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,
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
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.
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
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
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
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.
- a bottle of hydrochloric acid is labelled with a concentration of 0.1 mol/dm3. This solution
has a pH of 1.
- a bottle of ethanoic acid is labelled with the SAME concentration of 0.1 mol/dm3. This solution
this time has a pH of 2.9
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
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.
Another area which often causes confusion is with strong acids and concentrated
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.
- A strong acid is one which dissolves fully and dissociates fully (100%) in water, weak acids are only partly dissociated in water, often
much less than 5% dissociated. This is also true for weak and strong bases.
- Strong acids have low pH values (1,2 or 3). Strong bases have high pH values (12, 13, 14)
- pH is a measure of the concentration of hydrogen ions in an aqueous solution. Acids and alkalis are solution in water.
- When the pH decreases by 1 the concentration of hydrogen ions increases by a factor of x10.
- Weak acids and bases react in a similar but slower way to strong acids and bases.