Weak vs Strong Acid
The strength of an acid is its ability to ionize or donate the hydrogen ion in an aqueous solution reacting with water. It is not, what many wrongly presume to be the concentration of an acid in a solution. The more an acid ionizes, the stronger it is and the less production of hydrogen ions indicates a weak acid. This is the difference between a strong and a weak acid. Let us take a closer look. The concentration of ions in an aqueous solution of an acid tells how strong or weak an acid it is. So, you can have a concentrated solution of a weak acid and it is perfectly possible to have a dilute solution of a strong acid.
When an acid is added to water, it dissolves, transferring a hydrogen ion (proton) to a water molecule producing hydroxonium ion and a negative ion which depends upon which acid we are starting with. In general, the equation is represented as following.
HA + H2O ↔ H3O+ + A-
Generally these reactions are irreversible but some acids are so strong that nearly 100% of the acid gets ionized with the result that you have only Hydroxonium ion and negative ions at the end of the reaction. Talking about hydrochloric acid, it is considered a very strong acid and dissolves 100% in water. Thus its solution contains only hydroxonium ions and the resulting chloride ions. On the basis of its ability to produce ions, HCl is considered a strong acid. Some other acids that ionize to a great degree are sulphuric acid and nitric acid.
HCl [aq] → H+[aq] + Cl-[aq]
Remember that it is water that is involved in the reaction and thus H+ is actually hydroxonium ion, written as H3O.
It becomes clear then that a weak acid is one that does not fully ionize in a water solution. For example, Ethanoic acid is a weak acid which implies that it does produce hydroxonium ions and ethanoate ions but the forward reaction is less successful than the back reaction and the ions produced get back into acid molecules and water molecules easily so that in the end, there are very few hydroxonium ions in the solution. In this case, only about 1% of hydroxonium ions are found in the solution as against 100% H+ ions in the case of hydrochloric acid. This means that ethanoic acid is a very weak acid in comparison to hydrochloric acid.
Difference Between Weak and Strong Acid
• Not all acids dissolve in water to the same extent with the result that some ionize almost 100% while some others ionize very little.
• The ability of an acid to ionize more in water means it is a strong acid while its inability to ionize indicates its weakness.
• Strong acids produce more H+ ions while weak acids produce very few H+ ions in water.