Ionic Equilibria Past Paper PDF 2024-2025 (Physical Pharmacy I, 2nd Stage)

University of Basrah college of pharmacy - 24 ،‫ كانون اﻷول‬01 Department of Pharmaceutics Physical pharmacy I 2nd stage Ionic equilibria...

University of Basrah college of pharmacy - 24 ،‫ كانون اﻷول‬01 Department of Pharmaceutics Physical pharmacy I 2nd stage Ionic equilibria 2024-2025 Outlines  Objectives  Theories  Acid-base equilibria  Calculation of pH, acidity constants  The effect of ionic strength. 2 Ionic equilibria 1 University of Basrah college of pharmacy - 24 ،‫ كانون اﻷول‬01 Department of Pharmaceutics Objectives  Define of acids and bases  Concept of Sörensen's pH scale.  Understanding different terminology such as Ampholytes, Aprotic , etc  Ionization of Polyprotic electrolytes.  pKa and pH calculation of aqueous solutions with different composition 3 Theories  Arrhenius Theory  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. 4 Ionic equilibria 2 University of Basrah college of pharmacy - 24 ،‫ كانون اﻷول‬01 Department of Pharmaceutics Brönsted–Lowry theory  According to the Brönsted–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. 5  The strength of an acid or a base varies with the solvent.  HCl 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.  Consequently, the strength of an acid depends: 1. not only on its ability to give up a proton 2. but also on the ability of the solvent to accept the proton from the acid. This is called the basic strength of the solvent. 6 Ionic equilibria 3 University of Basrah college of pharmacy - 24 ،‫ كانون اﻷول‬01 Department of Pharmaceutics In the Brönsted–Lowry classification, acids and bases may be anions such as HSO4- and CH3COO-, cations such as NH4+ and H3O+, or neutral molecules such as HCl and NH3. Water can act as either an acid or a base and thus is amphiprotic. Solvent classification 1. A protophilic or basic solvent  is one that is capable of accepting protons from the solute. Eg. acetone, ether, and liquid ammonia. 2. A protogenic solvent is a proton-donating compound and is represented by acids such as formic acid, acetic acid, sulfuric acid, liquid HCl, and liquid HF. 3. Amphiprotic solvents act as both proton acceptors and proton donors, and this class includes water and alcohols. 4. 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. 8 Ionic equilibria 4 University of Basrah college of pharmacy - 24 ،‫ كانون اﻷول‬01 Department of Pharmaceutics Proteolytic reactions or protolysis. Acid-base reactions occur when an acid reacts with a base to form a new acid and a new base, called conjugates. So it is the reaction that involve a transfer of a proton, and are known as protolytic reactions or protolysis. Proton transfer reactions are also known as protonation–deprotonation reactions. hydronium ion. 9 Several examples illustrate these types of reactions, as shown in Table 7-1. Ionic equilibria 5 University of Basrah college of pharmacy - 24 ،‫ كانون اﻷول‬01 Department of Pharmaceutics Lewis Electronic Theory.  According to the Lewis 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 eg boron trifluoride and aluminum chloride,  Lewis base eg amines, ethers, and carboxylic acid anhydrides, are classified as bases according to the Lewis definition. 11  The Lewis acid theory is widely used for describing the mechanism of many organic and inorganic reactions.  It is referred to simply as a form of electron sharing rather than as acid–base reactions.  It will be important in solubility and complexation.  The Brønsted–Lowry nomenclature is particularly useful for describing ionic equilibria and is used extensively in this chapter Ionic equilibria 6 University of Basrah college of pharmacy - 24 ،‫ كانون اﻷول‬01 Department of Pharmaceutics Sörensen's pH  H ion concentration of a solution varies from approximately 1 in a 1 M solution of a strong acid to about 1 × 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. 13 Ionic Equilibria  Equilibrium can be deffined as a balance between two opposing forces or actions. TRUE or FALSE?  This statement does not imply cessation of the opposing reactions. Rather, it suggests a dynamic equality between the velocities of the two reactions. Tor F?  Equilibrium is the condition where the standard free energy difference between the two sides of reaction equation (7-4) is zero (ΔG° = 0). T or F ? Ionic equilibria 7 University of Basrah college of pharmacy - 24 ،‫ كانون اﻷول‬01 Department of Pharmaceutics The arrows pointing in the forward and reverse directions indicate that reactions are proceeding to the right and left simultaneously Rate of forward Rate of backward At equilibrium 15 Ka : ionization constant or or the dissociation constant of acetic acid At large conc. Of water (diluent) 16 Ionic equilibria 8 University of Basrah college of pharmacy - 24 ،‫ كانون اﻷول‬01 Department of Pharmaceutics where c is large in comparison with x. The term c – x can be replaced by c without appreciable error, giving the equation EXAMPLE 7-1  In a liter of a 0.1 M solution, acetic acid was found by conductivity analysis to dissociate into 1.32 × 10−3 moles each of hydronium and acetate ions at 25°C. What is the acidity (or dissociation) constant Ka for acetic acid?  H.W Ionic equilibria 9 University of Basrah college of pharmacy - 24 ،‫ كانون اﻷول‬01 Department of Pharmaceutics Relationship Between Ka and Kb A simple relationship exists between the dissociation constant of a weak acid HB and that of its conjugate base B−, or between BH+ and B, when the solvent is amphiprotic. This can be obtained by multiplying the following equations: x Ka= Kw/Kb Kb =Kw/Ka Kw. known as the autoprotolysis constant, or the ion product of water 19 EXAMPLE 7-4 Calculate Ka  Ammonia has Kb = 1.74 × 10−5 at 25°C. Calculate Ka for its conjugate acid, NH4+. We have Ionic equilibria 10 University of Basrah college of pharmacy - 24 ،‫ كانون اﻷول‬01 Department of Pharmaceutics Ionization of Polyprotic Electrolytes.  Acids that donate a single proton and bases that accept a single proton are called monoprotic electrolytes.  A polyprotic (polybasic) acid is one that is capable of donating two or more protons, and a polyprotic base is capable of accepting two or more protons.  A diprotic (dibasic) acid, such as carbonic acid, ionizes in two stages, and a triprotic (tribasic) acid, such as phosphoric acid, ionizes in three stages.  In any polyprotic electrolyte, the primary protolysis is greatest, and succeeding stages become less complete at any given acid concentration. 21 Ionization of Polyprotic Electrolytes. 22 Ionic equilibria 11 University of Basrah college of pharmacy - 24 ،‫ كانون اﻷول‬01 Department of Pharmaceutics Ampholytes Zwitterion A species that can function either as an acid or as a base is called an ampholyte and is said to be amphoteric in nature. Amino acids and proteins are ampholytes of particular interest in pharmacy. glycine hydrochloride is dissolved in water,: react as acid Amphoteric with H2O compound react as base with H2O 23  The amphoteric species +NH3CH2COO− is also called a zwitterion because?  Ans.: it carries both negative and positive charge; the whole molecule is electrically neutral Ionic equilibria 12 University of Basrah college of pharmacy - 24 ،‫ كانون اﻷول‬01 Department of Pharmaceutics 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 ----- ------. e.g. pH of milk 6.6 (casein IEP=4.6) 25 Solutions containing Strong acids  Strong acid concentration of H is equal to initial concentration of acid. Thus, they are considered to ionize fully in aqueous solutions. 26 Ionic equilibria 13 University of Basrah college of pharmacy - 24 ،‫ كانون اﻷول‬01 Department of Pharmaceutics Solutions containing Only a Weak Acid  Conc. of base (Cb) is zero.  [H3O+] is generally much greater than [OH−] In many instances ,Ca is much greater than [H3O+] , and the eq simplifies further to : EXAMPLE 7-12 calculate the pH  Calculate the pH of a 0.01 M solution of salicylic acid, which has Ka = 1.06 × 10−3 at 25°C. (a) Using equation (7-102), (b) Using equation (7-101), we find =0.00326M Compare to the conc of acid, The approximation that Ca >> [H3O+] is not valid. Ionic equilibria 14 University of Basrah college of pharmacy - 24 ،‫ كانون اﻷول‬01 Department of Pharmaceutics EXAMPLE 7-13 Calculate pH  Calculate the pH of a 1 g/100 mL solution of ephedrine sulfate. The molecular weight of the salt is 428.5 g/mol, and Kb for ephedrine base is 2.3 × 10−5.  (a) The ephedrine sulfate, (BH+)2SO4, dissociates completely into two BH+ cations and one SO42− anion. Thus, the concentration of the weak acid (ephedrine cation) is twice the concentration, Cs, of the salt added. Ka= Kw/Kb Solutions Containing Only a Weak Base  Conc. Of acid (Ca) is zero,  and [OH−] is generally much greater than [H3O+]. and if Cb is much greater than [OH−], which is generally true for solutions of weak bases, Ionic equilibria 15 University of Basrah college of pharmacy - 24 ،‫ كانون اﻷول‬01 Department of Pharmaceutics 31 Solutions Containing a Single Conjugate Acid–Base Pair  In a solution composed of a weak acid and a salt (conjugate base) of that acid (e.g., acetic acid and sodium acetate)  or a weak base and a salt (conjugate acid) of that base (e.g., ephedrine and ephedrine hydrochloride),  Ca and Cb are generally much greater than either [H3O+] or [OH−]. Ionic equilibria 16 University of Basrah college of pharmacy - 24 ،‫ كانون اﻷول‬01 Department of Pharmaceutics EXAMPLE 7-17 Calculate pH Ionic equilibria 17 University of Basrah college of pharmacy - 24 ،‫ كانون اﻷول‬01 Department of Pharmaceutics Solutions Containing Two Weak Acids  In systems containing two weak acids, Cb1 and Cb2 are zero,  For all systems of practical importance, Ca1 and Ca2 are much greater than K1 and K2, so the equation simpli es to Ionic equilibria 18 University of Basrah college of pharmacy - 24 ،‫ كانون اﻷول‬01 Department of Pharmaceutics Solutions Containing a Salt of a Weak Acid and a Weak Base  The salt of a weak acid and a weak base, such as ammonium acetate, dissociates almost completely in aqueous solution to yield NH4+ and Ac−, the NH4+ is an acid and can be designated as HB1, and the base Ac− can be designated as B2− in equations.  In most instances, however, Cs >> [H3O+], EXAMPLE 7-22 Ionic equilibria 19 University of Basrah college of pharmacy - 24 ،‫ كانون اﻷول‬01 Department of Pharmaceutics 39 Ionic strength 40 Ionic equilibria 20 University of Basrah college of pharmacy - 24 ،‫ كانون اﻷول‬01 Department of Pharmaceutics Ionic strength  ionic strength---I or μ ---a measure of the total ion concentration in solution----but ions with more charge are counted more due to stronger electrostatic interactions with other ions (I.e., can influence the increase “ionic atmosphere” greater than singly charged ions) 41 For zwitterion molecule For monotropic molecule Kr=Salting in constant=0.32 for amino acid in water 42 Ionic equilibria 21 University of Basrah college of pharmacy - 24 ،‫ كانون اﻷول‬01 Department of Pharmaceutics 0.1 43 44 Ionic equilibria 22 University of Basrah college of pharmacy - 24 ،‫ كانون اﻷول‬01 Department of Pharmaceutics Thanks for your attention 45 Ionic equilibria 23