Hydrolysis is the name for a substance chemically reacting with water.
Hydrolysis should be distinguished from solvation, which is the process of water molecules associating themselves with individual solute molecules or ions.
I. Salts of Weak Acids
In general, all salts of weak acids behave the same, therefore we can use a generic salt to represent all salts of weak acids.
Let NaA be a generic salt of a weak acid and A¯ its anion.
Two specific examples of salts of weak acids:
Substance ---Formula---The anion portion (A¯)
The generic chemical reaction may be written thusly:
A¯ + H2O --> HA + OH¯
This reaction is of a salt of a weak acid (NOT the acid) undergoing hydrolysis, the reaction with water.
1) The Na+ IS NOT involved. Its source is the strong base (NaOH) that helped form the salt (called NaA in the generic example) and it DOES NOT affect the pH. Its presence in both writing the chemical reactions and doing the calculations is deleted. However, keep in mind that Na+ is present in the solution.
2) HA is the UNDISSOCIATED acid.
It is not the acid that makes the acidic pH of a solution, it is the amount of hydrogen ion (or hydronium ion, H3O+).
In order to produce the hydrogen ion, the acid must dissociate.
3) There is free hydroxide ion (OH¯) in the solution. This is the thing that makes the pH greater than 7.
If there is acid (HA) and base (OH¯), why don't they just react and give back the reactants on the left side?
The answer? This reaction is an equilibrium.
When a chemical reaction comes to equilibrium, there is a mixture of all involved substances in the reaction vessel. This mixture is characterized by a constant composition. (constant composition DOES NOT imply equal composition.)
The key point that makes a reaction come to equilibrium is that it is reversible. This means that both the forward reaction and the reverse reaction can happen,
The reaction comes to equilibrium when the rates of the two reactions (forward and reverse) become equal.
So, while it is true that the HA and OH¯ will react in the reverse direction, so can the A¯ and the H2O in the forward direction. The key point is that the reaction happens in such a way that a small amount of HA and OH¯ are present at equilibrium.
When calculations are done, the important points will be (1) how much OH¯ is formed and (2) what is the pH of the solution?
II. Salts of Weak Bases
In general, all salts of weak bases behave the same, therefore we can use a generic salt to represent all salts of weak bases.
Let B be a generic base and HB+ its salt. (Compare how this is worded compared to the "salt of weak acid" discussion.)
HB+ is a cation, but that word is not used as much in discussions as is "anion" is above.
Two specific examples of salts of weak bases:
Substance---Formula---The cation portion (HB+)
methyl ammonium nitrate---CH3NH3NO3---CH3NH3+
The chloride ion, Cl¯, and the nitrate ion, NO3¯ tend to be used in examples.
The chemical reaction may be written as:
HB+ + H2O --> B + H3O+
This reaction is of a salt of a weak base (NOT the base) undergoing hydrolysis, the reaction with water.
It is very important that you notice several things:
1) There is an anion involved, but it is not usually written. For example Cl¯ could be the anion, but it IS NOT involved. Its source is a strong acid (HCl) that helped form the salt and it DOES NOT affect the pH. However, Cl¯ is present in the solution.
2) B is the UNPROTONATED base. Keep in mind that it is not the base that makes the basic pH of a solution, it is the amount of hydroxide ion (OH¯). In order to produce it, the base must protonated by the water.
3) There is free hydronium ion (H3O+) in the solution!! This is the thing that makes the pH less than 7.
If there is base (B) and acid (H3O+), why don't they just react and give back the reactants on the left side?
The answer, of course, is given in above in the discussion of salts of weak acids. It would be the same explanation here.
Of course, when calculations are done, the important points will be (1) how much H3O+ is formed and (2) what is the pH of the solution?