chemguide: CIE A level chemistry support: Learning outcome 4(i)

Chemguide: Support for CIE A level Chemistry

Learning outcome 4(i)

This statement is about the effect of hydrogen bonding on the physical properties of a compound in which hydrogen bonds are present.

Before you go on, you should find and read the statement in your copy of the syllabus.

First, re-read the Chemguide page about hydrogen bonding, revising hydrogen bonding, but mainly concentrating on the bits about water and alcohols.

Then re-read the bit about ice and water just over half-way down the page about molecular structures.

Then have a quick look at the physical properties of alcohols in some more detail, by reading the physical properties section of the page an introduction to alcohols.

The new bit here for you to think about is the effect of hydrogen bonding on the solubility of alcohols in water.

Looking at the specific points made in the syllabus statement:

Boiling and melting points

A compound containing molecules which can hydrogen bond will have higher boiling and melting points than one with similarly sized molecules which can't hydrogen bond.

Viscosity

The viscosity of a liquid is a measure of how runny it is. This is affected by the size of the molecules and the strength of the forces between them.

You normally think of water as not being very viscous - it seems to flow very easily. But there is a simple experiment which has surprising results.

You have a glass tube about a metre long and sealed at both ends. It is full of water apart from an air bubble one or two centimetres long. You have another tube just the same except that it is filled with ethoxyethane (diethyl ether), C₂H₅-O-C₂H₅. Ethoxyethane doesn't form hydrogen bonds.

If you hold the tubes vertically so that the bubbles are at the bottom, the bubbles will rise to the top again along the length of the tube. What is happening, of course, is that the liquids are flowing down to fill the air space.

Students seeing this for the first time expect that the larger ethoxyethane molecules will flow much more slowly than the little water molecules, and so expect the bubble in the water tube to get to the top much faster.

In fact the exact opposite happens. The hydrogen bonding in the water slows the molecules down, and it takes ages for the bubble in the water to get to the top.

So in this case, the attractions between the water molecules due to the hydrogen bonding completely outweigh the effect of the molecular sizes.

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