Properties of hydrogen bonding

Hydrogen bonding (2)

A strong type of intermolecular force that occurs when a hydrogen atom is covalently bonded to a very electronegative atom

This is specifically fluorine (F), nitrogen (N), or oxygen (O)

Why does hydrogen bonding occur specifically in N,O,F (3)

High electronegativity - strongly attract bonding electrons - highly polarise the bond

Small atomic radii - allows close approach to other molecules - creates shorter, stronger bonds

This creates a large dipole in the bond - gives the hydrogen atom a significant partial positive

Why does the boiling/melting point of alkanes increase as they get bigger? (4)

Between alkane molecules there are London forces

As the molecular size increases the number of electrons per molecule increases

This increases strength of the instantaneous and induced dipoles that form

The London forces increase in strength

How does the length of the carbon chain increasing affect the boiling/melting points of alkanes? (3)

As the length of the carbon chain increases the number of points of contact between the molecules increases

London forces exist at each point of contact between the molecules

The more points of contact the greater the overall London forces of attraction

Why do alcohols have higher boiling points with the same amount of electrons as alkanes? (3)

Alcohols contain a hydroxyl group (-OH)

They can form hydrogen bonds between their molecules

These strong hydrogen bonds + permanent dipole forces require more energy to overcome than London forces alone

Why does the difference in boiling point of alcohols and their corresponding alkane become smaller as the alcohol chain length increases? (4)

All alcohols can form hydrogen bonds due to their –OH group

The strength of London forces increases significantly with more electrons, and greater molecular surface area

In smaller molecules like methanol, hydrogen bonding dominates, leading to a large boiling point difference compared to methane

However, in larger molecules like hexanol and hexane, London forces become much more significant, reducing the relative impact of hydrogen bonding on boiling point

Why is water more dense than ice? (4)

In ice, hydrogen bonds hold water molecules in a rigid, open lattice with longer bond lengths, creating more space between molecules

This maximises the number of hydrogen bonds that can form between molecules

When ice melts, these hydrogen bonds break and molecules move closer together, shortening bond lengths and increasing density

This is why liquid water is denser than its solid form.