Differences between strong and weak acids

What are the differences between strong and weak acids?

Acids can be categorised into two groups: strong and weak acids.

Strong acids are acids that dissociates completely in water to produce a high concentration of H+ ions. Whereas, weak acids are acids that dissociates partially in water to produce a low concentration of H+ ions.

In other words, the strength of an acid is the measure of the acid molecules ability to dissociate in water to form ions.

Sulfuric acid, nitric acid and hydrochloric acid are strong acids. In a solution of strong acid, all the acid molecules become ions in water.

For example, a solution of sulfuric acid only contains hydrogen ions, H+ and sulfate ions SO42-. It does not contain hydrogen sulfate molecules.

The chemical equation to illustrate the dissociation of a strong acid in water is shown below:

HNO3(aq) → H+(aq) + NO3-(aq)

In this reaction, a molecule of nitric acid (HNO3) dissociates in water to produce a high concentration of H+ and NO3- ions. The NO3- ions do not "further dissociate" to give nitrogen and oxygen ions as it is a polyatomic ion!

An important point to note is that a solution of strong acid contains hydroxide OH- ions too. But a very small amount compared to the H+ ions.

On the other hand, weak acids only dissociate partially in water to form ions. 

Example of a weak acid is ethanoic acid, CH3COOH. When ethanoic acid is dissolved in water, only the hydrogen ions in the -COOH group can dissociate in water.

To illustrate the dissociation of a weak acid using chemical equations, a double arrow ⇄ is used. This means that the dissociation is not complete and there is only a partial dissociation.

An example of the dissociation of a weak acid (ethanoic acid) is shown below:

CH3COOH(aq) ⇌ CH3COO (aq)+ H+(aq)

In a solution of weak acid, most of the acid molecules remain unchanged in water. Few acid molecules ionize to form hydrogen ions.

pH scale

pH a figure expressing the concentration of hydrogen ions in the solution. The higher the concentration of hydrogen ions, the lower the pH.

For pH less than less 7, it is an acidic solution. There is more hydrogen ions than hydroxide ions.

Acidic solution → more H+ ions than OH- ions → pH < 7

For pH more than 7, it is an alkaline solution. There is more hydroxide ions than hydrogen ions. Alkaline solution → more OH- ions than H+ ions → pH > 7 .

Important: pH is not a measure of the strength of acid! It is a measure of the concentration of hydrogen ions in a solution. Normally, strong acids have low pH value of around 1 and 2 as the molecules dissociate complete in water to produce a high concentration of H+ ions.

However, a solution of strong acid can also have high pH and less than 7 if the solution is very diluted with water.

Difference between monobasic and dibasic acid

The basicity of an acid refers to the number of hydrogen ions that can be produced per molecule of acid when dissolve in water.

Dibasic acid is an acid that dissociates to produce 2 hydrogen ions per molecule. Examples of dibasic molecules are sulfuric acid, carbonic acids and phosphoric acids. Note that although phosphoric acid has three hydrogen atoms per molecule (H3PO4), the last hydrogen is not able to dissociate in water,




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