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.
In water, salts will dissociate completely to form ions.
NH4Br(s)→ NH4+(aq) + Br-(aq)
Here, the salt NH4Br is put into water and dissociates into NH4+and Br-.
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:
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 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.
A base hydrolysis reaction will resemble the reaction for base dissociation. A common weak base that dissociates in water is ammonia:
NH3 + H2O 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 H3O+ +HCO3-
Solutions to Example Problems
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.
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