Chemguide: Core Chemistry 14 - 16

Reactions between acids and metal oxides and hydroxides

This page introduces the word "base" and looks briefly at the reactions between acids and metal oxides and hydroxides producing salts. It assumes that you have already read the page about salts.

You will find more experimental detail about these reactions in the next page in this sequence.

Metal oxides and hydroxides as bases

Metal oxides

Most metal oxides are ionic, contining the oxide ion, O2-. Oxide ions can react with the hydrogen ions that acids produce in solution to form water.

2H+(aq) + O2-(s)     H2O(l)

Note:  Notice that I have described the oxide ion as being solid. Metal oxides are solid and most are insoluble in water. Those that do dissolve in water are actually reacting with the water to produce metal hydroxides.

A substance that combines with the hydrogen ions in an acid is described as a base.

So, for example, copper(II) oxide is a base because it contains the oxide ion which behaves in this way.

Oxides like this are described as basic oxides.

The overall equation for the reaction between copper(II) oxide and dilute sulfuric acid is

CuO(s) + H2SO4(aq)     CuSO4(aq) + H2O(l)

Oxide ions from the copper(II) oxide and hydrogen ions from the acid have combined to make water and aren't there any longer.

That leaves the copper(II) ions and the sulfate ions to make the salt copper(II) sulfate.

Reactions between metal oxides and dilute acids usually need to be heated gently.

Metal hydroxides

Metal hydroxides contain the hydroxide ion, OH-. Hydroxide ions also react with hydrogen ions from an acid to make water.

H+(aq) + OH-(aq)     H2O(l)

Note:  Although at this level we tend to think about all metal hydroxides as containing hydroxide ions, it is actually a simplification - but it works OK!

Notice that this time, I have described the hydroxide ion as being in solution. That's because we normally use this reaction for soluble hydroxides - mainly sodium, potassium and ammonium hydroxides.

All metal hydroxides, solid or in solution, are bases because they contain hydroxide ions. A soluble base like sodium hydroxide is called an alkali.

Metal hydroxides like this are described as basic hydroxides.

The full equation for the reaction between sodium hydroxide solution and dilute sulfuric acid is

2NaOH(aq) + H2SO4(aq)     Na2SO4(aq) + 2H2O(l)

These reactions happen instantly in the cold.

If you are trying to make, say, pure sodium sulfate crystals in this way, it is tricky to get the proportions right, because everything is a colourless solution.

We will explore this in detail on the next page.

A comment about dilute nitric acid

You will probably remember that nitric acid reacts differently with metals from the other common acids.

With metal oxides and hydroxides, nitric acid behaves in exactly the same way as the other acids, this time producing nitrates.

So, for example, with copper(II) oxide:

CuO(s) + 2HNO3(aq)     Cu(NO3)2(aq) + H2O(l)

. . . and with sodium hydroxide solution:

NaOH(aq) + HNO3(aq)     NaNO3(aq) + H2O(l)

A comment about these reactions and the Reactivity Series

The reactivity series is completely irrelevant to reactions of the acids apart from their reactions with metals.

Oxides or hydroxides of metals below hydrogen in the reactivity series react in exactly the same way as compounds of metals in the middle or at the very top of the reactivity series.

A summary equation

metal oxide or metal hydroxide + acid    salt + water

Where would you like to go now?

To the next page about making salts. . .

To the acid, bases and salts menu . . .

To the Chemistry 14-16 menu . . .

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© Jim Clark 2020