chemguide: CIE A level chemistry support: Learning outcome 9.3(b)

Chemguide: Support for CIE A level Chemistry

Learning outcome 9.3(b)

This statement is about the bonding, shape and volatility of the tetrachlorides of the Group 4 elements.

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

Shapes and bonding

These all have the formula XCl₄.

They are all simple covalent molecules with a typical tetrahedral shape. All of them are liquids at room temperature. (Although even at room temperature, lead(IV) chloride will tend to decompose to give solid lead(II) chloride and chlorine gas.)

CCl₄ is typical:

Remember that simple lines show bonds in the plane of the paper (or screen), dotted lines show bonds going back into the paper, and wedges show bonds coming out towards you.

Volatility

The more volatile a substance is, the lower its boiling point. The boiling points of these compounds are:

	B Pt (°C)
CCl₄	76
SiCl₄	57
GeCl₄	87
SnCl₄	114
PbCl₄	decomposes at 105

Note: Several sources quote 105° C as the boiling point of PbCl₄ - it isn't. It decomposes explosively at this temperature. That isn't the same as boiling!

You can see that, apart from CCl₄, the boiling points increase as you go down the Group. This is because of increasing van der Waals dispersion forces as the molecules get bigger. Bigger molecules have more electrons which can be polarised, and more space to polarise them over.

So volatility decreases as you go down the Group.

Note: If you aren't sure about van der Waals dispersion forces, you should revise it by following this link before you go any further.

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In the teacher support material, CIE mention CCl₄ as an oddity, but don't suggest any explanation for it. I have no idea what the explanation is, and haven't been able to find one either. There are several sites on the web where this question is raised, but none that I have found come up with a good answer. In fact, many of the answers are rubbish!

Don't worry about this.

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