STRONG AND WEAK BASES

This page explains the terms strong and weak as applied to bases. As a part of this it defines and explains K_b and pK_b.

We are going to use the Bronsted-Lowry definition of a base as a substance which accepts hydrogen ions (protons).

The usual way of comparing the strengths of bases is to see how readily they produce hydroxide ions in solution. This may be because they already contain hydroxide ions, or because they take hydrogen ions from water molecules to produce hydroxide ions.

Strong bases

Explaining the term "strong base"

A strong base is something like sodium hydroxide or potassium hydroxide which is fully ionic. You can think of the compound as being 100% split up into metal ions and hydroxide ions in solution.

Each mole of sodium hydroxide dissolves to give a mole of hydroxide ions in solution.

Some strong bases like calcium hydroxide aren't very soluble in water. That doesn't matter - what does dissolve is still 100% ionised into calcium ions and hydroxide ions. Calcium hydroxide still counts as a strong base because of that 100% ionisation.

Working out the pH of a strong base

Remember that:

Since pH is a measure of hydrogen ion concentration, how can a solution which contains hydroxide ions have a pH? To understand this, you need to know about the ionic product for water.

Wherever there is water, an equilibrium is set up. Using the simplified version of this equilibrium:

In the presence of extra hydroxide ions from, say, sodium hydroxide, the equilibrium is still there, but the position of equilibrium has been shifted well to the left according to Le Chatelier's Principle.

There will be far fewer hydrogen ions than there are in pure water, but there will still be hydrogen ions present. The pH is a measure of the concentration of these.

An outline of the method of working out the pH of a strong base

• Work out the concentration of the hydroxide ions.

• Use K_w to work out the hydrogen ion concentration.

• Convert the hydrogen ion concentration to a pH.

An example

To find the pH of 0.500 mol dm^-3 sodium hydroxide solution:

Because the sodium hydroxide is fully ionic, each mole of it gives that same number of moles of hydroxide ions in solution.

[OH^-] = 0.500 mol dm^-3

Now you use the value of K_w at the temperature of your solution. You normally take this as 1.00 x 10^-14 mol² dm^-6.

[H⁺] [OH^-] = 1.00 x 10^-14

This is true whether the water is pure or not. In this case we have a value for the hydroxide ion concentration. Substituting that gives:

[H⁺] x 0.500 = 1.00 x 10^-14

If you solve that for [H⁺], and then convert it into pH, you get a pH of 13.7.

Weak bases

Explaining the term "weak base"

Ammonia is a typical weak base. Ammonia itself obviously doesn't contain hydroxide ions, but it reacts with water to produce ammonium ions and hydroxide ions.

However, the reaction is reversible, and at any one time about 99% of the ammonia is still present as ammonia molecules. Only about 1% has actually produced hydroxide ions.

A weak base is one which doesn't convert fully into hydroxide ions in solution.

Comparing the strengths of weak bases in solution: K_b

When a weak base reacts with water, the position of equilibrium varies from base to base. The further to the left it is, the weaker the base.

You can get a measure of the position of an equilibrium by writing an equilibrium constant for the reaction. The lower the value for the constant, the more the equilibrium lies to the left.

In this case the equilibrium constant is called K_b. This is defined as:

pK_b

The relationship between K_b and pK_b is exactly the same as all the other "p" terms in this topic:

The table shows some values for K_b and pK_b for some weak bases.

As you go down the table, the value of K_b is increasing. That means that the bases are getting stronger.

As K_b gets bigger, pK_b gets smaller. The lower the value of pK_b, the stronger the base.

This is exactly in line with the corresponding term for acids, pK_a - the smaller the value, the stronger the acid.