Reactions Between Acids And Ammonia - Chemguide

Chemguide: Core Chemistry 14 - 16

Reactions between acids and ammonia

This page looks at the reactions between acids and ammonia to give ammonium salts.

Ammonia gas

Ammonia, NH3, is a gas with a very sharp smell which most people would recognise as the smell of wet nappies (diapers) which have been left a bit too long! The ammonia comes from bacteria acting on urea in the urine.

You can easily test for ammonia gas because it is the only alkaline gas you will meet at this level. It turns red litmus paper blue.

The reaction between ammonia gas and hydrogen chloride gas

Hydrochloric acid is a solution of hydrogen chloride gas in water; ammonia solution is a solution of ammonia gas in water. Concentrated solutions give off hydrogen chloride and ammonia gases.

The next bit of video shows what happens if you hold a cotton wool bud soaked in concentrated hydrochloric acid over a flask of concentrated ammonia solution.

Where the two gases mix, you get a white smoke of solid ammonium chloride.

NH3(g) + HCl(g) NH4Cl(s)

This is a simple example of ammonia reacting with an acid to make a salt.

Ammonia solution

Ammonia doesn't just dissolve in water; it reacts with it.

NH3(g) + H2O(l) NH4+(aq) + OH-(aq)

This new arrow symbol shows that the reaction is reversible.

The ammonia gas reacts with water to give ammonium ions and hydroxide ions. At the same time, ammonium ions and hydroxide ions are reacting to give ammonia and water.

Almost instantly in this reaction, you will get to the point that the rates of these two reactions become the same. When that happens there is no further change in the amounts of ammonia, water, ammonium ions and hydroxide ions present.

We say that the reaction has reached dynamic equilibrium. Dynamic because the reactions are still going on; equilibrium because the amounts of everything present aren't changing.

In ammonia solution of the sort of concentrations used in the lab, less than 1% of the ammonia has actually reacted at any one time.

Traditionally, ammonia solution was treated as ammonium hydroxide solution, but it is actually more accurate to just think of it as a solution of ammonia itself.

Note: We will come back to dynamic equilibrium later in the course. All you really need to know at the moment is that most of the ammonia in ammonia solution is just there as NH3(aq). However, it is sometimes convenient to treat it as if it was ammonium hydroxide solution.

The pH of ammonia solution is typically between 11 and 12 depending on its concentration. It is alkaline, and so you would expect it to react with acids.

The reaction between ammonia solution and acids

You have seen ammonia gas reacting with hydrogen chloride gas above. Ammonia solution reacts with hydrochloric acid (a solution of hydrogen chloride in water) in a similar way.

You could write this equation in two ways.

NH3(aq) + HCl(aq) NH4Cl(aq)

NH4OH(aq) + HCl(aq) NH4Cl(aq) + H2O(l)

Both are valid ways of writing the equation.

In the first one, you are showing the acid reacting with the majority of the ammonia which still exists as ammonia molecules.

The second one shows the reaction between the acid and the ammonium hydroxide produced by ammonia reacting with the water.

If you did the same thing with the reaction between ammonia solution and dilute sulfuric acid:

2NH3(aq) + H2SO4(aq) (NH4)2SO4(aq)

2NH4OH(aq) + H2SO4(aq) (NH4)2SO4 + 2H2O(l)

It is definitely less bother writing these equations starting from ammonia itself rather than ammonium hydroxide. If you use the ammonium hydroxide version, be careful not to get the balancing of the water molecules wrong.

Whichever acid you use in these reactions, you start with two colourless solutions and end up with a colourless solution. It's not visually exciting!

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