As an example, let us take the case of acetic acid. PARTIAL IONIATION OF WEAK ACIDS AND BASES. • Ionization of a compound refers to a process in which a neutral molecule splits into charged ions when exposed in a solution. The value of dissociation constants of some of the monoprotic acids have been given in Table 25.1. The strong acids generally have weaker conjugate base whereas strong bases have weaker conjugate acids. This is because of the high degree of ionization of strong acids and bases. the expression of Ka then becomes, Knowing the value of Ka it is possible to calculate the degree of ionization of the acid at any particular concentration C. From the degree of ionization, hydronium ion concentration can be calculated as: The relative strengths of bases are also compared m terms of their dissociation or ionization constants. Thus, addition of acetate ions reduces the degree of ionization of acetic acid. The following equation represents the ionization of acetic acid in aqueous solution: Suppose C moles of CH3COOH are dissolved per litre of solution and let a be the degree of ionization of CHFOOH, then at equilibrium the concentration of various species would be as follows: Therefore , ka = (Ca ) (Ca)/ C (l-a) = Ca2 / (l-a), Since for weak acid a is very small as compared to 1, a in the denominator can be neglected. On the basis of the degree of ionization of acids and bases, we can determine the strength of acids and bases. For example. The fraction of the total number of molecules of the acid dissolved that ionizes is known as degree of ionization of the acid. The degree of ionization of the acid decreases if some strong electrolyte that can provide any of the product ions is added to the solution of the acid. According to Arrhenius theory, acids are the compounds that dissociate in an aqueous medium to generate hydrogen ions, \(H^+\) (aq). Larger the value of Ka , higher is the concentration of H30+, and stronger is the acid. The Ionization Constants of Some Bases at 298 K, Base Ionization Constant, Kb, Ammonia, NH3 1.8 x 10-5, Methylamine, CH3NH2 4.6 x 10-4, Dimethyle, (CH3) 2NH 5.4 x 10-4, Trimethylamine, (CH3)3N 6.5 x 10-5, Aniline, C6H5NH2 4.3 x 10-10, Pyridine, C5H5N 1.8 x 10-9. If \(HA\) has a higher tendency to donate proton in comparison to \(H_3O^+\), \(HA\) acts as a strong acid in comparison to \(H_3O^+\). Therefore, the numerical value of K a is a reflection of the strength of the acid. On the other hand, bases are the compounds which furnish hydroxyl ions, \(OH^{-}\)(aq) in an aqueous medium. For example, the ionization equilibrium of an acid HA may be represented as: Since concentration of water is very large and remains almost constant in solution, it can be combined with K to give another constant Ka. These acids are termed as strong acids. Ionization of these bases yields hydroxyl ions, (\(OH^-\)). Your email address will not be published. If you are on a personal connection, like at home, you can run an anti-virus scan on your device to make sure it is not infected with malware. Acids like perchloric acid (HCIO4), hydrochloric acid (HCl), hydrobromic acid (HBr), hydroiodic acid (HI), trioxonitrate(V) acid (HNO3) and tetraoxosulphate(VI) acid H2SO4) are termed strong because they are almost completely dissociated into their constituent ions in an aqueous medium, thereby acting as strong proton (H+) donors. This is because of the high degree of ionization of strong acids and bases. Addition of some strong electrolyte which provides either of the ions (OH- or M+) reduces the degree of ionization of the weak base in accordance with Le-Chatelier’s principle. For example, the degree of ionization of ammonia is suppressed by addition of ammonium chloride or sodium hydroxide. Here Ka is called ionization constant of acid. The degree of ionization differs for different acidic and basic compounds. Also, a strong acid implies good proton donor whereas strong base implies good proton acceptor. The acid ionization represents the fraction of the original acid that has been ionized in solution. Some acids such as perchloric acid (\(HClO_4\)), hydrochloric acid (\(HCl\)) dissociate completely into their constituent ions in aqueous medium. A strong acid is completely ionized while a weak acid is only partially ionized. From the knowledge of Ka it is possible to calculate hydronium ion concentration and degree of ionization of a weak acid. Similarly, the degree of ionization of a weak base is suppressed by common ion effect. The Ionization Constants of Some Common Weak Acids at 298 K, Calculation of [ H3O +] and Degree of Ionization. Weak acids with relatively higher K a values are stronger than acids with relatively lower K a values. The strength of acids or bases is experimentally measured by determining its ionization constants or dissociation constants. Similarly, some bases like lithium hydroxide (\(LiOH\)), sodium hydroxide (\(NaOH\)) too completely dissociate into their ions in an aqueous medium. Your email address will not be published. But weak acids are not completely ionized. If sodium acetate is added to the above solution, the concentration of acetate ions increases and the above equilibrium shifts in backward direction according to Le-Chatelier’s principle. Arrhenius theory plays an important role in explaining the ionization of acids and bases as mostly ionization takes place in an aqueous medium. The ionization constant Kb for a weak base B can be represented as follows: Smaller the value of ionization constant for a base, weaker is the base. These bases are termed as strong bases. PARTIAL IONIATION OF WEAK ACIDS AND BASES Strung acids ionize almost completely into hydrogen ion and the corresponding anion in water and, therefore, the molar concentration of hydrogen ions in the solution is same as that of acid itself. According to the Arrhenius theory, the acids are the compounds that dissociate in the aqueous medium in order to generate the hydrogen ions, H + in the aqueous medium. This reaction implies that acid dissociation equilibrium is dynamic in nature where the transfer of proton can take place in both forward and backward direction. An acid or base's strength refers to its degree of ionization. If dissociation constants of two acids are known, their relative strengths can be predicted. Since KHF > KCH3COOH , therefore, HF is stronger acid than CH3COOH. Strung acids ionize almost completely into hydrogen ion and the corresponding anion in water and, therefore, the molar concentration of hydrogen ions in the solution is same as that of acid itself. The degree of ionization of a weak acid or a weak base is suppressed (reduced) by common ion effect. Ionization of acids yields hydrogen ions, thus these compounds act as proton donors. The equilibrium moves in the direction of formation of a weaker acid and weaker base. For example: dissociation of weak acid HA, \(HA ~(aq)~ + ~H_2O~ (l)~ ⇌~H_3O^+~(aq)~ + ~A^-~ (aq)\). Strong acids and bases have high degree of ionization in comparison to weak acids and bases. Similarly, if H+ ions are provided from some source, such as dilute hydrochloric acid, the degree of ionization of the weak acid is suppressed. For example, consider the ionization of acetic acid. In case of weak acids the ions formed are in equilibrium with the unionized molecules of the acid. Required fields are marked *. Similarly, strong bases like sodium hydroxide (NaOH), potassium hydroxide (KOH), caesium hydroxide (CaOH) and barium hydroxide Ba(OH)2 almost completely dissociate into ions in an aqueous medium giving hydroxyl ions, OH- According to Arrhenius concept they are strong bases as they are able to completely dissociate and produce OH – in the medium. If you are at an office or shared network, you can ask the network administrator to run a scan across the network looking for misconfigured or infected devices. An acid on ionization gives hydrogen ion and the corresponding anion. A strong acid will completely ionize in water while a weak acid will only partially ionize. For detailed discussions on ionization of acids and bases in an aqueous medium, download BYJU’S – The Learning App. For detailed discussions on ionization of acids and bases in an aqueous … Since there are different degrees of ionization, there are different levels of weakness. 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