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Hydrolysis

Written by tutor Kathie Z.

Just as the study of water is “hydrology”, hydrolysis means water + separation. Most often, you will be studying the breaking of chemical bonds by adding a water molecule which means the substance you are observing is breaking down.

This process involves several Chemistry steps that you have already learned:

You may have also learned about hydrolysis in Biology when you learned about ATP and digestion. You may want to review these topics in your textbook or in the Wyzant Help pages.

Hydrolysis

This is a chemical process of adding a water molecule to a compound which most often causes both parts to break their chemical bonds resulting in each fragment becoming either an anion (negatively charged ions like hydroxide ions, OH-) or a cation (positively charged ions like hydrogen ions H+). The oppositely charged ions attach to each other synthesizing into new substances. Often these reactions take place when ionic compounds dissolve in water.

There are five types of hydrolysis reactions which you may encounter in your studies: Salts, ATP, Digestion (enzymes), Soap (saponification) and Weathering (rocks).

Salts are compounds that result from the reaction of an acid and a base. Ionic compounds can be either an acid or a base and are the foundation of a salt which is their resultant compound. Depending on whether the salt was originally a strong acid and weak base, or the reverse (weak acid and strong base) when it dissolves in water (undergoes hydrolysis) it will either result in a strong acidic solution or conversely a strong alkaline solution. The water molecules spontaneously ionize into hydroxyl anions and hydrogen cations in the process.

a) You can safely combine vinegar and sodium bicarbonate and create sodium acetate at home:

CH3COOH + NaHCO3 -> CH3COO-Na+ + H2O + CO2

The sodium acetate dissociates in water into sodium and acetate ions. Sodium ions react very little with the hydroxyl ions whereas the acetate ions combine with the hydrogen ions resulting in a basic (alkaline) solution and releasing carbon dioxide gas.

b) Ammonium chloride (NH4Cl) is the salt of a weak base, ammonia (NH3) and a strong acid, hydrochloric acid (HCl).

NH4Cl + H2O -> NH4+ + H2O + Cl- -> NH3 + H3O+

When the ammonium chloride is dissolved in water it hydrolyzes splitting into the cation NH4+ and anion Cl-. The NH4+ will react with the H2O by losing a hydrogen ion becoming H3O+. Since this chemical reaction produces the hydronium ions (H3O+) it results in an acidic solution.

Remember, the salt of a strong base and a strong acid will NOT hydrolyze in water because the cation of the base and the anion of the acid do NOT react with water.

ATP is the molecule adenosine triphosphate. As you will remember from biology, the release of energy by ATP is essential to life. Cells use ATP to store energy and release it as needed through hydrolysis. So hydrolysis is related to both energy metabolism and storage. ATP undergoes hydrolysis two ways. 1) ATP removes one phosphate group and forms ADP (adenosine diphosphate) and 2) then removal of another phosphate group results in AMP (adenosine monophosphate) and pyrophosphate which usually breaks down even further. These biosynthesis reactions often occur in chains as the phosphate bonds undergo hydrolysis.

Digestion is the breakdown of food so that our bodies can more easily absorb nutrients. Hydrolysis is the process of adding water to the organic compounds in food. However, because most of the organic compounds do not easily combine with water, there needs to be a catalyst to jump start the process. Catalysts in organic reactions are called enzymes. You will remember from biology that each enzyme is a specific key to unlock its matching process. So lipases match fats, carbohydrases match carbohydrates and proteases match proteins with water in the hydrolysis process.

The specificity of the enzymes is very important in matching the “key” and “lock” so every protease key does not catalyze the hydrolysis of every protein. The positioning and orientation of the protein matter and can even be corrupted. This becomes very important in the activation of hormones in the endrocrine system of the reproductive system. It is also thought to be part of the insulin resistance issues of type-2 diabetics relating to their pancreas.

The breakdown of starch is a fairly well understood type of hydrolysis. Starch is catalyzed by the enzyme amylase. Starch which is a complex carbohydrate, is broken down into smaller molecules of maltose (a simple sugar). Maltose is then broken down into glucose molecules in the presence of the enzyme maltase. At each step of hydrolysis, water molecules break down into ions and add a hydroxyl anion or hydrogen cation to the new molecules formed on each fragment of the broken bond.

Soap is one of the industrial hydrolysis processes which is done in a laboratory. The raw materials do not react with water and require a catalyst. In addition, the process is more efficient when conducted in a high pressure tank and at high temperatures. The catalyst for soap is usually a strong base. (For other laboratory compounds to be hydrolyzed they may require a strong acid.) When fat is hydrolyzed with sodium hydroxide in a reaction with water the process is called saponification. The “soap” that results is actually a fatty acid salt.

Very old oil paintings can develop white deposits and lumps on their surface. As some art works have been cleaned and restored, these deposits have been discovered to be saponification. It is believed that the fatty acids in the old oil paints reacted with the metals in the paint pigments that were used. It is not known why it only happened on some pieces of art and not others.

Weathering is a part of the rock cycle. Don't make the mistake some students make and confuse rock weathering with weather related to climate. This form of weathering is NOT related to the water cycle and the weather outside.

Hydrolysis is an important process in the weathering of rocks and reducing rocks in their form from igneous and metamorphic rocks into sedimentary types. Essentially from large hard rocks into smaller fragments. All rocks are formed of minerals. Many minerals on earth's crust are silicates. Some of these silicates, like the feldspars, and can hydrolyze slowly reacting with water to form new and different minerals along with water soluble compounds. As the new minerals are formed the rocks change into sediments which may be compacted into sedimentary rocks like claystone and siltstone. If they are not formed into rock they become soils which provide essential nutrients to plants.

Dehydration Synthesis (opposite of Hydrolysis)

Another way to think of the opposite of hydrolysis is that you are synthesizing two chemicals together and making a water molecule which is a form of dehydration or water molecule removal from the reacting substance.

By combining the two ingredients (synthesis) you are removing two hydrogen atoms and one oxygen atom which then combine together as one water molecule (H2O). You take a hydroxyl ion (OH-) from one molecule and a hydrogen ion (H+) from the other and synthesize them into a new substance plus water. As always in chemistry it can become much more complicated but this is the underlying concept.

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