Lesson 4: Physical Properties | CHE 2.5

Lesson at a Glance | He Tirohanga Whakamua

Do NowWhy ethanol mixes with water while hexane floats on top10 min

Intermolecular SpectrumDispersion vs permanent dipoles vs hydrogen bonds15 min

Chain Length TrendsBoiling point increase & water solubility decrease15 min

Portfolio EntryWrite Section 4: Organic Physical Properties & Solubility10 min

Exit VerificationPredict boiling points for propane, chloropropane, propan-1-ol5 min

Ngā Whāinga Ako | Learning Intentions

Students will know

How intermolecular forces govern physical properties: Alkanes/alkenes have weak temporary dipoles; Haloalkanes have permanent dipoles; Alcohols, carboxylic acids, and amines form strong hydrogen bonds.
Why boiling points increase with carbon chain length (larger electron cloud size leads to stronger temporary dipole dispersion forces requiring more thermal energy to overcome).
The "Like dissolves like" rule: Small polar molecules (ethanol, ethanoic acid) dissolve in polar water via hydrogen bonding; non-polar hydrocarbons do not dissolve in water (forming 2 layers) but dissolve in non-polar solvents.

Students will demonstrate

By completing a physical properties comparison matrix across 7 homologous series.
By completing Section 4 of their Level 2 Organic Chemistry Mastery Portfolio.

Curriculum alignment

NZC (2007) · Science · Level 7 · Material World: “Investigate and measure the chemical and physical properties of a range of groups of substances, for example, acids and bases, oxidants and reductants, and selected organic and inorganic compounds.”
NZC (2007) · Science · Level 7 · Material World: “Relate properties of matter to structure and bonding.”

Do Now | Tīmatanga Whakaaro (10 min)

Solubility Prompt:

"Methanol ( ${CH}_{3} OH$ ) mixes completely with water in all proportions. Why does octan-1-ol ( ${CH}_{3} ({CH}_{2})_{7} OH$ ) float insolubly on top of water like salad oil?"

Unpack: Both have a polar $- O H$ group capable of hydrogen bonding. However, octan-1-ol has a massive 8-carbon non-polar hydrocarbon tail. The non-polar hydrophobic tail disrupts water's hydrogen bonding network, making the long-chain alcohol insoluble in water.

Boiling Point & Intermolecular Forces Spectrum (15 min)

1. Hydrocarbons (BP: ~-42°C)

Propane ( $C_{3} H_{8}$ ): Non-polar. Weak temporary dipole-dipole forces only. Low boiling point, insoluble in water.

2. Haloalkanes (BP: ~46°C)

1-chloropropane ( $C_{3} H_{7} Cl$ ): Polar $C - C l$ bond. Permanent dipole-dipole forces. Higher BP, insoluble in water.

3. Alcohols (BP: ~97°C)

Propan-1-ol ( $C_{3} H_{7} OH$ ): Polar $- O H$ group forms strong hydrogen bonds. High BP, soluble in water.

4. Carboxylic Acids (BP: ~141°C)

Propanoic acid ( $C_{2} H_{5} COOH$ ): Forms stable hydrogen-bonded dimers. Highest BP, soluble in water.

Chain Length Trends (15 min)

Where this usually goes wrong: students write "bigger molecule, higher boiling point" with no mechanism. The mark is for the reason, not the trend.

Boiling point rises with chain length

A longer chain has more electrons and more surface area, so instantaneous dipoles are stronger and more numerous. More energy is needed to separate the molecules. Note this is about the forces BETWEEN molecules — the covalent bonds inside them are untouched by boiling.

Water solubility falls with chain length

An $- O H$ group hydrogen-bonds with water, but the hydrocarbon tail cannot. As the tail lengthens it outweighs the polar head, so solubility drops sharply. Same functional group, opposite trend to boiling point.

Do this now: rank ethanol, butan-1-ol and octan-1-ol for boiling point, then for water solubility. Explain in one sentence why the two rankings are not the same.

📁 Organic Chemistry Portfolio — Section 4: Physical Properties & Intermolecular Forces

Students open their Level 2 Chemistry Portfolio and complete Section 4:

Section 4 Requirements:

1. Boiling Point Trend Graph: Plot Molar Mass vs Boiling Point for Alkanes vs Primary Alcohols (1 to 5 carbons), annotating intermolecular force types.

2. Water & Non-Polar Solvent Solubility Table: Predict and justify solubility for 6 molecules (butane, 1-chlorobutane, propan-1-ol, hexan-1-ol, ethanoic acid, propan-1-amine).

3. Excellence Intermolecular Rationale: 1-paragraph explanation of why ethanoic acid has a significantly higher boiling point than propan-1-ol despite identical molar mass (60 g/mol).

Exit Verification | Ka Mutu Hoki (5 min)

Exit Check:

"My Section 4 correctly explains hydrogen bonding in alcohols and carboxylic acids, and justifies why water solubility decreases as carbon chain length increases."

Teacher Planning & NCEA Alignment

NCEA Level 2 Chemistry Alignment (4 Credits External):

Physical Properties: Explain trends in melting point, boiling point, and solubility of organic compounds in water and non-polar solvents.
Intermolecular Forces: Relate physical properties to temporary dipoles, permanent dipoles, and hydrogen bonding.

Vocabulary: Physical properties, boiling point, melting point, solubility, intermolecular forces, temporary dipole, permanent dipole, hydrogen bonding, hydrophobic, hydrophilic.

Other teaching approach: Guided Viewing & Practical Chemistry →

Lesson 4: Physical Properties & Intermolecular Forces