Higher tier
Strong and weak acids
Before starting a discussion on weak and strong acids
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, the state symbol (aq)- short for aqueous or water is telling us that acids and alkalis are all aqueous solutions, this is what the (aq) state symbol is telling you.
Acids are substances which dissolve in water to release
an excess of hydrogen ions (H+(aq)). You may see the formula for an
acid often simplified to HA. Now when
an acidic 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 type of arrow used in the equations; that is → 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 year 7
science. However it might surprise you that very few reactions actually go to completion, most
reactions are reversible and involve equilibrium.
- This arrow ⇌ tells you that the reaction contains a mixture both reactants
and products and that this reaction is reversible.
At any
one time there is a mixture of the reactants and
products present. Different reactions using different chemicals will produce different
amounts of reactants and products in these equilibrium mixtures.
Strong and weak acids
Strong acids which include hydrochloric, sulfuric and nitric
acids are acids which fully
dissociate or break up in water; that is they are 100% dissociated into ions.
This means when they are added to water the molecules that make them up
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 soluble in water and when it dissolves it completely dissociates or breaks up 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 molecules dissociate in
water, 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 in
water 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).
Recall that 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. This means that when the weak acid molecules are dissolved in water very few of them break apart or dissociate. That is to say the reactions as mentioned earlier are
reversible and consist of
equilibrium mixtures of reactants and products. This means that 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. Not many of the ethanoic acid molecules breaks apart or dissociate when it dissolves in water and almost all of the
ethanoic acid molecules remain intact with only a very few actually breaking up to form ions, we can 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. The image below summarises the differences between weak and strong acids in terms of their ability to ionise in water.
It is a similar story with weak bases. Ammonia for example dissolves readily in water to form a weakly 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 (H+(aq)) 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 that a
solution
with a pH of 1 has 10 times more hydrogen ions present than a
solution of pH2.
Concentration and pH
It can sometimes be confusing when discussing concentrations of solutions of weak and strong acids
and alkalis. For example:
- 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
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 labels 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 present 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
acids.
Some students get these terms mixed up. A hydrochloric acid solution with a concentration
of 0.1mol/dm3 is a fairly dilute solution; that is 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
solutions the acid is a strong one that is fully dissociated in the water.
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. Ethanoic acid with a concentration of 5 mol/dm3 is a concentrated solution of a weak acid.
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 or number 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 hydrochloric (a strong acid) and ethanoic (a weak acid) acids with marble chips.
Key Points
- 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 fashion but slower rate than strong acids and bases.
- The terms dilute and concentrated refer to the amount of water present in the acid or alkali and NOT to how strong it is.
Practice questions
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