This statement is about the reactions of the tetrachlorides of the Group 4 elements with water.

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

The facts

Carbon tetrachloride (tetrachloromethane)

Carbon tetrachloride has no reaction with water. If you add it to water, it simply forms a separate layer underneath the layer of water. It doesn't react on heating either.

Silicon, germanium and tin tetrachlorides

These are also called silicon(IV) chloride, germanium(IV) chloride and tin(IV) chloride.

Silicon tetrachloride reacts violently with water to give white solid silicon dioxide and steamy fumes of HCl.

Liquid SiCl₄ fumes in moist air for this reason - it is reacting with water vapour in the air.

The germanium and tin compounds behave similarly. The CIE teacher support material says that hydrolysis happens increasingly readily as you go down this part of the Group, but they are all rapid reactions.

Lead(IV) chloride

The CIE teacher support material doesn't mention this. The reaction is complicated by the fact that all of these reactions give off a lot of heat. In the case of lead(IV) chloride, the heat is enough to decompose the lead(IV) chloride into lead(II) chloride and chlorine.

Any unchanged lead(IV) chloride would behave exactly like the silicon tetrachloride, producing solid lead(IV) oxide and HCl. But you would also get a precipitate of lead(II) chloride formed and chlorine gas.

I wouldn't bother to spend much time learning this. I suspect that if it did come up, there would be enough information in the question to help you through it.

The explanations

We need to explain two things - why the reactivity of the others increases down the Group, and why the CCl₄ behaves completely differently.

Why does the reactivity increase as you go down the Group?

As the Group 4 elements gets bigger, the bonds between them and the chlorine atoms get longer. Longer bonds are weaker and more easily broken. During the reactions, you have to break element-chlorine bonds and replace them by element-oxygen bonds. If the bonds to the chlorines are weaker, that will happen more easily.

Why doesn't CCl₄ react with water?

CCl₄, of course, has the strongest bonds to the chlorines, but that isn't the main problem.

Suppose a water molecule is going to react with the carbon tetrachloride. The reaction would have to start by the water molecule's oxygen attaching itself to the carbon atom via the oxygen's lone pair. A chlorine atom would get pushed off the carbon in the process.

There are two problems with this.

First, the chlorines are so bulky and the carbon atom so small, that the oxygen can't easily get at the carbon atom.

. . . and even if it did, there will be a stage where there is considerable cluttering around that carbon atom before the chlorine atom breaks away completely. There is going to be a lot of repulsion between the various lone pairs on all the atoms surrounding the carbon.

That cluttering is going to make this half-way stage (properly called a "transition state") very unstable. A very unstable transition state means a very high activation energy for the reaction.

The other problem is that there isn't a convenient empty orbital on the carbon that the oxygen lone pair can attach to.

If it could attach before the chlorine starts to break away, that would be an advantage. Forming a bond releases energy, and that energy would therefore be readily available for breaking a carbon-chlorine bond. But in the case of a carbon atom, that isn't possible.

The situation with silicon (and the other) tetrachlorides is different.

The silicon atom is bigger, and so there is more room around it for the water molecule to attack, and the transition state will be less cluttered.

But silicon has the additional advantage that there are empty 3d orbitals available to accept a lone pair from the water molecule. Carbon doesn't have 2d orbitals because there are no such things. There are no empty 2-level orbitals available in the carbon case.

This means that the oxygen can bond to the silicon before the need to break a silicon-chlorine bond. This makes the whole process energetically easier. The activation energy for the reaction will be significantly less.

Go to the Section 9.3 Menu . . .

To return to the list of learning outcomes in Section 9.3

Go to the CIE Main Menu . . .

To return to the list of all the CIE sections

Go to Chemguide Main Menu . . .

This will take you to the main part of Chemguide.