Pharmaceutical Analytical Chemistry I Lecture 7 PDF

11/22/2024 Pharmaceutical analytical chemistry I Lecture 7 Dr. Reem Youssif Shahin 187 Aqueous solutions An aqueous solution is a solution in which the solvent is water Seawater...

11/22/2024 Pharmaceutical analytical chemistry I Lecture 7 Dr. Reem Youssif Shahin 187 Aqueous solutions An aqueous solution is a solution in which the solvent is water Seawater Rain Cola Juice Vinegar 188 1  11/22/2024 Aqueous Solutions and Solubility How do solids such as salt and sugar dissolve in water? Water dissolve salts by separation of cations and anions and formation of new interactions between water and ions: when sodium chloride is put into water, it separates into Na+ cations and Cl - anions. As a result of the polar nature of the water molecule, the oxygen atom in water is electron- rich, giving it a partial negative charge (Ƃ- ), while The hydrogen atoms, are electron-poor, giving them a partial positive charge (Ƃ+),thus a dipole is formed. As a result, the positively charged sodium ions are strongly attracted to the oxygen side of the water molecule and the negatively charged chloride ions are attracted to the hydrogen side of the water molecule (ion dipole interaction). 189 Hydration shell (hydration sphere) is formed surrounding solute molecules with water molecules. the attraction between the separated ions and the water molecules overcomes the attraction of sodium and chloride ions to each other, and the sodium chloride finally dissolves in the water In summary, water can dissolve ionic substances (universal solvent) by two mechanism ion- dipole interaction and hydrogen bond formation. Sodium Chloride Dissolving in Water :The attraction between water molecules and the ions of sodium chloride causes NaCl to dissolve in the water. 190 2  11/22/2024 In contrast to sodium chloride, sugar is a molecular compound. Most molecular compounds dissolve in water as intact molecules. Sugar dissolves because the attraction between sugar molecules and water molecules overcomes the attraction of sugar molecules to each other A Sugar Solution 191 Electrolyte and Nonelectrolyte Solutions Electrolyte: substance able to conduct electricity as it ionizes greatly in water. Two types of electrolytes are well known strong and weak electrolytes. Item Strong electrolyte Weak electrolyte Ionization Higher Lower Electric conduction Higher Lower Acids HCl, H2SO4, HNO3, HClO4 HCN, H3BO3, CH3COOH, H3PO4, H2CO3 Bases NaOH, KOH Al(OH)3 Non-electrolyte: Substance not ionized in water and no ability to conduct electricity e.g. sugars and glycerol. Note that: All salts are strong electrolyte such as NaCl, KCl, CH3COONa. 192 192 3  11/22/2024 comparison between electrolytes and non electrolytes. Electrical conductivity of aqueous solutions: The circuit will be completed and will allow current to flow only when there are charge carriers (ions) in the solution. (a) A hydrochloric acid solution, which is a strong electrolyte, contains ions that readily conduct the current and give a brightly lit bulb. (b) An acetic acid solution, which is a weak electrolyte, contains only a few ions and does not conduct as much current as a strong electrolyte. The bulb is only weakly lit. (c) A sucrose solution, which is a nonelectrolyte, contains no ions and does not conduct a current. The bulb remains unlit. 193 ` 2-Weak electrolytes: when dissolved in water , it is weakly ionized or ionized to a much less extent, so it is a weak conductor for electricity 1-Strong electrolytes: when.Example: acetic acid: CH3COOH dissolved in water , it is completely ionized as NaCl HCl is assumed to be 100 percent dissociated into ions in solution 3-Non electrolytes: dissolve as molecules in water, so do so it is a good conductor for not produce ions, and do not electricity conduct electricity. Example A solution’s sugar ability to conduct electricity depends on number of ions it contains 194 4  11/22/2024 Representing Aqueous Reaction equations: Molecular and Ionic Equations A molecular a chemical equation showing the complete, neutral formulas for every equation compound in a reaction. A complete a chemical equation showing all of the species as they are actually present in ionic solution: strong electrolytes are therefore equation represented as their component ions. 195 1- a molecular equation: a chemical equation showing the complete, neutral formulas for every compound in a reaction. KOH+HCl→KCl+H2​O 2- A complete ionic equation: a chemical equation showing all of the species as they are actually present in solution: strong electrolytes are therefore represented as their component ions. H+(aq) + Cl -(aq) + K+(aq) + OH-(aq) →H2O(l ) + K+(aq) + Cl -(aq) 196 5  11/22/2024 Acids and bases Definitions of Acids and Bases: I- The Arrhenius Definition: Acid: A substance that produces H+ ions in aqueous solution. Base: A substance that produces OH- ions in aqueous solution. According to the Arrhenius definition, HCl is an acid because it produces H+ ions in solution : HCl(aq)→ H+ (aq) + Cl-(aq) According to the Arrhenius definition, NaOH is a base because it produces OH- ions in solution: NaOH(aq)→ Na+ (aq) + OH-(aq) 197 ✓ Limitations of Arrhenius theory: 1. Acids and bases e.g. HCl, H2SO4, NaOH according to these definitions, include only electrically neutral substances, and do not include ions such as NH4+ (ammonium ion), HSO4- (acid sulphate), and HCO3- (bicarbonate). 2. Limited to aqueous media. 198 6  11/22/2024 II- The Brønsted–Lowry Definition: Acid  H+ + Base ✓ An acid is a substance, either ion or molecule, that can donate a proton (H+) to another substance. Substances which are acids according to this concept are: HCl, HNO3, H2SO4, H3O+, NH4+, HSO4-, HCO3- and H2O. ✓ A base is a substance, also either ion or molecule, that is capable of accepting a proton. The following are typical bases: NH3, OH-, CH3COO-, CN-, HCO3- and H2O. 199 HCl is an acid because, in solution, it donates a proton to water: HCl(aq) + H2O(l )→H3O+ (aq) + Cl-(aq) According NH3 is a base because it accepts a proton from water: to this NH3(aq) + H2O NH+4(aq) + OH-(aq) definition some substances—such as water in the previous two equations—can act as acids or bases (amphoteric). 200 7  11/22/2024 Conjugate Acid–Base Pairs The substance that was the base (NH3) has become the acid (NH4 +) and vice versa. NH4 + and NH3 are called conjugate acid–base pair, two substances related to each other by the transfer of a proton A conjugate acid is : any base to which a proton has been added. A conjugate base is: any acid from which a proton has been removed. Summarizing the Brønsted–Lowry Definition of Acids and Bases: A base accepts a proton and becomes a conjugate acid. An acid donates a proton and becomes a conjugate base. 201 Auto ionization of water and pH Water is amphoteric; it can act as an acid or a base. Even when pure, water acts as an acid and a base with itself, a process called autoionization: H2O + H2O H3O+ + OH- Acid Base (proton (proton donor) acceptor) So water ionization equation is : 2H2O H3O+ + OH- For simplicity, it can be written as: H2O H+ + OH- and the ionization constant can be expressed as: 202 8  11/22/2024 In dilute solutions concentration of water is a constant, and we may combine [H2O] with constant Kc, Thus Kc [H2O] = [H+] [OH-] Kc [H2O] is called ion product of water or water dissociation constant, and is given the symbol Kw. Kw = [H+] [OH-] = 1.0 x 10-14 In pure water, [H+] = [OH-] = x (molar conc.) Kw = [H+] [OH] = X2 = 1.0 x 10-14 x = 1.0 x 10-7 M In neutral solution and in pure water [H+] = [OH-] = 1.0 x 10-7 M at 25oC In an acid medium the [H+] is > 1.0 x 10-7 > [OH-] In an alkaline medium the [OH-] is > 1.0 x 10-7 > [H+] 203 The pH Concept and Scale for Acidity and Basicity. Acidity of a solution is expressed in terms of its hydrogen ion concentration, and not in terms of concentration of the dissolved acid. The pH Concept:Concentration of H+ in a solution may be expressed in terms of the pH scale. In general p Anything = - log Anything pH of a solution is defined as pH = log1/ [H+]= -log [H+] for example In a solution of [H+] = 10-3 M pH = - log [H+] = - log [10-3] = - (-3) = 3 For a neutral solution [H+] = 1 x 10-7 [OH-] = 1 x 10-7 pH = - log [H ]= -log 10-7 = 7 + pOH = -log [OH-]= - log 10-7 = 7 204 therefore, pH + pOH = 14 204 9  11/22/2024 The pH Concept: Solution 205 205 pH meters are used to measure acidity 206 206 10  11/22/2024 Problem on pH Example 1: Calculate the pH of HNO3 whose hydrogen ion concentration is 0.76 M Solution: pH = -log [ 0.76 ] = - ( – 0.119) = 0.119 ~0.12M Example 2: In a NaOH solution [OH-] is 2.93 x 10-4 M. Calculate the pH of the solution. Solution: pOH = -log [ 2.93 x 10-4 ] = 3.54 pH + pOH = 14 pH = 14- 3.54 = 10.46 207 207 Problem on pH If pH = 1.46 and you are asked to calculate [H+]. pH = -log [H+] 1.46 = -log [H+] We must take the antilog So [H+ ] = 10 -1.46 =0.035 Example 3: pH of rainwater collected in a certain region was 4.82. Calculate the H+ ion concentration of the rainwater. Solution: 4.82 = -log [H+] We must take the antilog So [H+] = 10 – 4.82 = 0.000015 = 1.5 x 10-5 208 208 11  11/22/2024 pH scale [H+] 10-1 10-2 10-3 10-4 10-5 10-6 10-7 10-8 10-9 10-10 10-11 10-12 10-13 10-14 pH 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Increasing acidity Increasing basicity Neutral ❖ Many common substances are either acidic or basic, and their degree of acidity or basicity is conveniently expressed in terms of pH. ❖ Amino acids are organic acids that contain NH2 group and acid COOH group. ❖ Neutral amino acids (monoamino and monocarboxylic; e.g glycine, and alanine), Acidic amino acids (monoamino and dicarboxylic; e.g glutamic acid and aspartic acid), and basic amino acids (diamino and monocarboxylic; e.g. ornithine and lysine). 209 209 Strength of Acids and Bases Strong acids : are strong electrolytes that ionize completely in water. Strong acids are inorganic acids: hydrochloric acid (HCl), nitric acid (HNO3), perchloric acid (HClO4), sulfuric acid (H2SO4), hydrobromic acid (HBr) and hydroiodic acid (HI). 210 210 12  11/22/2024 Weak acids: ionize only to a limited extent in water. Examples of weak acids are hydrofluoric acid (HF), acetic acid (CH3COOH), and the ammonium ion (NH4+). HF(aq) + H2O(l) H3O+(aq) + F-(aq) Strong acid as HCl is completely or 100 % ionized Weak acid as Hf is weakly ionized 211 211 Strong bases: are strong electrolytes that ionize completely in water: Hydroxides of alkali metals and certain alkaline earth metals are strong bases. Weak bases, like weak acids, are weak electrolytes that ionize partially in water. Ammonia is a weak base. It ionizes to a very limited extent in water: NH3(aq) + H2O(l) NH4 +(aq) + OH-(aq) 212 212 13  11/22/2024 Dissociation of weak acids Consider a weak monoprotic acid, HA. Its ionization in water is represented by: HA(aq) + H2O(l) H3O+(aq) + A-(aq) or simply HA(aq) + H2O(l) H+(aq) + A-(aq) e.g. acetic acid (CH3COOH): CH3COOH H+ + CH3COO- The equilibrium expression for this ionization is: Ka=[H+][CH3COO-]/[CH3COOH] where Ka, the acid ionization constant (the equilibrium constant for the ionization of an acid). The strength of the acid HA is measured quantitatively by the magnitude of Ka: - The larger Ka, the stronger the acid—that is, the greater the concentration of H+ ions at equilibrium due to its ionization. - Note that: only weak acids have Ka values associated with them. 213 213 Dissociation of weak bases The ionization of weak bases is treated in the same way as the ionization of weak acids: When ammonia dissolves in water: NH3 + H2O NH+4 + OH- So K = [NH+4 ][OH-]/[NH3][H2O] Compared with the total concentration of water, very few water molecules are consumed by this reaction, so we can treat [H2O] as a constant. Thus Kb = K[H2O] = The 214 smaller the value of Kb, the smaller the extent of ionization and the weaker the base. 214 14  11/22/2024 Dissociation of weak acids and bases: Relative strengths of acids and bases can also be indicated by their pKa's. The smaller the value of pKa or pKb the stronger is the acid or base. For example, the pKa's for: HC2H3O2 (acetic acid) pKa = 4.74 HC2H2ClO2 (chloroacetic acid) pKa = 2.85 HC2HCl2O2 (dichloroacetic acid) pKa = 1.30 Order of increasing acidity is therefore, dichloroacetic acid ˃ chloroacetic ˃ acetic 215 215 15