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Strong and Weak Acids


What are acids?

All acids contain a positive hydrogen

ion (H+) bound to a negative ion (A-).

When in solution, the acid

ionises. This means that it

forms its charged ions, which

are released into solution:

The general formula of an acid is

HA, where ‘H’ represents the

hydrogen ion and ‘A’ represents

the accompanying negative ion.


Strong acids

Acids can be strong or weak depending on how much they

ionise in solution.

Strong acids ionise fully in solution, so that all of their H+ ions

are released into the mixture.

Hydrochloric acid is a strong acid

as it fully ionises in solution:

Other strong acids include nitric acid

(HNO3) and sulfuric acid (H2SO4).


The ionisation of a weak acid is a reversible reaction:

Weak acids

Weak acids do not fully ionise in solution. This is because

some of their H+ ions stay attached to the acid molecule, (HA).

Ethanoic acid (vinegar) is a

typical weak acid as it does

not fully ionise in solution:

Other weak acids include

citric acid in lemons and

carbonic acid (H2CO3).


Identifying strong and weak acids


Strong or weak?


3 40 1 2 5 6 7 8 9 10 11 12 13 14

What does the pH scale show?

The pH scale runs from 0 to 14. The numbers 0 to 6 represent

acidic conditions and 8 to 14 represent alkaline conditions.

A pH value 7 is neutral.

What are the pH values of some everyday items?

increasing acidity increasing alkalinity


pH of strong and weak acids

have higher pH values

pH is a measure of the concentration of H+ ions in solution. A

lower pH means more H+ ions are present in solution.

As strong acids ionise fully in solution, they contain many H+

ions per molecule of acid, producing a lower pH.

Weak acids do not fully ionise. They therefore contain

fewer H+ ions per molecule of acid and:

higher H+

concentration

lower H+

concentration

are worse conductors of electricity

react less quickly.


Acidity and acid strength

The acidity of an aqueous solution depends on the number of

H+ ions present. It is a measure of how acidic the solution is,

and indicates the strength of an acid.

Acid strength is measured in pH. It

is the concentration of H+ ions in

solution. The higher the

concentration of H+ ions, the lower

the pH, and the stronger the acid.

The pH scale also measures

alkalinity. As the concentration of

OH– ions increases, the

concentration of H+ decreases. This

results in an increase in pH.


Neutrality

When an acid reacts with a base, a neutralisation

reaction occurs:

As the base (OH-) is added,

the number of free hydrogen

ions in solution decreases,

causing an increase in pH.

A solution is neutral at pH 7 when the concentration of

H+ ions equals the concentration of OH- ions

This means the neutrality of a solution is determined by

the concentration of hydrogen ions present.


Diluting acids

Strong acids have a relatively high concentration of H+

ions, and therefore a low pH.

When a strong acid is diluted, the concentration of H+ falls and

the pH rises. This decreases the acid strength.

As the concentration of H+ ions decreases by a factor

of 10, the pH increases by one unit.

10-710-610-510-410-310-210-11

decreasing H+ concentration

H+concentration

(mol/dm3)

pH


Diluting acids


Reactivity of strong and weak acids

A solution of ethanoic acid reacts more slowly than

hydrochloric acid of the same concentration.

Hydrochloric acid is fully

dissociated, so all the hydrogen

ions are available to react.

Ethanoic acid is partially

dissociated, so there are fewer

hydrogen ions to react.

The lower concentration of H+

ions in ethanoic acid leads to a

lower collision frequency

between reactant molecules,

and so a slower reaction.


Acids and carbonates

When a carbonate is mixed with an acid, the products are

a salt, carbon dioxide and water.

For example, the reaction of nitric acid

(a strong acid) and copper carbonate:

What would you expect to observe in this reaction?

What difference might you see in the reaction

of ethanoic acid and copper carbonate?


Weak acids and carbonates

As ethanoic acid is a weak acid, only moderate fizzing

would be expected.

What would happen if the concentration

of ethanoic acid was increased?

This is because ethanoic

acid has fewer H+ ions in

solution than a strong acid

of the same concentration.

This means there are fewer

collisions occurring with the

carbonate and so a slower

rate of reaction.


Reacting with metals


Glossary


Multiple-choice quiz

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