Chemguide: Support for CIE A level Chemistry

Learning outcome 9.2(d) and part of 9.2(f)

This statement - 9.2(d) - deals with the acid-base behaviour of the oxides and hydroxides of the Period 3 elements. 9.2(f) 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 oxides:

Na2O, MgO, Al2O3, SiO2, P4O10, SO2 and SO3.

We will look at the hydroxides separately, further down this page.

The oxides

The acid-base reactions of these oxides are found on the page acid-base behaviour of the period 3 oxides. You will have visited this page before, looking at the reactions of the oxides with water.

Remember that you don't need to know about phosphorus(III) oxide, or the oxides of chlorine.

The reactions

Take your time, and work down through the list of oxides, just looking at the acid-base behaviour of the oxides in the list above. Keep it simple! You have already looked at the reactions with water. Ignore them this time around.

The most important statement is under the heading "The trend in acid-base behaviour" near the top of the page.

You should be able to describe, and write equations for, the reactions of:

  • sodium oxide with dilute acids;

  • magnesium oxide with dilute acids;

  • aluminium oxide (acting as a base) with dilute acids;

  • aluminium oxide (acting as an acid) with sodium hydroxide solution;

  • silicon(IV) oxide with sodium hydroxide solution;

  • phosphorus(V) oxide with sodium hydroxide solution;

  • sulphur dioxide (sulphur(IV) oxide) with sodium hydroxide solution;

  • sulphur trioxide (sulphur(VI) oxide) with sodium hydroxide solution;

Note:  You may well complain that I haven't given the equations for the reactions between phosphorus(V) oxide and sodium hydroxide solution, or sulphur trioxide and sodium hydroxide solution. That is perfectly true - and I have commented on the phosphorus case on the page.

The truth is that you can't possibly learn all the huge number of different equations that you might be asked for. You have to be able to work them out as and when you need them. So do that!

Start by doing the relatively easy reaction between sulphur trioxide and sodium hydroxide solution to make sodium sulphate, Na2SO4.

Then, in the phosphorus(V) oxide case, work out the equation for the reaction between it and sodium hydroxide solution to give sodium phosphate(V), Na3PO4, and then do the other possible products as well. Asking you to do this in an exam is a perfectly valid question. (Hint: Start by balancing the number of phosphorus atoms in each case. To be honest, if you need this hint, you could do with a lot more practice at writing equations!)

Explaining the reactions

The ionic oxides (sodium, magnesium and aluminium oxides) all contain O2- ions. These oxide ions are strongly basic, reacting with hydrogen ions from an acid to make water.

The reason that the oxides become less basic as you go from sodium to aluminium can be accounted for by the increasing strength of the ionic bond between the metal ions and the oxide ions as the charge on the metal ions increases.

It takes increasing amounts of energy to free the ions from the crystal lattices.

The covalent oxides are acidic. Silicon(IV) oxide has no reaction with water, but the rest react with water to give acids which become stronger as you go across the period - provided you always compare the oxides with the highest oxidation state. You will find an explanation for this when looking at the "hydroxides" below.

The hydroxides

The acid-base reactions of the hydroxides are found on the page about the period 3 hydroxides.

I am fairly sure that this page contains far more information than you will need, because I haven't found much about the Period 3 hydroxides in the exam papers I have available. I will keep a check on new papers as they are produced, and update this as necessary.

What is important is that you know the trends:

Taking the "hydroxides" as meaning not only simple ionic hydroxides like sodium hydroxide, but also covalently bound -OH groups as in sulphuric acid:

  • The simple ionic hydroxides (sodium and magnesium hydroxides) are basic due to the presence of the basic hydroxide ions. Hydroxide ions react with hydrogen ions to make water.

  • Aluminium hydroxide is amphoteric - it reacts with both acids and bases.

  • The covalent hydroxides are acidic, and the acidity increases depending on the number of element-oxygen double bonds also attached to the central atom. As long as you are looking at the element in its highest oxidation state, this means that the acidity of the covalent hydroxides increases as you go across the period. So, sulphuric acid is a strong acid, whereas silicic acid is very weak.

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© Jim Clark 2010 (last modified May 2014)