Forces between molecules — AQA A-Level Chemistry
Test yourself on Forces between molecules with AQA A-Level practice questions.
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Forces between molecules explained
Three forces act between molecules. Induced dipole-dipole forces, also called van der Waals, dispersion or London forces, act between every molecule: the constantly moving electrons create an instantaneous dipole which induces a matching dipole in a neighbour, and the strength grows with the number of electrons and with the surface area in contact. Permanent dipole-dipole forces act in addition wherever the molecule has a permanent dipole, with δ⁺ of one molecule attracted to δ⁻ of the next. Hydrogen bonding is the strongest and needs two things: a hydrogen atom bonded directly to nitrogen, oxygen or fluorine, and a lone pair on a nitrogen, oxygen or fluorine atom of another molecule.
The melting and boiling points of molecular substances are influenced by the strength of these intermolecular forces.
Melting and boiling a molecular substance separates whole molecules, so only the intermolecular forces are overcome; the covalent bonds inside each molecule are untouched. Stronger intermolecular forces therefore mean a higher melting or boiling point. Comparisons follow a fixed order: identify every force present in each substance, decide which is strongest, and only then compare. Hydrogen bonded liquids boil highest, polar molecules of similar size come next, and non-polar molecules lowest. Where the same force operates in both substances, compare the number of electrons and the surface area in contact, so a longer unbranched chain boils higher than a shorter or more branched isomer with the same formula.
The importance of hydrogen bonding in the low density of ice and the anomalous boiling points of compounds.
Water is exceptional because each molecule can form up to four hydrogen bonds, two through its hydrogen atoms and two through the lone pairs on its oxygen. When water freezes, these hydrogen bonds hold the molecules in a rigid, open tetrahedral cage, keeping them further apart on average than in the liquid, so ice is less dense than water and floats. On melting, some hydrogen bonds break and the structure collapses inwards. The same bonding explains the anomalous boiling points of water, ammonia and hydrogen fluoride, each far above the trend set by the other hydrides of its group. Water tops even hydrogen fluoride because it forms more hydrogen bonds per molecule.
Students should be able to: explain the existence of these forces between familiar and unfamiliar molecules explain how melting and boiling points are influenced by these intermolecular forces.
For an unfamiliar molecule, work from the structure. Induced dipole-dipole forces are always present, so start there and judge their size from the number of electrons and the shape. Then look for a permanent dipole: polar bonds in an unsymmetrical arrangement. Finally check for a hydrogen atom bonded directly to nitrogen, oxygen or fluorine, which allows hydrogen bonding to a lone pair on a neighbouring molecule. Boiling point then follows from the strongest force present. Ethanol and ethylamine both hydrogen bond, so both boil above methoxymethane, which has only permanent dipole-dipole and induced dipole forces; ethanol beats ethylamine because oxygen is more electronegative than nitrogen.
Your focus
- Identify every intermolecular force acting in a named substance and justify each from its structure.
- Draw a diagram of a hydrogen bond between two molecules, showing partial charges and the lone pair involved.
- Explain why induced dipole-dipole forces increase in strength as a homologous series is ascended.
Show all 12 objectives
- Explain a difference in boiling point between two substances by naming and comparing their intermolecular forces.
- State precisely what is overcome when a molecular solid melts.
- Predict which of two isomers with the same molecular formula boils at the higher temperature and justify the prediction.
- Explain why ice is less dense than liquid water in terms of the number and arrangement of its hydrogen bonds.
- Account for the boiling point of water lying far above that of the other Group 6 hydrides.
- Draw the hydrogen bonding between two water molecules, showing lone pairs and partial charges.
- Deduce, from the displayed structure of an unfamiliar molecule, which intermolecular forces act between its molecules.
- Explain a trend in boiling point across three related compounds by naming the strongest force in each.
- Justify which of two hydrogen bonded liquids has the higher boiling point using the electronegativity of the donor atom.
Forces between molecules exam tips
Quick Revision Summary (Key Takeaway)
Forces between molecules, or intermolecular forces, comprise induced dipole-dipole (van der Waals), permanent dipole-dipole, and hydrogen bonds, determining key physical properties like boiling points and solubility. Understanding how molecular shape, electron count, and electronegativity dictate these electrostatic attractions is essential for mastering AQA A-Level Chemistry physical trends.
Topic Overview
Forces between molecules encompass the electrostatic attractions existing between discrete covalent molecules, categorised into van der Waals (induced dipole-dipole) forces, permanent dipole-dipole forces, and hydrogen bonds. These non-covalent interactions directly govern macroscopic physical properties including melting points, boiling points, enthalpy of vaporisation, viscosity, and solubility.
Within AQA A-Level Chemistry, mastering this topic allows students to explain periodic anomalies and predict how changes in molecular structure influence chemical behaviour. It forms the foundational underpinning for understanding organic reaction mechanisms, polymer characteristics, and complex biochemical structures such as DNA and proteins.
Key Concepts
- →Van der Waals (induced dipole-dipole) forces arise from temporary electron density fluctuations that induce dipoles in adjacent molecules; their strength increases with greater total electron count and larger surface contact area.
- →Permanent dipole-dipole forces operate exclusively between polar molecules that possess an overall permanent dipole caused by asymmetrical electronegativity differences.
- →Hydrogen bonding is the strongest type of intermolecular force, occurring when hydrogen is covalently bonded to highly electronegative nitrogen, oxygen, or fluorine atoms bearing lone pairs.
- →Physical phase changes such as boiling and melting involve overcoming intermolecular attractions, leaving internal covalent bonds unaltered.
Marking Points
- one mark for identifying induced dipole-dipole forces as present between all molecules
- one mark for adding permanent dipole-dipole forces only where the molecule has a permanent dipole
- one mark for hydrogen bonding identified only where hydrogen is bonded directly to nitrogen, oxygen or fluorine
- one mark for a diagram showing the lone pair and the hydrogen bond drawn to it
- one mark for the order of strength, hydrogen bonding strongest and induced dipole forces weakest for small molecules
- one mark for stating that only intermolecular forces are overcome on melting or boiling
- one mark for naming the strongest intermolecular force present in each substance
- one mark for comparing the number of electrons or surface contact where induced dipole forces dominate
- one mark for the conclusion about which substance has the higher melting or boiling point, with a reason
- one mark for each water molecule forming up to four hydrogen bonds using its hydrogen atoms and the lone pairs on oxygen
- one mark for the rigid open or tetrahedral lattice holding the molecules further apart than in the liquid
- one mark for the resulting lower density of ice so that it floats on water
- one mark for hydrogen bonding being stronger than the dipole-dipole and induced dipole forces in the other hydrides of the group
- one mark for naming every intermolecular force acting in each molecule, not only the strongest
- one mark for justifying hydrogen bonding by identifying a hydrogen bonded to nitrogen, oxygen or fluorine and a lone pair to accept it
- one mark for identifying which force is strongest in each substance
- one mark for the comparison of boiling points with a reason based on those forces
Examiner Tips
- 💡List every force present, not just the strongest; induced dipole forces are always there.
- 💡In a hydrogen bond diagram show the lone pair, the partial charges and a roughly linear arrangement.
- 💡Check the hydrogen is bonded directly to nitrogen, oxygen or fluorine before claiming a hydrogen bond.
- 💡Open the answer with the words intermolecular forces are overcome; it protects you from the covalent bond error.
- 💡Name the forces in both substances before comparing; a one-sided answer rarely scores full marks.
- 💡If the only difference is chain shape, argue from surface contact between molecules.
- 💡Draw the open lattice with the hydrogen bonds marked as dashed lines and the lone pairs shown.
- 💡Say more hydrogen bonds per molecule when comparing water with hydrogen fluoride.
- 💡For an anomalous boiling point question, always compare with the trend in the rest of the group.
- 💡Annotate each structure with its polar bonds and any hydrogen attached to nitrogen, oxygen or fluorine before writing.
- 💡Compare like with like: molecules of similar electron count isolate the effect of the stronger force.
- 💡When two molecules both hydrogen bond, decide between them on electronegativity.
- 💡When explaining trends involving van der Waals forces, always link the increase in strength to 'more electrons' rather than 'greater Mr' or 'heavier molecule'.
- 💡Always state that forces between molecules are 'overcome' or 'broken' and explicitly state that this requires 'more energy' or 'less energy'.
- 💡When drawing hydrogen bonds, ensure you include three features: partial charges (delta+ and delta-), the relevant lone pair of electrons on the electronegative atom, and a dashed line drawn directly from the lone pair to the delta+ hydrogen atom.
Common Mistakes
- claiming that molecules which hydrogen bond have no induced dipole-dipole forces
- assuming any molecule containing hydrogen can hydrogen bond, so including methane or hydrogen chloride
- drawing the hydrogen bond to the wrong atom or omitting the lone pair it must reach
- describing intermolecular forces as bonds within the molecule being broken
- writing that covalent bonds break, which is explicitly refused by the mark scheme
- claiming a branched isomer boils higher than its straight-chain isomer
- comparing relative molecular masses rather than electron count and contact area
- ignoring hydrogen bonding when it is present and arguing only from molecular size
- saying the water molecules themselves expand on freezing
- claiming the hydrogen bonds lengthen rather than the lattice holding molecules apart
- explaining the high boiling point of water by strong covalent bonds
- stating that hydrogen fluoride boils higher than water, ignoring the number of hydrogen bonds per molecule
- naming only one force when several act at once
- ignoring induced dipole-dipole forces in a polar or hydrogen bonded molecule
- using bond enthalpy data to argue about boiling points, which concern intermolecular forces
- referring to breaking covalent bonds anywhere in an answer about boiling
- Believing that hydrogen bonds are covalent bonds found inside a molecule rather than electrostatic attractions acting between separate molecules.
- Assuming that polar molecules only experience permanent dipole-dipole forces, overlooking that van der Waals forces act between all molecular substances and often contribute the largest portion of total attraction.
- Attributing higher boiling points in larger molecules to 'higher molecular mass' instead of the correct scientific rationale: 'a greater number of electrons resulting in stronger van der Waals forces'.
Revision Plan
- 1Day 1-2: Review electronegativity and 3D molecular shapes to reliably predict overall molecular polarity.
- 2Day 3-4: Learn the physical origins and relative magnitudes of van der Waals, permanent dipole-dipole, and hydrogen bonds.
- 3Day 5-6: Practice sketching hydrogen bonding diagrams to examiner specifications and explaining Group 15, 16, and 17 hydride boiling point anomalies.
- 4Day 7: Complete past AQA exam questions focusing on boiling point comparisons between branched versus unbranched isomers and polar versus non-polar species.
Exam Question Types
- 📋Comparative boiling point questions: Explaining differences between hydrides, isomers, or homologous series using comparative intermolecular force arguments.
- 📋Structural diagram questions: Drawing two molecules showing all partial charges, relevant lone pairs, and a dashed line representing the hydrogen bond.
- 📋Anomalous properties of water: Explaining why ice is less dense than liquid water or why water has an unexpectedly high boiling point relative to Group 16 hydrides.
Command Word Expectations (AQA)
Identify the specific intermolecular forces present in each substance, state which force is stronger, and link this difference directly to the quantity of energy required to overcome them.
Identify both similarities and differences in the intermolecular forces operating within two or more substances and state the resulting differences in their physical properties.
Use the provided formula, shape, or electronegativity data to infer the predominant intermolecular force and predict relative physical behaviour without being explicitly told the answer.
How Students Lose Marks (Examiner Pitfalls)
Step-by-Step Worked Solutions
Question: Explain the trend in boiling points of the Group 4 hydrides: CH4 (-161.5 °C), SiH4 (-111.8 °C), GeH4 (-88.5 °C), and SnH4 (-51.8 °C).
- 1.Step 1: Identify the molecular shape and polarity. All Group 4 hydrides are tetrahedral symmetrical molecules; individual C-H/Si-H/Ge-H/Sn-H dipoles cancel out, so they are non-polar molecules.
- 2.Step 2: Identify the intermolecular forces present. The only intermolecular forces operating between these non-polar molecules are van der Waals (induced dipole-dipole) forces.
- 3.Step 3: Relate molecular structure to force strength. Moving down Group 4 from CH4 to SnH4, the number of electrons per molecule increases significantly (CH4 has 10 electrons, SnH4 has 54 electrons).
- 4.Step 4: Explain the consequence on boiling point. A larger electron cloud leads to greater electron fluctuations and stronger temporary/induced dipoles, resulting in stronger van der Waals forces that require more thermal energy to overcome.
Question: Ethanol (CH3CH2OH) and methoxymethane (CH3OCH3) have the same molecular formula (C2H6O) and identical Mr (46.0). Explain why the boiling point of ethanol (78 °C) is significantly higher than that of methoxymethane (-24 °C).
- 1.Step 1: Identify the functional groups and bonds present. Ethanol contains an -OH group where hydrogen is directly bonded to an electronegative oxygen atom. Methoxymethane contains a C-O-C ether linkage without any hydrogen bonded directly to oxygen.
- 2.Step 2: Determine the strongest intermolecular forces in each compound. Ethanol forms hydrogen bonds between molecules as well as permanent dipole-dipole and van der Waals forces. Methoxymethane forms permanent dipole-dipole and van der Waals forces, but cannot form hydrogen bonds with itself.
- 3.Step 3: Compare the relative strengths of these interactions. Hydrogen bonds are significantly stronger than permanent dipole-dipole and van der Waals interactions.
- 4.Step 4: Conclude regarding energy requirements. Considerably more energy is required to overcome the strong hydrogen bonds between ethanol molecules than to overcome the weaker permanent dipole-dipole forces between methoxymethane molecules.