Trends in Physical Properties Within Homologous Series (VCE SSCE Chemistry): Revision Notes

Trends in Physical Properties Within Homologous Series

Introduction

The functional groups present in organic compounds play a crucial role in determining their physical properties. For example, ethylene glycol contains two hydroxyl groups ($-\text{OH}$), which give it a relatively high boiling point of $197°\text{C}$. This property makes it useful as a coolant and antifreeze agent when mixed with water in car engines.

Physical properties of alkanes, alkenes and haloalkanes

Melting and boiling points of alkanes

Alkanes and alkenes are hydrocarbons and are non-polar molecules. Because they lack polarity, the only intermolecular forces between them are weak dispersion forces (also called van der Waals forces). These weak forces significantly influence their melting and boiling points.

The table below shows the melting and boiling points of the first six alkanes:

Alkane	Molecular formula	Melting point ($°\text{C}$)	Boiling point ($°\text{C}$)
Methane	$\text{CH}_4$	$-182$	$-162$
Ethane	$\text{C}_2\text{H}_6$	$-183$	$-89$
Propane	$\text{C}_3\text{H}_8$	$-188$	$-46$
Butane	$\text{C}_4\text{H}_{10}$	$-138$	$-0.5$
Pentane	$\text{C}_5\text{H}_{12}$	$-130$	$36$
Hexane	$\text{C}_6\text{H}_{14}$	$-95$	$69$

The graph below illustrates how melting and boiling points increase with chain length:

Effect of molecular shape on boiling points

Molecular shape significantly influences the strength of dispersion forces. Straight-chain alkanes can fit together more closely than branched-chain alkanes, leading to higher boiling points.

Melting and boiling points of alkenes and haloalkanes

Alkenes, like alkanes, are hydrocarbons with non-polar molecules. They exhibit similar melting and boiling points to alkanes with the same number of carbon atoms, as shown in the table below:

Compound	Melting point ($°\text{C}$)	Boiling point ($°\text{C}$)
Butane ($\text{C}_4\text{H}_{10}$)	$-138$	$-0.5$
But-1-ene ($\text{C}_4\text{H}_8$)	$-185.3$	$-6.3$
Chlorobutane ($\text{C}_4\text{H}_9\text{Cl}$)	$-123$	$78$

Haloalkanes, however, contain polar bonds. For example, chloromethane contains a polar carbon-chlorine bond:

The chlorine atom is more electronegative than carbon, creating a permanent dipole. This allows dipole-dipole attractions to occur between molecules in addition to dispersion forces.

Viscosity of alkanes, alkenes and haloalkanes

Viscosity is defined as a liquid's resistance to pouring or flowing. A liquid that pours slowly (like honey or tomato sauce) is described as viscous or having high viscosity. Petrol, by contrast, has low viscosity and pours quickly.

The viscosity of a liquid depends on the interactions between its molecules. Like boiling point, viscosity increases as the forces of attraction between molecules increase.

As the carbon chain length increases:

• More points of contact exist between molecules
• Dispersion forces become stronger
• Viscosity increases

The diagram below illustrates this concept:

Shorter hydrocarbon chains:

• Fewer contact points between molecules
• Weaker overall dispersion forces
• Lower viscosity

Longer hydrocarbon chains:

• More contact points between molecules
• Stronger overall dispersion forces
• Higher viscosity

Industrial application: petroleum fractions

Crude oil contains a mixture of hydrocarbons with different chain lengths. During fractional distillation, molecules of similar size are separated based on their boiling points. The table below shows different fractions and their uses:

Fraction	Composition	Boiling range ($°\text{C}$)	Uses
Gas	$\text{C}_1-\text{C}_4$	$<30$	Heating fuel, LPG
Petrol	$\text{C}_5-\text{C}_{12}$	$30-250$	Motor fuel/petrol
Kerosene	$\text{C}_{12}-\text{C}_{16}$	$250-300$	Jet and diesel fuel
Heating fuel oils	$\text{C}_{16}-\text{C}_{18}$	$>300$	Diesel fuel, heating fuel oil
Lubricating oils	$\text{C}_{18}-\text{C}_{20}$	$>350$	Lubrication
Paraffin waxes	$\text{C}_{20}-\text{C}_{40}$	—	Candles, wax
Bitumen	Above $\text{C}_{40}$	—	Roofing tar, road surfaces

Effect of temperature on viscosity

As temperature increases, viscosity decreases. This occurs because:

• Molecules gain more kinetic energy
• Molecules move more quickly
• Contact time between molecular chains decreases
• Dispersion forces are disrupted
• Weaker forces of attraction result in decreased viscosity

Physical properties of alcohols, carboxylic acids, amines and amides

These four homologous series are considered together because their molecules contain functional groups capable of forming hydrogen bonds. This ability has a profound effect on their physical properties.

Understanding hydrogen bonding

Hydrogen bonds are the strongest type of intermolecular force. Molecules that can form hydrogen bonds generally exhibit much higher boiling points than those that cannot.

Compare the following compounds with similar molecular masses:

Homologous series	Compound	Formula	Molar mass ($\text{g mol}^{-1}$)	Melting point ($°\text{C}$)	Boiling point ($°\text{C}$)
Alkane	Butane	$\text{C}_4\text{H}_{10}$	$58$	$-138$	$-0.5$
Alcohol	Propan-1-ol	$\text{C}_3\text{H}_7\text{OH}$	$60$	$-126$	$97.2$
Carboxylic acid	Ethanoic acid	$\text{CH}_3\text{COOH}$	$60$	$16.6$	$118$
Amine	Propan-1-amine	$\text{C}_3\text{H}_7\text{NH}_2$	$59$	$-83$	$49$
Amide	Ethanamide	$\text{CH}_3\text{CONH}_2$	$59$	$80$	$210$

Melting and boiling points of alcohols

Alcohols contain the hydroxyl functional group ($-\text{OH}$). The oxygen atom is more electronegative than hydrogen, making the $\text{O}-\text{H}$ bond polar. This creates a permanent dipole that enables hydrogen bonding between molecules.

The diagram shows hydrogen bonding between ethanol molecules. The partially positive hydrogen atom ($\delta^+$) in one $-\text{OH}$ group is attracted to a lone pair of electrons on the oxygen atom ($\delta^-$) of a neighbouring molecule. These hydrogen bonds are much stronger than dispersion forces, resulting in higher melting and boiling points for alcohols.

Viscosity of alcohols

Alcohols are more viscous than alkanes with the same number of carbon atoms due to the strength of intermolecular hydrogen bonding.

Melting and boiling points of amines and amides

Both amines and amides contain polar nitrogen-hydrogen bonds, enabling hydrogen bond formation.

In amines: Hydrogen bonds form between the lone pair of electrons on the electronegative nitrogen atom and the partially positive hydrogen atom on another amine molecule.

In amides: Hydrogen bonds form between the lone pair of electrons on the oxygen atom of one molecule and the partially positive hydrogen atom on a neighbouring molecule.

Melting and boiling points of carboxylic acids

Carboxylic acids exhibit particularly high boiling points due to dimer formation. In the liquid state, two carboxylic acid molecules can form a stable unit called a dimer, held together by two hydrogen bonds:

The dimer has double the molar mass of a single carboxylic acid molecule. This increased size leads to:

• Stronger dispersion forces between dimers
• Combined effect of hydrogen bonds and dispersion forces
• Higher boiling points and viscosity compared to other organic molecules of similar size

The graph below compares the boiling points of carboxylic acids and alcohols:

Effect of hydrocarbon chain branching on boiling points

The position and degree of branching in a molecule affects its boiling point. This is particularly noticeable in alcohols.

Physical properties of aldehydes, ketones and esters

Aldehydes, ketones, and esters are considered together because their molecules are held together by dipole-dipole attractions. Unlike alcohols and carboxylic acids, these molecules cannot form hydrogen bonds with each other because they lack a hydrogen atom bonded directly to oxygen or nitrogen.

Melting and boiling points of aldehydes, ketones and esters

All three groups contain a carbon-oxygen double bond ($\text{C}=\text{O}$), called a carbonyl group. Since oxygen is much more electronegative than carbon, this bond is polar, creating a permanent dipole in the molecule.

The diagram shows how dipole-dipole attractions occur between ketone molecules. The partially positive carbon atom ($\delta^+$) of one molecule is attracted to the partially negative oxygen atom ($\delta^-$) of another.

The table below compares boiling points across different functional groups:

Homologous series	Semi-structural formula	Molar mass ($\text{g mol}^{-1}$)	Boiling point ($°\text{C}$)
Alkane	$\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_3$	$58$	$-0.5$
Alcohol	$\text{CH}_3\text{CH}_2\text{CH}_2\text{OH}$	$60$	$97$
Aldehyde	$\text{CH}_3\text{CH}_2\text{CHO}$	$58$	$48$
Ketone	$\text{CH}_3\text{COCH}_3$	$58$	$56$
Ester	$\text{HCOOCH}_3$	$60$	$32$

Effect of chain length on boiling point

As with other homologous series, increasing the hydrocarbon chain length in aldehydes, ketones, and esters leads to increased boiling points. This occurs because:

• More contact points exist between longer hydrocarbon chains
• Dispersion forces increase overall
• More energy is required to separate molecules

Remember!