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Strong and Weak Acids

The concept of acid and base is very old. Acids are part of our life and they can be differentiated from base by their taste. First it was purposed by Svante Arrhenius and called as Arrhenius acid-base theory. According to this theory; an acid is a substance that releases hydrogen ions in its aqueous solution. For example; hydrochloride gas is an acid because it releases hydrogen ions in its aqueous solution. The reaction can be written as given below;

$HCl + HOH \rightarrow Cl^{-} + H_{3}O^{+}$

But this theory could not be applicable on all substances such as many substances do not have any hydrogen like carbon dioxide. Then how will be possible to explain their nature. Another issue was with aqueous solution. If a substance is acidic in its aqueous solution, does it mean that it will not be acidic in its solid or gaseous state? 

Bronsted-Lowry concept was purposed after this theory. They did not continue with aqueous solution but show the acidic and basic nature with the help of hydrogen ions. If a substance can give hydrogen ion, it will be acidic. If a substance can accept hydrogen ion, it will be basic in nature. In other words; an acid is a proton donor and a base is a proton acceptor. For example; hydrochloride gas can give hydrogen ion whereas ammonia can accept hydrogen ion. Therefore hydrochloride gas is acidic whereas ammonia is basic in nature.
$HCl \rightarrow  H^{+} + Cl^{-}$ 

$NH_{3} + H^{+} \rightarrow NH_{4}^{+}$

This theory overcome many limitation of previous theory but could not explain many important concepts like acidic and basic nature of those substances which can’t accept or donate hydrogen ions. The latest concept is given by Lewis that is called as Lewis acid-base theory. According to this theory; an acid is an electron pair acceptor whereas a base is an electron pair donor. Now let’s discuss about the strength of acids. There are two kinds of acids; strong acid and weak acid.

 

Definition of Strong and Weak Acids

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We know that acids can give hydrogen ions. Hence strength of acids depends on the concentration of hydrogen ions given by them. In other words; degree of dissociation of an acid determines the strength of it. Remember strength is not related to concentration of an acid. It can be possible to get concentrated and dilute solution of a strong or weak acid.

If an acid show 100% dissociation in water to give hydrogen ions, it is called as strong acid whereas a weak acid show partial dissociation and some un-dissociated molecules of acid remain in solution.  When an acid release hydrogen ion in its aqueous solution, this hydrogen ion forms hydronium ion with water molecule as given below;

$HA + H_{2}O \rightleftharpoons H_{3}O^{+} + A^{-}$ 

This is a reversible reaction. If an acid is strong enough, the reaction will move to right side whereas in case of weak acid, it will move to left side and remain some un-dissociated molecules. Like in case of hydrogen chloride with water the reaction can be written as;

$H_{2}O_{(I)}+HCL_{(g)}\rightarrow H_{3}O^+_{(aq)}+Cl^-_{(aq)}$

This is because the backward reaction is negligible and can be ignored. Therefore hydrogen chloride is considered as strong acid. Sulphuric acid and nitric acid are other examples of strong acids. In case of weak acids, the dissociation or ionisation is not 100% in water.

For example acetic acid reacts with water to form hydroxonium ions and acetate ion. In this case, the backward reaction is more successful and reform acid with water.

$CH_{3}COOH+H_{2}O\rightleftharpoons CH_{3}COO^-+H_{3}O^+$

Hydrogen fluoride and many inorganic acids are good examples of weak acids.

List of strong and weak acids:
   Strong acid        Weak acid    
 Sulphuric acid Acetic acid 
 Nitric acid Hydrogen fluoride 
 Hydrogen chloride  Carbonic acid 
 Hydroiodic acid Phosphoric acid 
 Chloric acid Benzoic acid 
 Perchloric acid Oxalic acid 

Difference Between Strong and Weak Acids

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Strong and weak acid can be differentiated with the help of pH. It is a measure of the concentration of hydrogen ions present in the solution. The range of pH is 0 to 14 in which 0 to 7 represents acidic nature whereas 7 to 14 represent basic nature of solution. 

Low of pH indicates high concentration of hydrogen ions. In other words, a strong acid will show low pH value. A weak acid will release less number of hydrogen ions therefore the pH of solution will be high but less than 7. 

The acid dissociation constant, Ka is another way to measure the strength of an acid. For an acid HA; acid dissociation constant can be written as,
$HA+H_2O \rightleftharpoons H_3O^++A^-$

$K_C=\frac{[H3O^+][A^-]]}{[HA][H_2O]}$

Hence low value of acid dissociation constant represents the more equilibrium lies to the left or dissociation is less, means acid is weak. On the contrary, high value of Ka represents a strong acid.
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