Higher tier

Strong and weak acids

Before starting a discussion on weak and strong acids and alkalis you should already know that acidity is a property of water. Acids and alkalis are solutions with an excess of H+(aq) and OH-(aq) in water. This is what the (aq) is telling you. Acids are substances which dissolve in water to release hydrogen ions, H+(aq). The formula for acids is often simplified to HA. When an acid is added to water there are two reactions that can happen, these are:

Reaction 1: HA → H+(aq) + A-(aq)
Reaction 2: HA ⇌ H+(aq) + A-(aq)
(note the water has been missed out in these equations just to simplify things)

At first glance these two equations might seem similar but in fact they are quite different. The reason for these differences is due to the arrow, → or ⇌.

This arrow, → , tells you that the reaction goes to completion. This means that 100% of the reactants are turned into products. This is the type of arrow that you have probably been using since year7 science. However it might surprise you that very few reactions actually go to completion, most reaction involve equilibrium reactions.

This arrow, ⇌,tells you that the reaction mixture contain both reactants and products. At any one time there is a mixture of the two. This mixture could be 95% reactants and 5% products or vice versa. Different reactions using different chemicals will produce different equilibrium mixtures. Strong acids which include hydrochloric, sulfuric and nitric acids are acids which fully dissociate in water, they are 100% dissociated into ions. This means when they are added to water they fully break-up or dissociate or ionise into hydrogen ions and an anion (a negatively charged ion) e.g. Hydrochloric acid is made by dissolving hydrogen chloride gas in water. Hydrogen chloride gas is very very soluble in water and when it dissolves it completely dissociates or ionises to form hydrogen ions, H+(aq) and chloride ions Cl-(aq).

HCl(g) → H+(aq) + Cl-(aq)
(I have left out the water to simplify the equation)
The hydrogen chloride gas consists of small covalent molecules which break apart and form ions when it dissolves in water. All of the hydrogen chloride dissociates, that is the reaction goes to completion. Hydrochloric acid is a strong acid because it is fully dissociated into H+(aq) and Cl-(aq).

Strong alkalis are the same, they are made when a base dissolves and fully dissociates in water to release an excess of hydroxide ions. Metal oxides from group I of the periodic table such as sodium oxide and potassium oxide are strong bases e.g. They dissolve completely and dissociate or ionise fully to give a solution of sodium ions and hydroxide ions:
NaOH(s) → Na +(aq) + OH-(aq)
(I have left out the water to simplify the equation).
Strong acids have low a pH of 1, 2 or 3 and strong alkalis have a pH of 12, 13 and 14. This is shown on the colour chart for universal indicator below.

colour chart for universal indicator Weak acids are formed when a substance only partly ionises or dissociates when it dissolves in water. That is to say the reactions as mentioned earlier involve equilibrium and the acid is NOT fully ionised. Weak acids include carbonic acid and carboxylic acids such as ethanoic and methanoic acid e.g. When the carboxylic acid ethanoic acid is added to water it form a weak acid. The ethanoic acid molecule only partly breaks apart when it dissolves in water and most of the ethanoic acid molecule remain intact, we show this as:
CH3COOH(l) ⇌ CH3COO-(aq) + H+(aq)
The important point is that there a very few hydrogen ions, H+(aq), most of the molecules of the ethanoic acid remain intact. It is a similar story with weak bases. Ammonia for example dissolves readily in water to form a weak alkaline solution of ammonium hydroxide.
NH3(g) + H2O(l) ⇌ NH4+(aq) + OH-(aq)
Again this is an equilibrium mixture and does not go to completion as is the case with strong acids and bases.

pH scale

The pH scale as you probably already guessed from the letters p and H, has something to do with hydrogen ions. The pH scale is a measure of the concentration of hydrogen ions in a solution. The lower the pH the higher the concentration of hydrogen ions present. In fact every time the pH changes by 1 the concentration of hydrogen ions changes by a factor of x10. This means a solution with a pH of 1 has 10 times more hydrogen ions present than a solution of pH2, provided the volumes remain the same!

Concentration and pH

It can sometimes be confusing when discussing concentrations of solutions of weak and strong acids and alkalis. For example: The difference of almost nearly 2 pH values means that the concentration of hydrogen ions in the ethanoic acid is almost 100 times less than in the hydrochloric acid, but the label on the bottles are both 0.1 mol/dm3.
For the hydrochloric acid, which is a strong acid that is completely dissociated in water, the concentration of 0.1 mol/dm3 refers to the concentration of hydrogen ions. For the ethanoic acid, a weak acid which is only partly dissociated in water the concentration refers to the "undissociated" CH3COOH molecules and NOT to the small number of hydrogen ions in the solution. So take care with concentrations when you are considering weak and strong acids and alkalis.

concentrated and dilute orange juice Another area which often causes confusion is with strong acids and concentrated acids. Some students get these terms mixed up. A hydrochloric acid solution with a concentration of 0.1 mol/dm3 is a fairly dilute solution, it contains mostly water and only a little acid. A hydrochloric acid solution with a concentration of 5 mol/dm3 is much more concentrated than the 0.1 mol/dm3 solution, that is it contains less water and more acid. However in both solution the acid is a strong one, it is fully dissociated.

Strong and weak refers to how ionised or dissociated the acid molecules are. Concentrated or dilute refers to how much water is present. If the acid is dilute then it contains a large amount of water and as the concentration rises the amount of water present reduces.

Reactions of strong and weak acids

The reactions of weak and strong acids are similar but with a weak acid they are slower and less violent. This is simply because the characteristic reactions of acids are based on the concentrations and numbers of hydrogen ions, H+(aq) present and since weak acids have less hydrogen ions present their reactions will be slower. The diagram below shows this by comparing the reactions of the hydrochloric and ethanoic acid with marble chips.

comparing the reactions of weak and strong acid with marble chips

Key Points

Practice questions

Check your understanding - Questions on weak and strong acids