. BF₃ has three bonding pairs and no lone pair, so it is trigonal planar, 120° [1]. The pairs repel equally [1]. NF₃ has four electron pairs: three bonding and one lone pair [1], so it is trigonal pyramidal [1]. The lone pair repels more than a bonding pair, which squeezes the bond angle to less than 109.5° [1]. Examiner insight: A shape and an angle without a reason about repulsion between pairs earns less than a full explanation.

8. (a) Sulfur is surrounded by six bonding pairs and no lone pair [1]. Equal repulsion pushes the pairs to maximum separation [1], giving octahedral with all angles 90° [1]. (b) Xenon has 8 + 4 = 12 outer electrons, so six pairs: four bonding and two lone [1]. The two lone pairs sit opposite each other to reduce lone pair-lone pair repulsion [1], so the shape is square planar with 90° angles. A tetrahedron has only four pairs [1]. Examiner insight: For six pairs with two lone pairs, state where the lone pairs are placed, not only the name of the shape.

9. (a) Phosphorus has five bonding pairs and no lone pair [1], giving trigonal bipyramidal [1] with angles of 120° and 90° [1]. (b) Iodine has 7 + 2 + 1 = 10 electrons, so five pairs: two bonding and three lone [1]. The lone pairs lie in the trigonal plane, leaving the shape linear, 180° [1]. Examiner insight: Counting the extra electron from the negative charge is the step that is often missed in questions on ions.

10. (a) The ability of an atom to draw the bonding pair of electrons towards itself in a covalent bond [1]. (b) Chlorine is more electronegative than carbon, so the bond is C(δ+)-Cl(δ-) [1]. (c) B-F bonds are polar [1], but BF₃ is symmetrical (trigonal planar) so the dipoles cancel [1]. CH₂Cl₂ has two different types of outer atom, so its dipoles do not cancel and it has a net dipole [1]. Examiner insight: A bond-polarity answer should name the more electronegative atom and use partial-charge labels on the right atoms.

11. (a) From HCl to HI the number of electrons increases [1], so the induced dipole-dipole forces strengthen [1], and a greater energy input is needed to separate molecules [1]. HF has hydrogen bonds [1], involving hydrogen on a highly electronegative fluorine and a lone pair on a neighbouring fluorine, which are stronger than the other forces present [1]. (b) Ethanol has an O-H bond, so its molecules form hydrogen bonds [1]. Methoxymethane has no hydrogen on oxygen, so only permanent dipole-dipole and induced dipole-dipole forces act [1]. With equal relative molecular mass the induced forces are similar, so the extra hydrogen bonding makes ethanol’s boiling point higher [1]. Examiner insight: A comparison of two forces should state which is stronger, and extended answers are easier to credit when they follow the order: force, strength, energy needed.

Where marks are usually lost

• Leaving out lone pairs when counting electron pairs for an ion.
• Giving a shape name without any reference to repulsion between pairs.
• Referring to “breaking bonds” when a molecular substance changes state.
• Treating polar bonds as meaning a polar molecule.
• Naming hydrogen bonding where the hydrogen is bonded to carbon.
• Failing to make a comparison explicit when asked “why is X higher than Y”.
• Drawing a co-ordinate arrow in the wrong direction.

Frequently asked questions

Which part of the AQA Chemistry specification do these bonding questions cover? They cover section 3.1.3 Bonding, points 3.1.3.1 to 3.1.3.7, of the AQA AS and A-level Chemistry specification (7404/7405). Use them after the bonding study guide and revision notes.

Why is “bonds break” a weak answer for melting a molecular substance? It is ambiguous: when a simple molecular substance melts or boils, intermolecular forces are overcome, not covalent bonds. State which type of force or bond is involved.

How do you explain a molecular shape for full marks? Count all the bonding pairs and lone pairs, including those in ions, and refer to repulsion between electron pairs. A shape name with no reference to repulsion is unlikely to earn full credit.

Does a molecule with polar bonds have to be polar? No. A molecule with polar bonds can be non-polar if its shape makes the dipoles cancel, so treating polar bonds as meaning a polar molecule is a common error.

How should comparison questions about melting points be answered? Address both substances explicitly, naming the bonding or force in each and saying which is stronger. Describing only one substance does not explain the difference.

Next steps

• Review the revision notes and the study guide.
• See the AQA A-level Chemistry hub and checklist.
• Take the free 10-minute diagnostics.
• Book a free trial class.

Official syllabus

Source: AQA AS and A-level Chemistry (7404/7405) specification, version 1.1, December 2015, published by AQA, section 3.1.3 Bonding.