Why is aluminium oxide insoluble in water

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All you really need to know for this topic is that the lower the pK a value, the stronger the acid. It is pretty unlikely that you would ever react phosphorus III oxide directly with a base, but you might need to know what happens if you react the phosphorous acid formed with a base. In phosphorous acid, the two hydrogen atoms in the -OH groups are acidic, but the other one isn't.

That means that you can get two possible reactions with, for example, sodium hydroxide solution depending on the proportions used. In the first case, only one of the acidic hydrogens has reacted with the hydroxide ions from the base. In the second case using twice as much sodium hydroxide , both have reacted. If you were to react phosphorus III oxide directly with sodium hydroxide solution rather than making the acid first, you would end up with the same possible salts.

Note: Check your syllabus, past papers and mark schemes before you get too bogged down in this! Follow this link to find out how to get hold of them if you haven't already got them UK-based syllabuses only. Phosphorus V oxide reacts violently with water to give a solution containing a mixture of acids, the nature of which depends on the conditions.

We usually just consider one of these, phosphoric V acid, H 3 PO 4 - also known just as phosphoric acid or as orthophosphoric acid. Phosphoric V acid is also a weak acid with a pK a of 2. That makes it fractionally weaker than phosphorous acid. Solutions of both of these acids of concentrations around 1 mol dm -3 will have a pH of about 1.

Once again, you are unlikely ever to react this oxide with a base, but you may well be expected to know how phosphoric V acid reacts with something like sodium hydroxide solution. If you look back at the structure, you will see that it has three -OH groups, and each of these has an acidic hydrogen atom. You can get a reaction with sodium hydroxide in three stages, with one after another of these hydrogens reacting with the hydroxide ions.

Again, if you were to react phosphorus V oxide directly with sodium hydroxide solution rather than making the acid first, you would end up with the same possible salts. Note: If you get a question in an exam which just asks you to write an equation for the reaction of sodium hydroxide with phosphoric V acid, which equation should you write? It shouldn't really matter - all of them are perfectly valid. In each case, it just depends on the proportions of the two reagents you are using.

If you really want to be certain, check past papers and mark schemes. I found one question about the reaction between sodium oxide and phosphoric V acid where the mark scheme accepted any of the possible equations - which is what I would expect.

I know I haven't given you that particular set of equations, but they aren't difficult to work out as long as you understand the principle, and I can't possibly give every single acid-base equation. This already long page would go on for ever, and everybody would give up in despair well before the end! That's why you are trying to understand chemistry rather than learn it parrot-fashion.

Please don't waste time learning equations - or at least, not until you know and understand all the rest of the chemistry that you need to know and understand! Any one equation stands a very small chance of coming up in an exam, even if it is on your particular syllabus. Life is too short to waste time learning equations.

Know how to work them out if you need to. We are going to be looking at sulphur dioxide, SO 2 , and sulphur trioxide, SO 3. Sulphur dioxide is fairly soluble in water, reacting with it to give a solution known as sulphurous acid, and traditionally given the formula H 2 SO 3.

However, the main species in the solution is simply hydrated sulphur dioxide - SO 2 , xH 2 O. It is debatable whether any H 2 SO 3 as such exists at all in the solution. Sulphurous acid is also a weak acid with a pK a of around 1. A reasonably concentrated solution of sulphurous acid will again have a pH of about 1. Note: There is some variability in the pK a value quoted for sulphurous acid by various sources - ranging from 1.

I have no way of knowing which of these is right. The ionisation of the "sulphurous acid" involves ionisation of the hydrated complex, and you shouldn't need to worry about this at this level.

Sulphur dioxide will also react directly with bases such as sodium hydroxide solution. If sulphur dioxide is bubbled through sodium hydroxide solution, sodium sulphite solution is formed first followed by sodium hydrogensulphite solution when the sulphur dioxide is in excess.

Note: Sodium sulphite is also called sodium sulphate IV. Sodium hydrogensulphite is also sodium hydrogensulphate IV or sodium bisulphite. Notice that the equations for these reactions are different from the phosphorus examples. In this case, we are reacting the oxide directly with the sodium hydroxide, because that's the way we are most likely to do it.

Another important reaction of sulphur dioxide is with the base calcium oxide to form calcium sulphite calcium sulphate IV. This is at the heart of one of the methods of removing sulphur dioxide from flue gases in power stations. Sulphur trioxide reacts violently with water to produce a fog of concentrated sulphuric acid droplets.

Note: If you know about the Contact Process for the manufacture of sulphuric acid, you will know that the sulphur trioxide is always converted into sulphuric acid by a round-about process to avoid the problem of the sulphuric acid fog. You will find details of the Contact Process elsewhere on this site if you are interested, but it isn't relevant to the current topic.

The acid reacts with water to give a hydroxonium ion a hydrogen ion in solution, if you like and a hydrogensulphate ion. The second hydrogen is more difficult to remove. In fact the hydrogensulphate ion is a relatively weak acid - similar in strength to the acids we have already discussed on this page. This time you get an equilibrium:. Sulphuric acid, of course, has all the reactions of a strong acid that you are familiar with from introductory chemistry courses.

For example, the normal reaction with sodium hydroxide solution is to form sodium sulphate solution - in which both of the acidic hydrogens react with hydroxide ions. In principle, you can also get sodium hydrogensulphate solution by using half as much sodium hydroxide and just reacting with one of the two acidic hydrogens in the acid.

In practice, I personally have never ever done it - I can't at the moment see much point! Sulphur trioxide itself will also react directly with bases to form sulphates. For example, it will react with calcium oxide to form calcium sulphate. This is just like the reaction with sulphur dioxide described above. Chlorine VII oxide is also known as dichlorine heptoxide, and chlorine I oxide as dichlorine monoxide.

It continues the trend of the highest oxides of the Period 3 elements towards being stronger acids. A concentrated solution of sodium oxide in water will have pH Reaction with acids: As a strong base, sodium oxide also reacts with acids.

For example, it reacts with dilute hydrochloric acid to produce sodium chloride solution. Magnesium oxide is another simple basic oxide, which also contains oxide ions. However, it is not as strongly basic as sodium oxide because the oxide ions are not as weakly-bound. Because of the higher charge on the metal, more energy is required to break this association. Even considering other factors such as the energy released from ion-dipole interactions between the cations and water , the net effect is that reactions involving magnesium oxide will always be less exothermic than those of sodium oxide.

Reaction with water: At first glance, magnesium oxide powder does not appear to react with water. However, the pH of the resulting solution is about 9, indicating that hydroxide ions have been produced. In fact, s ome magnesium hydroxide is formed in the reaction, but as the species is almost insoluble, few hydroxide ions actually dissolve.

The reaction is shown below:. Reaction with acids: Magnesium oxide reacts with acids as predicted for a simple metal oxide. For example, it reacts with warm dilute hydrochloric acid to give magnesium chloride solution.

Describing the properties of aluminum oxide can be confusing because it exists in a number of different forms.

One of those forms is very unreactive known chemically as alpha-Al 2 O 3 and is produced at high temperatures. The following reactions concern the more reactive forms of the molecule. Aluminium oxide is amphoteric. It has reactions as both a base and an acid.

Reaction with water: Aluminum oxide is insoluble in water and does not react like sodium oxide and magnesium oxide. The oxide ions are held too strongly in the solid lattice to react with the water. Reaction with acids: Aluminum oxide contains oxide ions, and thus reacts with acids in the same way sodium or magnesium oxides do.

Aluminum oxide reacts with hot dilute hydrochloric acid to give aluminum chloride solution. Reaction with bases: Aluminum oxide also displays acidic properties, as shown in its reactions with bases such as sodium hydroxide.

Various aluminates compounds in which the aluminum is a component in a negative ion exist, which is possible because aluminum can form covalent bonds with oxygen.

This is possible because the electronegativity difference between aluminum and oxygen is small, unlike the difference between sodium and oxygen, for example electronegativity increases across a period. Aluminum oxide reacts with hot, concentrated sodium hydroxide solution to produce a colorless solution of sodium tetrahydroxoaluminate:. Silicon is too similar in electronegativity to oxygen to form ionic bonds. Therefore, because silicon dioxide does not contain oxide ions, it has no basic properties.

In fact, it is very weakly acidic, reacting with strong bases. Reaction with water: Silicon dioxide does not react with water, due to the thermodynamic difficulty of breaking up its network covalent structure.

Reaction with bases : Silicon dioxide reacts with hot, concentrated sodium hydroxide solution, forming a colorless solution of sodium silicate:. In another example of acidic silicon dioxide reacting with a base, the Blast Furnace extraction of iron, calcium oxide from limestone reacts with silicon dioxide to produce a liquid slag, calcium silicate:. Phosphorus III oxide: Phosphorus III oxide reacts with cold water to produce a solution of the weak acid, H 3 PO 3 —known as phosphorous acid, orthophosphorous acid or phosphonic acid:.