CHEM20 Session 08 — Intermolecular forces and properties

Delivery id: CHEM20 (SCN2796)

Time: 75 minutes

Unit A: Diversity of Matter and Chemical Bonding

Alberta POS: https://open.alberta.ca/publications/3950147-updated-2014

Official outcomes

Code	Type	Role today
20-A2.7k	Knowledge	Explain IMF: London (dispersion), dipole-dipole, hydrogen bonding
20-A2.8k	Knowledge	Relate MP, BP, enthalpies of fusion and vaporization to predicted IMF
20-A2.2sts	STS	Relate properties to IMF using melting/boiling-point evidence (dataset, not a new flame lab)

Learning targets

1. Distinguish intramolecular (bonds inside a molecule or ionic lattice) from intermolecular (between molecules).
2. Rank IMF strength qualitatively: hydrogen bonding > dipole-dipole > London (for similar size); London increases with more electrons / larger polarizability.
3. Identify H-bonding only when H is bonded to N, O, or F and a lone pair on N, O, or F is available (the Chem 20 rule).
4. Use a boiling-point table to justify rankings: He < CH4 < HCl < H2O; I2 vs F2; ethanol vs dimethyl ether (isomers, same formula C2H6O).

Prerequisites

Lewis + VSEPR. Polar bonds preview: ΔEN > ~0.5. Full molecular polarity is Session 09; today use “HCl is a polar molecule; Cl2 is not” as given when needed.

Diagnostic/retrieval

1. Shape of H2O and CO2.
2. Define intramolecular force.
3. Which has the higher BP, H2O or H2S, if you had to guess from last week’s water-as-solvent talk?
4. True/false: boiling water breaks O–H covalent bonds into H and O atoms.

Instruction

Intramolecular vs intermolecular. Boiling and melting molecular substances overcome IMF, not covalent bonds. Liquid water is still H2O molecules. (Ionic melting breaks the ionic lattice — that is intramolecular ionic bonding, Session 03 — different story.)

London (dispersion) forces: instantaneous dipoles in all atoms and molecules. Only IMF in nonpolar substances (He, F2, Cl2, Br2, I2, CH4, CO2). Stronger for more electrons: F2(g) BP −188 °C; Cl2(g) −34 °C; Br2(ℓ) 59 °C; I2(s) 184 °C (sublimes). I2 is a solid at room temperature because of stronger London forces, not because it is ionic.

Dipole-dipole: between polar molecules (HCl, CH3Cl, SO2). Extra attraction beyond London.

Hydrogen bonding: a special strong dipole-dipole when H is attached to N, O, or F. Water, HF, NH3, alcohols (O–H). Hydrogen bonding is not a covalent bond to hydrogen in this unit’s IMF list; it is intermolecular (except we do not open DNA here).

Properties (20-A2.8k). Stronger IMF → higher melting point, higher boiling point, larger enthalpy of fusion (ΔHfus) and enthalpy of vaporization (ΔHvap), generally higher viscosity, lower vapour pressure at a given T.

Dataset (use; do not heat):

Substance	Dominant IMF	BP (°C)	Notes
He	London	−269	few electrons
CH4	London	−162
HCl	dipole-dipole + London	−85	polar molecule
H2S	dipole-dipole + London	−60	no H-bonding (H not on N/O/F)
NH3	H-bonding	−33
HF	H-bonding	20
H2O	H-bonding	100	two O–H + two lone pairs
C2H5OH	H-bonding	78	ethanol
CH3OCH3	dipole + London	−25	dimethyl ether, isomer of ethanol
NaCl	ionic lattice	1413 (MP 801)	not IMF

Enthalpies (typical, for qualitative talk): ΔHvap(H2O) 40.7 kJ/mol at 100 °C vs ΔHvap(CH4) 8.2 kJ/mol at its BP — water’s H-bonding.

Hydride anomaly: H2O, HF, NH3 have much higher BP than the trend of H2S, HCl, PH3. That is evidence for hydrogen bonding (20-A2.2sts).

Timed visuals

id	trigger	duration	alt	teacher cue	tutor cue
CHEM20-VIS-08-SCALE	After retrieval	55 s	Bond vs IMF energy cartoon (hundreds of kJ/mol vs tens)	“Boiling does not yield hydrogen gas.”	Refuse electrolysis-as-boiling.
CHEM20-VIS-08-HALOGENS	Minute 16	50 s	F2 gas, Br2 liquid, I2 solid	“Same bond type, different London.”	No iodine heating demo unless school fumehood protocol — not required.
CHEM20-VIS-08-HYDRIDES	Minute 28	70 s	BP vs period for Group 16 hydrides	Circle H2O off the trend	Quote table values only.

Modelling/worked examples

Example 1. Rank BP: F2, Cl2, Br2. All London. More electrons down the group → F2 < Cl2 < Br2. Matches the table.

Example 2. Why is H2O liquid and H2S gas at 25 °C? Both bent polar molecules; only water H-bonds (H on O). H2S has dipole-dipole + London but weaker overall than water’s H-bonding network.

Example 3. Ethanol vs dimethyl ether (both C2H6O). Ethanol has O–H → H-bonding → BP 78 °C. Ether has no O–H → BP −25 °C. Same molecular formula, different IMF — 20-A2.2k meets 20-A2.8k.

Example 4. CO2 vs H2O. CO2 is a nonpolar molecule (Session 09) with London only; sublimes at −78 °C (dry ice). Water H-bonds. Do not argue from “CO2 has double bonds so higher BP.” Intramolecular bond order is not the IMF.

Example 5 — enthalpy. To vaporize 1 mol water at 100 °C takes ~40.7 kJ (IMF), not the ~900+ kJ to break two O–H covalent bonds per mole.

Guided practice + answers

1. Dominant IMF in CH4, HCl, NH3, I2. Answers: London; dipole-dipole (+London); H-bonding; London.
2. Which has higher BP, C2H6 or C3H8? Answer: C3H8 (more electrons, stronger London).
3. Does CH4 hydrogen-bond? Answer: No. H is attached to C, not N/O/F.
4. Why is I2 a solid but F2 a gas? Answer: Stronger London in I2 (more electrons), not ionic bonding.
5. Does boiling water produce O2 and H2? Answer: No. That would be a chemical decomposition / electrolysis, not vaporization.

Independent practice

• FOUNDATION: Match substance → IMF for He, Cl2, HCl, H2O. State whether melting ice breaks covalent bonds (no).
• CORE: Rank BP and justify: (a) F2, Cl2, Br2 (b) CH4, NH3, H2O (c) H2S vs H2O. Identify IMF in ethanol.
• PROFICIENT: Explain the Group 16 hydride BP anomaly using the table. Compare ΔHvap qualitatively for H2O vs CH4. Explain why NaCl’s high MP is not an IMF example.
• ADVANCED: C2H5OH vs CH3OCH3 argument in six sentences with data. Predict which household liquid (water vs mineral oil, primarily London) has the higher BP. Project: a cleaner that evaporates quickly likely has weaker IMF — do not recommend solvent sniffing or distillation at home.

Project connection

Water’s hydrogen bonding explains high BP, high ΔHvap, and (next session) polarity/solubility of ionic salts. A “safe water” brief should not treat boiling as making hydrogen gas. Vinegar (acetic acid) also H-bonds (O–H); oil does not mix well — polarity Session 09.

Checks

True/false freeze: “Hydrogen bonding is the bond inside H2.” False. Inside H2 is a covalent single bond. Hydrogen bonding is intermolecular (H2 cannot H-bond anyway).

Misconceptions

Wrong	Repair	Do not say
“Boiling breaks molecules into atoms.”	Overcomes IMF.	“Sort of broken.”
“Hydrogen bonding is any molecule with hydrogen.”	H must be on N, O, or F.	“Nice reasoning.”
“I2 is ionic because it is a solid.”	London forces; still I–I covalent molecules.	—
“Stronger covalent bonds mean higher BP.”	BP tracks IMF for molecular substances.	—
“HF hydrogen-bonds so HF is ionic.”	HF is a polar covalent molecule with strong IMF.	—

AI Tutor prompts and boundaries

Allowed: “Rank IMF and BP using the class table.” “Why doesn’t CH4 hydrogen-bond?”

Forbidden: Home boiling-point labs with thermometers in oil/alcohol on a stove; iodine heating; praising “H2O decomposes at 100 °C”; inventing ΔH numbers not in the table.

Human-tutor handoff

If graphs intimidate: use the table, not a sketch. If a student wants to “try dry ice at home,” stop (CO2 burns / asphyxia in enclosed spaces; not assigned).

Exit ticket + answers

1. Name the three IMF in 20-A2.7k.
2. Dominant IMF in F2 and in H2O.
3. Why is H2O’s boiling point higher than H2S’s?
4. True/false: melting ice breaks O–H covalent bonds.

Answers: 1. London (dispersion), dipole-dipole, hydrogen bonding. 2. London; hydrogen bonding. 3. Water hydrogen-bonds; H2S does not (H on S). 4. False.

Homework

Complete CORE rankings. Optional: read the provided BP CSV (not a web hunt for random values).

Teacher guidance

Use one consistent data card so arguments do not fight over 1 °C. Do not run a Bunsen BP lab this week. Iodine demo is unnecessary.

Materials

IMF poster; hydride BP graph; ethanol vs ether card; data sheet with the table above.

Preparation

Print data sheets. Confirm no student still thinks Session 04 vinegar “proved ionic.”

Timing

Min	Move
0–6	Retrieval (boiling ≠ decomposition)
6–22	Three IMF + intramolecular contrast
22–38	Dataset worked examples
38–50	Guided
50–66	Independent
66–75	Exit

Online alternative

Spreadsheet of the same BP table + ranking quiz. PhET IMF if approved. No kitchen oil-heating.

Low-technology alternative

Board-copied table. Three coloured sticky notes (London / dipole / H-bond) placed on molecule cards.