17.50 Alkalis

Before you read this, I suggest you read post 17.49.

fig 1 cropped

In post 17.49, we saw that an acid is something that acts as a source of hydrogen ions; something that reacts with hydrogen ions is called a base. A base that can dissolve in water is called an alkali. An alkaline solution is a solution that contains a dissolved alkali.

For example, the hydroxide (OH) ion is a base because it reacts with a hydrogen (H+) ion to give a water molecule (H2O); see posts 16.39 and 16.40 for more information about ions. Sodium hydroxide is a mixture of equal numbers of sodium (Na+) and OH ions. Hydrochloric acid is a mixture of equal numbers of H+ and chloride (Cl) ions. When sodium hydroxide is mixed with hydrochloric acid, its OH ions react with the H+ ions from the acid, leaving Na+ and Cl ions. A mixture of Na+ and Cl ions dissolved in water is called a solution of sodium chloride (we usually call sodium chloride “salt”). Because sodium hydroxide dissolves in water, it is an alkali.

You may have noticed that the explanation is the last paragraph is not quite correct. This is because a very small number of water molecules break down into H+ and OH ions (see post 17.49).

The carbonate ion (CO32-) is also a base because it reacts with an H+ ion to give a water molecule (H2O) and a carbon dioxide molecule (CO2). This reaction can be represented by

H+ + CO32- → H2O + CO2

(see post 16.33). Sodium carbonate (“washing soda”) is a base (because it contains CO32- ions which react with H+ ions) and it is also an alkali (because it dissolves in water). Calcium carbonate (chalk, limestone and marble) is a mixture of calcium (Ca2+) ions with an equal number of CO32- ions and is also a base (because the CO32- ions react with H+ ions) but is not an alkali (because it does not dissolve in water).

Geologists use an acid test to identify rocks containing calcium carbonate. They add a few drops of dilute hydrochloric acid to the surface of a sample of a rock. If the rock contains carbonate ions, the acid fizzes because bubbles of carbon dioxide are formed.

In post 17.49, we saw that water can be considered as an acid because a (very few) water molecules break down to give hydrogen ions; it can also be considered to be a base. It is a base because a water molecule can react with a hydrogen ion to form a hydronium ion (H3O+), as described in post 16.40.

fig 2 cropped

The picture above is copied from post 17.49 where it is explained in more detail. It shows that acetic acid can break down into acetate and H+ ions but also that acetate ions react with H+ ions to form acetic acid. So the acetate ion is a base. We say that the acetate ion is the conjugate base of acetic acid and that acetic acid is the conjugate acid of the acetate ion. Let’s think about the base B reacting with an H+ ion from a water molecule. We can represent this chemical reaction (post 16.33) by

B + H2O → BH + OH.

In the same way as we measured the strength of an acid by the acid dissociation constant, we can define the base dissociation constant by

eqn Kb cropped

which represents the ability of the base (B) to extract H+ ions from water; as in post 17.49, [square brackets] represent molarity (post 17.48), for example, [BH] represents the molarity of the base after it has taken up an H+ ion. If you know something about chemical equilibrium, you may be worried why [H2O] doesn’t appear in this equation. If this doesn’t apply to you – go ahead to the next paragraph. The reason is that the number of H2O molecules breaking down to give H+ ions will be so small that water molecules outnumber all the other molecules and ions present, and the molarity of pure water is equal to 1 (see post 17.48 and then think about what you’ve just read).

I may have given you the impression that all bases are negative ions (like OH) or contain negative ions (like calcium carbonate): this isn’t true. Ammonia is a gas that contains neutral molecule (NH3 post 16.40). (But you may be more familiar with a solution of ammonia, that is sometimes used for cleaning windows and carpets – if you are, the strong smell is ammonia gas coming out of the solution). If ammonia is bubbled through hydrochloric acid, it reacts with H+ ions, in the acid to form the ammonium ion (NH4+ post 16.40).

We know that strong acids, like sulfuric acid, can be corrosive – so can strong bases. For example, sodium hydroxide solution is used in oven cleaner and to unblock drains because it breaks down grease. But you need to wear rubber gloves when you’re using it because it can also break down the flesh of your hands!

 

Related posts

17.49 Acids
17.48 Moles
16.40 Complicated ions
16.39 Ions
16.33 Chemical reactions
16.30 Molecules

 

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