Hydrolysis of salts

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A Brief Introduction to Hydrolysis Calculations

Hydrolysis happens when a substance chemically reacts 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. Here are two specific examples of salts of weak acids:

SubstanceFormulaThe anion portion (A¯)
sodium acetateNaC2H3O2C2H3O2¯
sodium benzoateC6H5COONaC6H5COO¯

By the way, the potassium ion, K+, (and several others) could also be used above without affecting any discussions of this topic. As a practical matter, only Na+ and K+ tend to get used in examples.

The generic chemical reaction (in net ionic form) for hydrolysis may be written thusly:

A¯ + H2O --> HA + OH¯

This reaction is of a salt of a weak acid (NOT the acid) undergoing hydrolysis, the name for a chemical reaction with water. The salt is NaAc and it is reacting with the water. Keep in mind that the acid (HAc) does not undergo hydrolysis, the salt does.

It is very important that you notice several things:

1) The Na+ (notice only OH¯ is written) IS NOT involved. Its source is the salt (NaA) that is dissolving in the water 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. Some teacher might want to ask a "sneaky" question on a test.

2) HA is the UNDISSOCIATED acid. Keep in mind that it is not the acid that makes the acidic pH of a solution, it is the amount of hydrogen ion (or hydronium ion, H3O+, if you wish). 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.

Now, I can see a question forming in your mind. If there is acid (HA) and base (OH¯), why don't they just react and give back the reactants on the left side? Now, that really is a good question.

The answer? This reaction is an equilibrium. Now, if you are taking chemistry for the first time, you probably just got done with equilibrium a few weeks ago and it might have been hard to understand. That's understandable, but please realize that equilibrium is one of more important concepts in chemistry. Keep up the work!!

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. (Keep in mind that 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, althought NOT initially with equal probability. 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 (as opposed to zero) 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?

Quick answers: (1) the amount of OH¯ formed will be greater than the 10¯7 M value present in pure water and (2) the pH will be greater than 7, so the solution of the salt of a weak acid will be basic.


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. Here are two specific examples of salts of weak bases:

SubstanceFormulaThe cation portion (HB+)
ammonium chlorideNH4ClNH4+
methyl ammonium nitrateCH3NH3NO3CH3NH3+

The notation HB+ might be a bit confusing. Think of NH4+ this way:

HNH3+

NH3 is the base (symbolized by B) and an H+ has been attached to it in a chemical reaction. the NH3 has been protonated and the result (NH4+) is now an acid. Why? Because it now has a proton to donate.

By the way, the chloride ion, Cl¯, and the nitrate ion, NO3¯ tend to be used in examples. Other anions of strong acids could be used, but their use is fairly uncommon.

The generic chemical reaction (in net ionic form) for hydrolysis reaction may be written thusly:

HB+ + H2O --> B + H3O+

This reaction is of a salt of a weak base (NOT the base) undergoing hydrolysis, the reaction with water. The salt in this case is HB+Cl¯ and it is reacting with the water. Remember, the most common specific example would be ammonium chloride (NH4+Cl¯). Keep in mind that the base (generic example = B, specific example = ammonia or NH3) does not undergo hydrolysis, the salt does.

It is very important that you notice several things:

1) There is an anion involved, but it is usually not written. For example Cl¯ could be the anion, but it IS NOT involved. Its source is the salt (HB+Cl¯) that is dissolving in the water and it DOES NOT affect the pH. Its presence in writing the appropriate chemical reactions and doing the calculations is deleted. However, keep in mind that Cl¯ is present in the solution. Some teacher might want to ask a "sneaky" question on a test.

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.

Now, I can see a question forming in your mind. If there is base (B) and acid (H3O+), why don't they just react and give back the reactants on the left side? Now, that really is a good question.

The answer, of course, is given in above in the discussion of salts of weak acids. It would be the same explanation here, so I won't repeat it. What you might want to do, however, is look at the different phrasing in part I as compared to part II.

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?

Quick answers: (1) the amount of H3O+ formed will be greater than the 10¯7 M value present in pure water and (2) the pH will be less than 7, so the solution of the salt of a weak base will be acidic.


A Brief Introduction to Hydrolysis Calculations

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