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Chemical dissociated into ions. For example, HCl, NaCl,

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Chemical reactions mostly take place in solutions. Solution chemistry plays a very significant role in chemistry. Allchemical substances are made up of either polar units (called ions) or non-polar units. The activity of these entities ismore evident and pronounced in solution. The behaviour of these substances depends upon their nature and conditionsof the medium in which they are added. It is therefore necessary to understand the principles that govern theirbehaviour in solution.ElectrolytesAll 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. Some of the characteristic features of these electrolytes are:? They conduct electricity in their molten states or when present in the form of their solutions in any polarsolvent.? The solution of an electrolyte on the whole is electrically neutral.? Electrolytes, when dissolved in a solvent of high dielectric constant such as water form ionic solutionsinvolving the formation of cations and anions.? The conducting power of an electrolytic solution depends on the nature of the electrolyte, its concentration andtemperature.Depending on their conducting power electrolytes can be classified as:Strong electrolyte : These electrolytes give highly conducting solutions when dissolved in water because they arecompletely dissociated into ions. For example, HCl, NaCl, Na 2 SO 4 , KNO 3 , H 2 SO 4 , NH 4 Cl etc.Weak electrolyte : Substances, which give poorly conducting solutions when dissolved in water because they areionized to a very small extent are called as weak electrolytes. For example, NH 3 , CH 3 COOH, H 2 CO 3 , organicbases etc.CH3COOH + H2O —-> CH3COO- + H3O+? Based on certain characteristic observable properties, electrolytes were classified into acids and bases.o Arrhenius Concept of Acids and BasesArrhenius in 1887 put forward this concept. Accordingly, an acid is a hydrogen-containing compound, which gives freehydrogen ions when dissolved in water. A base is a hydroxyl group containing compound which gives free hydroxylions (OH – ) when dissolved in water. Thus, according to the Arrhenius concept, hydrogen chloride, acetic acid, andsulphuric acid, are acids because all these compounds give free H + ions in aqueous solutions.HCl (g) + H2O ——-> H+(aq) + Cl-CH3COOH + H2O —-> CH3COO- + H+(aq)Compounds such as NaOH and NH 4 OH are bases, because these compounds give free OH – ions in aqueous solutions.NaOH + H2O ——–> Na+ (aq) + OH- (aq)NH4OH + H2O ———-> NH4+(aq) + OH-(aq)Thus, according to Arrhenius concept of acids and bases, the neutralization of an acid with a base involves the reactionbetween H +( a q ) and OH -( a q ) i.e., H+ (aq) {from acid} + OH- (aq) {from base} ——–neutralization———-> H2O (l)However, the Arrhenius concept is applicable to the acid-base behaviour only in the aqueous medium. It does notprovide any explanation to the acid-base behaviour in the absence of water. This concept defines acids andbases as compounds-containing hydrogen and hydroxyl group respectively. There are however, manycompounds, which act as acid even when there is no hydrogen in their molecule. Similarly, there are manybases, which do not contain hydroxyl group. For example, CO 2 acts as an acid in its reaction with NaOH;and NH 3 acts as a base although it does not contain OH – group.2NaOH + CO2 ——-> Na2CO3 + H2ONH3 + HCl ———–> NH4ClIn aqueous solutions, hydrogen ion exist as hydrated species which is described by a simple formula H 3 O +H+ (aq) + H2O ——> H3O+ (aq)o Bronsted – Lowry Concept of Acids and BasesIn 1923, J.N.Bronsted and T.M.Lowry proposed a more general theory known as the Bronsted-Lowry proton transfertheory. According to this concept, any hydrogen containing species (a molecule, a cation or an anion), which is capableof donating one or more protons to any other substance, is called an acid. Any species (molecule, cation or anion),which is capable of accepting one or more protons from an acid, is called a base. Thus, according to the Bronsted-Lowry concept, an acid is a proton-donor, and a base is a proton-acceptor.The reaction of an acid with a base involves transfer of a proton from the acid to the base. So, an acid and a base shouldbe present simultaneously in any system. The extent of an acid-base reaction is governed not only by the protondonatingability of the acid, but also by the proton-accepting tendency of the base. Acids and bases classified on thebasis of this concept are termed as Bronsted acids and bases.

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