Acids are proton donors. Bases are proton acceptors. Acids and bases react by proton transfer.
H2O can act as both an acid and a base: H2O(l) + H2O(l)
H3O+ + OH–
Substances referred to as acids are stronger than water.
The reaction of strong acids with water is product-favoured (in these cases all of the acid dissolved forms products).
Reaction of weak acids with water is reactant-favoured (the extent depending on the acid).
How do I know if an acid is weak or strong?
These are the Bronsted-Lowry definitions.
H2O can act as both an acid and a base: H2O(l) + H2O(l)
H3O+ + OH–This reaction is described as strongly reactant-favoured because it occurs to a very small (but measureable) extent in pure water.
Substances referred to as acids are stronger than water.
Strong acids are stronger than water and stronger than H3O+
Weak acids are stronger than water and weaker than H3O+
Weak acids are stronger than water and weaker than H3O+
The reaction of strong acids with water is product-favoured (in these cases all of the acid dissolved forms products).
HCl(aq) + H2O 
H3O+ + Cl– (aq)
The H3O+ concentration or [H3O+] in the aqueous solution of a strong acid is equal to the concentration of the acid.
Thus the pH can be directly calculated from the concentration.
H3O+ + Cl– (aq)The H3O+ concentration or [H3O+] in the aqueous solution of a strong acid is equal to the concentration of the acid.
Thus the pH can be directly calculated from the concentration.
For 1 HCl, [H3O+] is also 1 and the pH is 0.
Reaction of weak acids with water is reactant-favoured (the extent depending on the acid).
[H3O+] in their aqueous solutions is lower than the initial concentration of the weak acid.
The pH of these solutions is higher than a strong acid in the same concentration.
The pH of these solutions is higher than a strong acid in the same concentration.
How do I know if an acid is weak or strong?
There are very few common strong acids.
You probably already know most of them: HCl, HBr, HI, HNO3, H2SO4.
If you remember that these are strong, it is fair to assume that other acids that you may encounter are weak.
You probably already know most of them: HCl, HBr, HI, HNO3, H2SO4.
If you remember that these are strong, it is fair to assume that other acids that you may encounter are weak.