Chem 3 - Acids and Bases - Strong and Weak Acids and Alkalis

When dissolved in water, Acids form H+ ions. An H+ ion is a Hydrogen atom which has lost its electron - in other words, its simply a proton.

A proton produced as we dissolve an acid in water becomes surrounded by water molecules which keep it in solution - we say that is Hydrated. Hydrated Hydrogen ions are represented as H+(aq)

An Alkali is a base which dissolves in water. When a base dissolves in water it produces OH- ions (Hydroxide ions).

Because acids act as a source of protons, we often refer to the as proton donors.

The Hydroxide ions from an alkali (soluble base) combine readily with protons (H+ ions) to form water. Because they behave like this, they are known as proton acceptors.

We can classify Acids and Alkalis as 'Strong' and 'Weak'. This depends on the extent to which they ionise in water.

A strong Acid or Alkali is one that is 100% ionised in water. Hydrochloric, Sulfuric and Nitric Acids are all examples of strong Acids. Sodium Hydroxide and Potassium Hydroxide are examples of strong Alkalis.

On the other hand, a weak Acid or Alkali is only partly ionised in water. Examples of weak Acids are Ethanoic, Citric and Carbonic Acid. Weak Alkalis include Ammonia solution. 

To test whether an Acid is weak or strong, there are 2 main ways. The pH scale is a measure of the concentration of Hydrogen ions in a solution. So if we measure the pH of a weak Acid and a strong Acid which have the same concentration, and strong Acid will have a lower pH. Thats because it will be fully ionised. 

Another way to test the strength of an Acid, is to measure the rate of reaction when we add a reactive metal to the Acid.