Chemguide: Support for CIE A level Chemistry

The Periodic Table

Learning outcome 9.2.5 and part of 9.2.6

Statement 9.2.5 deals with the reactions of the chlorides of the Period 3 elements with water. 9.2.6 is involved in the explanation for this.

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

Remember that we are only dealing with the chlorides:

NaCl, MgCl₂, AlCl₃, SiCl₄, and PCl₅.

The reactions between these chlorides and water are found on the page period 3 chlorides. That page contains a lot of information, and it is important that you concentrate on what you need to know.

Remember that you don't need to know about phosphorus(III) chloride, or the chlorides of sulphur.

What you need to get from the page

What follows is just a summary of the important things. It isn't a substitute for reading the page.

The structure of the chlorides

You need to know that:

• Sodium and magnesium chlorides are simple ionic chlorides with giant ionic structures.

• Aluminium chloride can have various structures depending on the conditions. At room temperature, think of it as being ionic with a high degree of covalent character. You must, however, be able to draw dots-and-crosses diagrams for its covalent structures as both AlCl₃ and Al₂Cl₆

• The other chlorides are all simple molecular compounds involving covalent bonding.

You need to understand that the reason for this is that the electronegativity difference between the other element and chlorine falls as you go across the period. Sodium has a low electronegativity and that means that there is enough electronegativity difference between sodium and chlorine to get an ionic bond.

As you go across the period, the electronegativity of the elements increases, and by the time you get to aluminium, the electronegativity difference between aluminium and chlorine is barely enough to get ions formed, and aluminium chloride often exists as covalent molecules.

After aluminium, there isn't enough electronegativity difference to get ions, and so the compounds are all covalent.

The reactions with water

You should know that:

• Sodium chloride dissolves in water to give a simple solution with a pH of 7.

• Magnesium chloride dissolves in water to give a solution with a pH of about 6.5. That means that there must be a slight reaction with the water, for reasons explained on the page. (The page gives the pH of the solution as "about 6", but CIE mark schemes want an answer in the range 6.5 - 6.9. It will actually depend on the concentration of the solution formed, but learn the one that you will need in the exam.).

• All the others react violently with water in a strongly exothermic reaction to give steamy fumes of hydrogen chloride gas, and produce acidic solutions. In each case, if you use a small amount of compound and a large excess of water, you may not see any gas. Hydrogen chloride is very soluble in water to give hydrochloric acid.

• You should be able to explain (including equations) the reactions involving aluminium, silicon and phosphorus(V) chlorides. Use the final (combined) equation listed under phosphorus(V) chloride, producing phosphoric(V) acid and HCl.

• The pH of the solutions formed will depend on the amount of water you add. If you quoted somewhere in the region of pH 1 to 3, you would be about right. To be sure, you need to look at recent past papers and mark schemes to see what CIE examiners expect.

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