The Difference in the Chemistry of Hydrolysis & Hydration.
List of Topics Included
(1) What is “Hydrolysis"?
(1/A) Meaning of the term, “S. T. P."
(1/B) Meaning of the term, “Self-ionization of water".
(1/C) Characteristics of hydrolysis.
(1/D) Hydrolysis occurring in inorganic chemistry (salt hydrolysis)
(1/E) Some examples of salt hydrolysis.
(1/F) Hydrolysis occurring in organic chemistry.
(1/G) Some example of hydrolysis of organic compounds
(2) What is "Hydration"?
(2/A) Characteristics of various hydration reactions
(2/B) Some examples of Hydration occurring in inorganic chemistry
(2/C) Some examples of Hydration occurring in organic chemistry
(3) Similarities between hydrolysis and hydration
(4) Differences between hydrolysis and hydration
(5) Application of hydrolysis and hydration
(1) What is "Hydrolysis"?
The chemical reaction of a given compound with water so as to yield two peculiar products is called, “Hydrolysis”.
Both, organic as well as inorganic compounds can react with water to undergo hydrolysis. As, pure water contains both H+ as well as OH- ions, the actual reaction of hydrolysis can be considered as reaction of given compound with H+ and OH- ions of water.
The existence of H+ and OH- ions in pure water is result of a phenomenon called, “self-ionization of water". However, due to reversible nature of self-ionization (also called, "auto-ionization") of water, the concentration of H+ and OH- ions in pure water is extremely poor.
Experimentally, it is found that, at S.T.P. (means at standard temperature and pressure), the concentration of H+ and OH- ions in pure water is equal having value of about, 10-7 mole /liter.
It is due to this reason that the rate of hydrolysis reaction is generally very slow. It is possible to increase the rate of hydrolysis by increasing concentration of H+/OH- ions. This can be generally achieved by adding a small quantity of either strong acid (like sulphuric acid-H2SO4) or strong base (like potassium hydroxide-KOH) in water.
Due to the reason explained above, a small quantity of strong acid or strong base is generally added during the reaction of hydrolysis. This increases the concentration of H+/OH- ions and helps to maintain the rate of reaction within some practical limit.
In short, presence of small quantity of strong acid or strong base can catalyze the reaction of hydrolysis.
(1/A) Meaning of the Term , “S. T. P.”
All chemical reactions are either endothermic or exothermic in nature. Due to this, the extent of reaction and hence quantity of products are affected significantly if change in temperature takes place.
Further, some reactions involve gaseous reactants or products, quantity of which is affected by change in pressure.
It is due to above two reasons that a fixed condition of temperature and pressure needs to be maintained during the course of a chemical reaction so as to obtain comparable results.
As per guidelines issued by I.U.P.A.C. (means International Union of Pure and Applied Chemistry), the value of fixed temperature is taken as 250C (means 298 K) and that of fixed pressure is taken as 1 bar.
This set of fixed condition of temperature and pressure is called, “Standard Temperature and Pressure” (abbreviated as, “S. T. P.”).
[Note: It is interesting to notice that in some of branches of chemistry, the value of standard temperature is taken as 00 C (means 273 K). For example in the branch of gaseous state, standard temperature is taken as 273 K. This means, if you want to calculate numerical based on: Boyle's law or Charles law or Ideal Gas Equation (PV = nRT), you must take the value of standard temperature as 273 K otherwise you will not get the correct answer.
But, if you want to calculate numerical based on laws of Thermodynamics or Electrochemistry, you must take the value of standard temperature as 298 K, to get the correct answer! Such condition of 298 K temperature & 1 bar pressure is also known as Standard Ambient Temperature & Pressure, abbreviated as, "S. A. T. P."]
(1/B) Meaning of the Term, “Self-Ionization of Water”
Self-ionization of water is a chemical reaction in which undissociated water dissociates into its ions: H+ and OH-. It is an endothermic chemical reaction, means more is the temperature more will be the concentration of H+ and OH-ions.
Why it is called self-ionization?
The phenomenon of producing ions is called ionization. For example, crystal of sodium chloride can not conduct electricity, suggesting it does not contain ions. However, when we dissolve it in water, its aqueous solution conducts electricity, suggesting that it contains ions. This means phenomenon of ionization takes place in presence of water.
However, in case of water, it is found that pure water is poor conductor of electricity, suggesting that pure water contains ions. Here ions are produced from water without presence of any foreign material. This means ionization of water occurs at its own, hence is called self-ionization of water.
Equation showing Self-Ionization of Water
(1/C) Characteristics of Hydrolysis
The main characteristic of the chemical reaction of Hydrolysis is that, during it the given compound reacts with water to yield two products. This means, during the phenomenon of hydrolysis, the given compound splits into two new compounds, both of which may either be similar or dissimilar.
Characteristic of hydrolysis occurring in inorganic chemistry:
Consider the hydrolysis of a hypothetical salt BA. The ions B+ and A- produced by its ionization react with H+ and OH- ions of water.
This yields two compounds: acid HA and base BOH.
Characteristic of hydrolysis occurring in organic chemistry:
Consider the hydrolysis of an ester R-COO-R'. This yields two compounds: acid RCOOH and alcohol R'OH.
In short, hydrolysis results in splitting of given compound.
[Note: Exceptionally, hydrolysis of some oxides may yield only one product.
Hydrolysis of quick lime (CaO) gives only one product called slaked lime [Ca(OH)2].
Likewise hydrolysis of phosphorus pentoxide (P4O10) and sulphur dioxide (SO2) also yield only one product called phosphoric acid (H3PO4) and sulphurous acid (H2SO3) respectively.]
(1/D) Hydrolysis in Inorganic Chemistry (Salt Hydrolysis)
In inorganic chemistry, when a salt is dissolved in water, it undergoes hydrolysis. In fact, the phenomenon of salt hydrolysis can be regarded as an opposite phenomenon to that of acid base neutralization.
In acid base neutralization, acid and base react to yield salt and water. Conversely, in salt hydrolysis, salt and water react to produce corresponding acid and base.
During salt hydrolysis the characteristics of acid and base produced makes the resulting solution to be either acidic or basic or neutral.
(1/E) Some Examples of Salt Hydrolysis
(A) Hydrolysis of acidic salt:
Consider the hydrolysis of a salt like ammonium chloride (NH4Cl).
During this two compounds are produced, one of which is ammonium hydroxide (NH4OH) while another is hydrochloric acid (HCl). As, the acid produced is stronger than base, this phenomenon renders the resulting solution to be acidic.
(B) Hydrolysis of basic salt:
Consider the hydrolysis of a salt like sodium acetate (CH3COONa). During this two compounds are produced, one of which is acetic acid (CH3COOH) while another is sodium hydroxide (NaOH). As, the base produced is stronger than acid, this phenomenon renders the resulting solution to be basic.
(C) Hydrolysis of neutral salt:
Consider the hydrolysis of a salt like sodium chloride (NaCl). During this two compounds are produced, one of which is hydrochloric acid (HCl) while another is sodium hydroxide (NaOH). As, both the acid as well as base produced are strong, this phenomenon renders the resulting solution to be neutral.
(D) Hydrolysis of salt formed by neutralization between weak acid and weak base:
Consider the hydrolysis of a salt like ammonium formate (HCOONH4). During this two compounds are produced, one of which is formic acid (HCOOH) while another is ammonium hydroxide (NH4OH). As, both the acid as well as base produced are weak, the nature of resulting solution will depend upon the value of ionization constant of acid (Ka) and that of base (Kb). Here, Ka of HCOOH is 1.77x10-4 (pKa=3.75) while Kb of NH4OH is 1.75x10-5 (pKb=4.75). This values indicate that the acid produced is stronger than base. Hence resultant solution in this case will be acidic in nature.
[Note: The hydrolysis of some inorganic compounds produces two compounds both of which are acids. For example hydrolysis of phosphorus pentachloride (PCl5) yields one molecule of phosphoric acid (H3PO4) and one molecule of hydrochloric acid.]
Equation showing Hydrolysis of Ammonium Chloride Salt
(1/F) Hydrolysis in Organic Chemistry.
In organic chemistry, the compounds like: ester, anhydride, sugar etc. react with water to yield two different compounds.
♣ Esters yield one molecule of carboxylic acid and one molecule of alcohol.
♣ Anhydrides yield two molecules of carboxylic acid, either same or different depending upon characteristic of anhydride.
♣ Complex sugars (means disaccharides and polysaccharides) yield two or more molecules of simple sugars (means monosaccharides).
(1/G) Some Examples of Hydrolysis of Organic Compounds
(A) Hydrolysis of an ester
Consider the hydrolysis of methyl ethanoate having chemical formula: CH3-COO-CH3.
On hydrolysis it produces two compounds: acetic acid, also known as ethanoic acid having formula: CH3COOH and methanol having chemical formula: CH3OH.
[Note: It is important to note that, 10 to 15 % aqueous solution of acetic acid is known as, "Vinegar" which is a famous food additive.]
(B) Hydrolysis of an anhydride
Consider the hydrolysis of acetic anhydride having chemical formula: (CH3CO)2O.
On hydrolysis it produces two molecules of acetic acid.
(C) Hydrolysis of a sugar
Consider the hydrolysis of sucrose having chemical formula: C12H22O11.
Sucrose is a disaccharide. On hydrolysis it yields two molecules of monosaccharides, one of which is glucose having chemical formula: C6H12O6 and another is fructose having chemical formula: C6H12O6.
Likewise, the hydrolysis of a polysaccharide called starch having chemical formula: (C6H10O5)n, yields too many molecules of monosaccharides called glucose having chemical formula: C6H12O6.
[Note: Here, the value of "n" may vary from 200 to 4000 depending upon the source.]
equation showing Hydrolysis of Methyl Ethanoate
(2) What is "Hydration"?
It is an addition reaction in which water adds into substrate molecule to yield only one product.
The product obtained on hydration is generally known as, "Hydrate".
(A) In case of inorganic chemistry:
(1) Hydration may takes place when concentrated aqueous solution of ionic compound is cooled to obtain crystals. During this phenomenon the ionic compound separates out as a crystal along with some water molecules. The number of water molecules entering thus in the crystal of ionic solid is known as, "water of crystallization".
(2) When a salt is dissolved in water, its ions combine and get surrounded with opposite ions of water. This is called, "hydration of ions".
(B) In case of organic chemistry:
Hydration takes place when an unsaturated organic compound reacts with water to yield a saturated compound. In such cases phenomenon of hydration may take place either in acidic or in basic medium. The acid catalyzed hydration of organic compounds proceeds via formation of carbocation intermediate, while the base catalyzed hydration of organic compounds proceeds via formation of carbanion intermediate.
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(2/A) Characteristics of Various Hydration Reactions
(1) Characteristic of hydration occurring during crystal formation.
This type of hydration may give specific shape as well as color to the crystal. However, crystal formed thus can easily loose the water of crystallization when it is heated. On losing water thus the crystal losses its shape and color and get converted into white powder.
(2) Characteristic of hydration occurring during combination of opposite ions with water.
When a salt is dissolved in water, ions of opposite charge are produced.
As the molecules of water are polar in nature, the ions of salt get surrounded by water molecules due to electrostatic attraction between them. Such type of hydration is an exothermic reaction.
In case the heat released thus is greater than crystal lattice energy of the salt, salt
becomes soluble. Thus, solubility of salt depends upon such hydration.
(3) Characteristic of hydration occurring during addition reaction of organic compounds.
Such type of hydration takes place during addition reaction of water with unsaturated organic compounds like alkenes, alkynes etc. In such reaction the weak π bond of unsaturated organic compound is broken and "H & OH" parts of water are added along multiple bond. This yields some important industrial chemicals like alcohols, aldehydes, ketones etc.
Generally such reaction takes place in acidic medium, hence it is called, "acidic hydration". Further, as per the mechanism of this reaction, attack of H+ ion is slowest and rate determining step, hence this reaction is also known as, "electrophilic addition reaction".
However, it must be noted here that the major product obtained during this reaction, is as per "Markovnikov's rule".
(2/B) Some Examples of Hydration Occurring in Inorganic Chemistry
(1) Consider aqueous solution of copper sulphate (CuSO4) which is blue in color. If we want to obtain crystals of copper sulphate from it, we must concentrate it. On heating the solution, it loses water by evaporation and becomes saturated.
Now, when we allow it to cool, blue crystals of copper sulphate separate out having formula, "CuSO4. 5 H2O". Due to presence of water, crystalline copper sulphate obtained thus is also known as, "hydrated copper sulphate".
Now, if we heat these crystals of copper sulphate again, it looses the water of crystallisation and converted into white powder having formula, "CuSO4". Due to absence of water it is also known as, "anhydrous copper sulphate".
(2) Consider the phenomenon of dissolution of solid sodium chloride (NaCl) in water. On dissolution, it dissociates into Na+ and Cl- ions.
Due to electrostatic attraction, the positively charged Na+ ions remain surrounded with six water molecules, such that partial negatively charged oxygen atom of water remains closer to positively charged Na+ ion.
Likewise, the negatively charged Cl- ions also remain surrounded with six water molecules such that partly positive hydrogen of water remains closer to Cl- ion.
This phenomenon is called, "hydration of ions in aqueous solution".
(2/C) Some Examples of Hydration Occurring in Organic Chemistry
(1) Hydration of ethene in presence of acid:
This is an important industrial reaction because it yields very important compound, "Ethyl alcohol (C2H5OH)" or Ethanol.
The chemical equation of this reaction is given below.
(2) Hydration of ethyne (C2H2):
It yields an important industrial compound, "Ethanal (CH3CHO)" which is also known as acetaldehyde.
(3) Hydration of propyne (C3H4):
It yields an important industrial compound, "Propanone (CH3-CO-CH3)" which is also known as acetone.
(4) Hydration of "Pent-2-yne (C5H8):
It yields a mixture of two products, "Pentan-2-one" and "Pentan-3-one".
It is interesting to notice that the relative proportion of methyl ketone, "Pentan-2-one" predominates. (This is because the intermediate carbocation produced in first case is stabilized by the phenomenon of "Hyperconjugation".)
Equation showing Hydration of Ethene
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(3) Similarities Between Hydrolysis and Hydration
(1) Both of the phenomena involve chemical reaction of substrate with water.
(2) Both of the phenomena are observed in organic as well as in inorganic reactions.
(3) Both of the phenomena are useful to produce some important industrial chemicals.
(4) Both of the phenomena are catalyzed by acid.
(4) Differences Between Hydrolysis and Hydration
(1) Former is substitution reaction while later is addition reaction.
(2) Former phenomenon generally yields two products while later yields only one product.
(3) Former phenomenon is catalyzed by both acid as well as base, while later is generally catalyzed by acid.
(5) Application of Hydrolysis and Hydration
(1) Important industrial chemicals like: ethanol, ethanal, acetone, glucose, fructose etc. can be produced.
(2) Aqueous solution can be made acidic or basic without adding acid or base into it.
(3) Presence of water or moisture can be detected. (White anhydrous copper sulphate turns to blue when it comes in contact with water).
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