Introduction
Chemical reactions mostly take place in solutions. Solution chemistry plays a very significant role in chemistry. All chemical substances are made up of either polar units (called ions) or non-polar units. The activity of these entities is more evident and pronounced in solution. The behaviour of these substances depends upon their nature and conditions of the medium in which they are added. It is therefore necessary to understand the principles that govern their behaviour in solution.
Electrolytes
All ionic substances and highly polar non-ionic compounds, which when dissolved in water give ions, are electrolytes. These electrolytes can be salts, acids and bases.
Acid Base Equilibrium-Arrhenius Concept of Acids and Bases
Arrhenius Concept of Acids and Bases
Arrhenius in 1887 put forward this concept. Accordingly, an acid is a hydrogen-containing compound, which gives free hydrogen ions when dissolved in water.
A base is a hydroxyl group containing compound which gives free hydroxyl ions (OH-) when dissolved in water.
Bronsted - Lowry Concept of Acids and Bases
In 1923, J.N.Bronsted and T.M.Lowry proposed a more general theory known as the Bronsted-Lowry proton transfer theory. According to this concept, any hydrogen containing species (a molecule, a cation or an anion), which is capable of donating one or more protons to any other substance, is called an acid.
Lewis Concept of Acids and Bases
In 1938 G.N. Lewis put forward a more generalized acid-base concept.
According to this concept, an acid is a species (charged or uncharged), which can accept a pair of electrons and a base is a species (charged and uncharged), which can donate a pair of electrons towards the formation of a covalent bond. Thus, a Lewis acid is an electrophile, and a Lewis base is a nucleophile. For example, in the reaction
:NH3 is a donor of electron pair and BF3 accepts a pair of electrons. So, according to the Lewis concept, :NH3 is a Lewis base, and BF3 is a Lewis acid.
Ionic Equilibrium
This type of equilibrium is observed in substances that undergo ionization easily, or in polar substances in which ionization can be induced. Ionic and polar substances are more easily soluble in polar solvents because of the ease of ionization taking place in the solvent medium. With the dissolution of ionic and polar substances in the solvent, these solutions become rich in mobile charge carriers (ions) and thus can conduct electricity. Substances, which are capable of conducting electricity are called as electrolytes while those substances which are non-conducting are called as non-electrolytes.
Ionization of Water
Pure water being a weak electrolyte under goes self ionization to a small extent as follows:
The equilibrium constant for this reaction is:
Hydrogen Ion Concentration
In 1909, Sorensen introduced a term for expressing the concentration of hydrogen ions, which give an idea about the acidic and basic characters of the aqueous solution. This term was called 'pH' which means the 'power of hydrogen ions'. The pH is defined as "the negative logarithm of the H3O+ ion concentration in moles per litre".
Buffers
Certain solutions, such as that of ammonium acetate, have a tendency to resist any change in its hydronium ion concentration or pH, whenever a small amount of a strong acid or a strong base is added to it. This property of a solution is known as buffer action.
Hydrolysis of Salts
Dissolution of different salts in water does not always result in neutral solutions. For example, aqueous solution of copper sulphate is acidic whereas aqueous solution of sodium acetate is basic and aqueous solution of sodium chloride forms neutral solution. This is due to the dissociation of the salt in water to form ions. This process of the reaction of anion or cation of the salt with water to produce an acidic or an alkaline solution is called hydrolysis. Thus, hydrolysis is reverse of neutralization.
Salt of a Strong Acid and Weak Base
Here, cation B+ undergoes hydrolysis to give free H+ ions. Therefore the resulting solution will be acidic in character having pH less than 7. For example,
Salt of Weak Acid and a Weak Base
In this case both the cation and anion undergo hydrolysis to the same or different extents. The resulting solution may be neutral, acidic or basic depending upon the relative strengths of acids and bases. The hydrolysis may be written as:
Salt of Strong Acid and Strong Base
Here, neither the cation nor the anion undergoes hydrolysis. Therefore, the resulting solution is neutral. For example, aqueous solution of KCl is neutral because when dissolved in water, its ions K+ and Cl- have no tendency to react with the H+ and OH- ions of water. The products of such a reaction are KOH and HCl, which are themselves completely dissociated. As a result, there is no change in the concentration of H+ and OH- ions and hence the solution continues to remain neutral.
Acid Base Titrations and Indicators
Titration is an operation for the measurement of quantities of substances in solution by the method of 'volume analysis'. This process involves adding a solution of the reagent of known concentration (standard solution), taken in a burette (called 'titrant'), to an unknown solution (called analyte), taken in a flask known as titration flask. Titration is continued until the stoichiometric amounts of the two reagents (titrant and analyte) have been mixed. For example a titration of a strong acid such as HCl and a strong base such as NaOH involve the following reaction.
Indicators
Indicators are substances, which indicate the completion (equivalence point or end point) of a chemical reaction by change in colour. For example, in volumetric analysis, during the titration of sodium hydroxide and hydrochloric acid (taken in the burette), phenolphthalein turns pink to colourless when the whole of sodium hydroxide has been neutralized by hydrochloric acid. All indicators show change in colour over some pH range, which varies considerably from one indicator to another.
Universal Indicators
A mixture of several indicators with a wide range of pH is an universal indicator. These are obtained by mixing certain indicators that behave like a single indicator. The changes in colour may occur over a considerable portion of the pH range. These can be used for approximate pH of a solution. For example, a mixture of suitable amounts of alcoholic solutions of phenolphthalein, methyl red, methyl yellow, bromothymol blue and thymol blue in sodium hydroxide solution is yellow in colour.
Acid Base Titrations
In acid base titrations, a base is added to an acid successively (or vice versa) and the pH of the solution is noted after every addition. The plot of pH against the amount of base (or acid) added is called a titration curve. The shape of the curve depends upon the ionization constants of the acid and base used in the titration.
Solubility Product
Compounds such as AgCl, AgBr, AgI, BaSO4 etc., when placed in a polar solvent such as water, dissolve to a very limited extent to produce ions in the solution. These sparingly soluble ionic substances (salts) establish an equilibrium between the solid phase and the ions in the solution. For example, for an ionic substance AB. the equilibrium is,
Importance of Solubility Product
Solubility product is an important concept that is used in explaining phenomena like solubility and precipitation of compounds in analytical chemistry.
