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Hydrolysis

1. 1.1. Salt Hydrolysis
2. 1.2. Acid Hydrolysis
3. 1.3. Basic Hydrolysis
2. Examples & Practice
3. Solutions to Example Problems
4. Use of Hydrolysis in the "Real World"
5. References
6. Outside Links
7. Contributors

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.

1. 1.1. Salt Hydrolysis
2. 1.2. Acid Hydrolysis
3. 1.3. Basic Hydrolysis
2. Examples & Practice
3. Solutions to Example Problems
4. Use of Hydrolysis in the "Real World"
5. References
6. Outside Links
7. Contributors

Salt Hydrolysis

In water, salts will dissociate completely to form ions.

Example:

NH₄Br_(s)→ NH₄⁺_(aq) + Br^-_(aq)

Here, the salt NH₄Br is put into water and dissociates into NH₄⁺and Br^-.

Hydrolysis of NaCl.jpg

In the figure above,

NaCl_(s) → 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 ways of forming salts:

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.
If the salt is formed from a strong acid and weak base, the bonds in the salt solution will break apart and becomes acidic.
If the salt is formed from a strong base and weak acid, the salt solution is basic and hydrolyzes.
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 K_a and K_b. If the K_a value is greater than the K_b value, the resulting solution will be acidicand vice versa.

Acid Hydrolysis

H₂O 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.

CH₃COOH +H₂O $\rightleftharpoons$ H₃O⁺ + CH₃COO^-

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:

NH₃ + H₂O $\rightleftharpoons$ NH₄⁺ +OH^-

In the hydrolysis of ammonia, the ammonia molecule accepts a proton from the water (because 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.

Examples & Practice

1) H₂CO₃ + H₂O $\rightleftharpoons$ 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^- +H₂O $\rightleftharpoons$ CH₃COOH + OH^-
   What is the pH of 0.30 M of sodium acetate?
   (Hint: First find Kb value)
   Given: K_aof CH₃COOH= 1.8 x 10^-5
5)
   a. Does sodium acetate (from previous problem) hydrolyze?
   b. Is solution acidic or basic?

Solutions to Example Problems

1)
   a. The conjugate base is the HCO₃^-. The weak acid is the H₂CO₃.
    b. Yes it hydrolyzes.
₂₎a.HF + H₂O $\rightleftharpoons$ 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⁺ +H₂O $\rightleftharpoons$ 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^- +H₂O $\rightleftharpoons$ CH₃COOH + OH^-

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

K_b=      K_w
   K_a of CH₃COOH
K_b= 1.0 x10^-14      1.8 x 10^-5
K_b= 5.6 x 10^-10
K_b= [CH₃COOH][OH^-]
       [CH₃COO^-]
(Use the given K_b 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

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.

Outside Links

Hydrolysis: http://en.wikipedia.org/wiki/Hydrolysis
Hydrolysis of Ions: http://spinner.cofc.edu/genchemlab/hydrolysis.htm?referrer=webcluster&
Salt Hydrolysis: http://www.chemistry.nmsu.edu/studntres/chem112.spring.1997/Hydrolysis.html
Acid-Base Reaction: http://en.wikipedia.org/wiki/Acid–base_reaction
Salt Hydrolysis: http://ibchem.com/IB/ibnotes/full/aab_htm/18.4.htm

Contributors

Patrina Kim (UCD), Gretchen Hehir

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knnakajima says:

Dear contributor, I think it would be helpful to add a table of contents! Also, you should consider putting the "Real World" module above the practice problems =) I think it would help out your audience greatly!

Posted 00:08, 2 Mar 2010