Hydrolysis

Hydrolysis is a reaction involving the breaking of a bond in a molecule using water. The reaction mainly occurs between an ion and water molecules and often changes the pH of a solution. In chemistry, there are three main types of hydrolysis: salt hydrolysis, acid hydrolysis, and base hydrolysis.

Salt Hydrolysis

In water, salts will dissociate to form ions (either completely or incompletely depending on the respective solubility constant, $$K_{sp}$$. For example:

$NH_4Br_{(s)} \rightarrow NH^+_{4(aq)} + Br^-_{(aq)}\tag{1}$

Here, the salt $$NH_4Br$$ is put into water and dissociates into $$NH_4^+$$ and $$Br^-$$.

Figure 1. IThe hydrolysis of table salt is shown ($$NaCl \, (s) \rightarrow Na^+(aq) + Cl^-(aq)$$).Note that water is polar, causing O to be slightly negative and H to be slightly positive. The positively charged sodium ion is attracted to the O in water and the negatively charged chlorine ion is attracted to the H in water.

There are four possible mechanisms of forming salts:

1. If the salt is formed from a strong base and strong acid, then the salt solution is neutral, indicating that the bonds in the salt solution will not break apart (indicating no hydrolysis occurred) and is basic.
2. If the salt is formed from a strong acid and weak base, the bonds in the salt solution will break apart and becomes acidic.
3. If the salt is formed from a strong base and weak acid, the salt solution is basic and hydrolyzes.
4. If the salt is formed from a weak base and weak acid, will hydrolyze, but the acidity or basicity depends on the equilibrium constants of Ka and Kb. If the Ka value is greater than the Kb value, the resulting solution will be acidic and vice versa.

Acid Hydrolysis

Water can act as an acid or a base based on the Brønsted-Lowry acid theory. If it acts as a Bronsted-Lowry acid, the water molecule would donate a proton (H+), also written as a hydronium ion (H3O+). If it acts as a Bronsted-Lowry base, it would accept a proton (H+). An acid hydrolysis reaction is very much the same as an acid dissociation reaction.

$CH_3COOH +H_2O \rightleftharpoons H_3O^+ + CH_3COO^- \tag{2}$

In the above reaction, the proton H+ from CH3COOH (acetic acid) is donated to water, producing H3O+ and a CH3COO-. The bonds between H+ and CH3COO- are broken by the addition of water molecules. A reaction with CH3COOH, a weak acid, is similar to an acid-dissociation reaction, and water forms a conjugate base and a hydronium ion. When a weak acid is hydrolyzed, a hydronium ion is produced.

Basic Hydrolysis

A base hydrolysis reaction will resemble the reaction for base dissociation. A common weak base that dissociates in water is ammonia:

$NH_3 + H_2O \rightleftharpoons NH_4^+ +OH^- \tag{3}$

In the hydrolysis of ammonia, the ammonia molecule accepts a proton from the water (i.e., water acts as a Bronsted-Lowry acid), producing a hydroxide anion (OH-). Similar to a basic dissociation reaction, ammonia forms ammonium and a hydroxide from the addition a water molecule.

Use of Hydrolysis in the "Real World"

In nature, living organisms are only able to live by processing fuel to make energy. The energy that is converted from food, is stored into ATP molecules (Adenosine Triphosphate). Life requires many processes in order to sustain itself such as cellular respiration, respiration, muscle contraction, distribution of hormones, transmittance of neuro-transmitters in the brain, etc. All of these important processes require an input of energy. To distribute this energy, the energy from the ATP molecules must be released. To release the energy stored in the bonds of ATP molecules, hydrolysis must occur to break a phosphate group off of an ATP molecule, thus releasing energy from the bonds. ATP now becomes ADP (Adenosine Diphosphate) from losing a phosphate group through hydrolysis.

References

• Petrucci, et al. General Chemistry Principles and Modern Applications. 9th ed. New Jersey: Prentice Hall, 2007. 686-690.
• Freeman, Scott. Biological Sciences. 4th ed. San Francisco: Pearson Prentice Hall, 2011.

Problems

1) H2CO3 + H2$\rightleftharpoons$ H3O+ +HCO3-
a. Identify which of these is the conjugate base and which is the weak acid.
b. Does the weak acid hydrolyze?
2)
a. Write out the chemical equation for the hydrolysis HF.
b. Is water acting as a Bronsted-Lowry acid or Bronsted-Lowry base?
3)
a. Write out the equation for the dissociation of the salt NH4Br.
b.Write out the hydrolysis of the cation that is produced from the dissociation of the ammonium bromide.
c. From what kinds of acids and bases is ammonium bromide (NH4Br) made from? Strong acid/strong base? Strong acid/weak base?     Strong base/weak acid? Weak base/weak acid?
d. State whether salt hydrolyzes.
e. State whether solution is acidic or basic.
4). CH3COO- +H2$\rightleftharpoons$ CH3COOH + OH-
What is the pH of 0.30 M of sodium acetate?
(Hint: First find Kb value)
Given: Kaof CH3COOH= 1.8 x 10-5
5)
a. Does sodium acetate (from previous problem) hydrolyze?
b. Is solution acidic or basic?

Solutions

1)
a. The conjugate base is the HCO3-. The weak acid is the H2CO3
b. Yes it hydrolyzes.
2)

a.HF + H2$\rightleftharpoons$ H3O+ + F-
b.Water is acting as a Bronsted-Lowry base because it is accepting a proton (H+) from the HF.
3)
a. NH4Br → NH4++ Br-
b. Br- does not hydrolyze; it is an ion.
NH4+ +H2$\rightleftharpoons$ H3O+ +NH3-  <--- Hydrolysis of NH4+
c. HBr is a strong acid. Ammonia is a weak base. So NH4Br  is made of a strong acid and weak base.
d. Yes it hydrolyzes.
e. Acidic
4)
CH3COO- +H2$\rightleftharpoons$ CH3COOH + OH-

 CH3COO- CH3COOH OH- I 0.30M C -x x x E 0.30 - x x x

Kb      Kw
Ka of CH3COOH
Kb= 1.0 x10-14
1.8 x 10-5

Kb= 5.6 x 10-10
Kb= [CH3COOH][OH-]
[CH3COO-]
(Use the given Kb and the concentrations from the ICE table)
5.6 x 10-10 =     x2
0.30-x     (Assume x << 0.30)
x2= 1.68 x 10-10
x=1.30 x 10-5 = [OH-]
pOH= -log (1.30 x 10-5) = 4.89
pH = 14 - pOH
pH = 14.00 - 4.89 = 9.11
5)
a. Yes it hydrolyzes
b. Basic solution

Contributors

• Patrina Kim (UCD), Gretchen Hehir