7-ionic equilibria.pdf

Physical Pharmacy Ionic equilibria Dr.Khalid T Maaroof PhD Pharmaceutical sciences Ionic equilibria • • • • Why study ionic equilibria? Acid base theories Acid-base equilibria Calculation of pH, acidity constants Theories of acids and bases   Theory I: Arrhenius defined an acid as a substa...

Physical Pharmacy Ionic equilibria Dr.Khalid T Maaroof PhD Pharmaceutical sciences Ionic equilibria • • • • Why study ionic equilibria? Acid base theories Acid-base equilibria Calculation of pH, acidity constants Theories of acids and bases   Theory I: Arrhenius defined an acid as a substance that liberates hydrogen ions and a base as a substance that supplies hydroxyl ions on dissociation in aqueous media. Theory 2: According to the Bronsted-Lowry theory, an acid is a substance, charged or uncharged, that is capable of donating a proton, and a base is a substance, charged or uncharged, that is capable of accepting a proton from an acid. • • • The strength of an acid or a base varies with the solvent. HCI is a strong acid but it is a weak acid in glacial acetic acid. Acetic acid, which is a weak acid, is a strong acid in liquid ammonia.     Theory 3: Lewis Electronic Theory. According this theory, an acid is a molecule or an ion that accepts an electron pair to form a covalent bond. A base is a substance that provides the pair of unshared electrons by which the base coordinates with an acid. Lewis acid: e.g. boron trifluoride and aluminum chloride, Lewis base: e.g. amines, ethers, and carboxylic acid anhydrides, are classified as bases according to the Lewis definition. Solvent classification  A protophilic or basic solvent is one that is capable of accepting protons from the solute. Eg. acetone, ether, and liquid ammonia.  A protogenic solvent is a proton-donating compound and is represented by acids such as formic acid, acetic acid, sulfuric acid, liquid HCI, and liquid HF.  Amphiprotic solvents act as both proton acceptors and proton donors, and this class includes water and alcohols.  Aprotic solvents, such as the hydrocarbons, neither accept nor donate protons, and, being neutral in this sense, they are useful for studying the reactions of acids and bases free of solvent effects. Protolytic reactions (protolysis) based on Bronsted-Lowry theory Sorensen's pH    H ion concentration of a solution varies from approximately 1 in a 1 M solution of a strong acid to about 1 x 10-14 in a 1 M solution of a strong base The pH of a solution can be considered in terms of a numeric scale having values from 0 to 14, which expresses in a quantitative way the degree of acidity (7 to 0) and alkalinity (7-14). The value 7 at which the hydrogen and hydroxyl ion concentrations are about equal at room temperature is referred to as the neutral point, or neutrality. The neutral pH at 0°C is 7.47, and at 100°C it is 6.15. Ionic equilibria Kw. known as the autoprotolysis constant, or the ion product of water Polyprotic electrolytes      Monoprotic/monobasic Polyprotic/polybasic Diprotic/dibasic Triprotic/tribasic… etc In any polyprotic/polybasic electrolyte, the primary protolysis is greatest, and succeeding stages become less complete at any given acid concentration. Ampholyte? Zwitter ion & ampholyte  Isoelectric point IEP  It is pH at which zwitterion concentration at maximum It has been used for determination of protein and amino acids. The molecules have------------solubility at IEP.    The net charge of a molecule at its IEP is-------. Charge of a molecule at pH above its IEP is--------- and below is---------. Measurement of pH  Strong acid: concentration of H is equal to initial concentration of acid.  Only a weak acid: example 7-12 7-13  Only a weak base: example 7-14 Solutions Containing a Single Conjugate Acid-Base Pair: example 7-16and17   Solutions Containing Two Weak Acids: E7-21 Solutions containing salt of a weak acid and a weak base: E2-22  Solutions Containing a Weak Acid and a Weak Base: E7-24  Questions !