CHAPTER 15 ACIDS AND BASES 15 1 Br nsted Acids and Bases Conjugate acid base pair the conjugate base of a Br nsted acid is the species that remains when one proton has been removed from the acid o HCl Cl It is acceptable to represent the proton in aqueous solution as either H or H3O 15 2 The Acid Base Properties of Water Water acts as a proton donor acid in basic solutions and a proton acceptor base in basic solutions An H and OH ion in chemical equilibrium with H2O is sometimes called the autoionization of water Kw the ion product constant which is the product of the molar concentrations of H and OH ions at a particular temperature 15 3 pH A Measure of Acidity The pH of a solution is defined as a negative logarithm of the hydrogen 15 4 Strength of Acids and Bases Strong acids are strong electrolytes that are assumed to ionize completely in water Most strong acids are inorganic acids o Kw 1 0 x 10 14 At 25 C H OH 1 0 x 10 7 o pH 7 In an acidic solution H OH In a basic solution H OH ion concentration in mol L o pH log H o pOH log OH pH pOH 14 00 o HCl o HI o HBr o HNO3 o HClO4 o H2SO4 diprotic Most acids are weak acids which only ionize to a limited extent in water Like strong acids strong bases are strong electrolytes that ionize completely in water All alkali metal hydroxides are soluble o KOH o NaOH o Ba OH 2 is also soluble from the alkaline earth metal group As acid strength increases the conjugate base strength decreases o HClO4 strong acid ClO4 weak base o HF medium acid F medium base o NH3 weak acid NH2 strong base If we know the relative strengths of two acids we can predict the position of equilibrium between one of the acids and the conjugate base of the other 15 5 Weak Acids and Acid Ionization Constants Ka is the acid ionization constant which is the equilibrium constant of an o At a given temperature the strength of the acid HA is measured quantitatively by the magnitude of Ka One way to determine Ka is to measure the pH of the acid solution of known concentration at equilibrium 15 6 Weak Bases and Base Ionization Constants Kb is the base ionization constant which is the equilibrium constant of a acid base 15 7 The Relationship Between the Ionization Constants of Acids and Their Conjugate Bases For any conjugate acid base pair it is always true that Ka Kb Kw 15 8 Diprotic and Polyprotic Acids These acids ionize in a stepwise manner lose one proton at a time o Ionization constants decrease markedly for the second and third stages Ionization of H3PO4 phosphoric acid o H2PO4 Ka 7 5 x 10 3 o HPO4 Ka 6 2 x 10 8 o PO4 2 Ka 4 8 x 10 13 15 9 Molecular Structure and the Strength of Acids Acid strength depends on o Properties of the solvent o Temperature o Molecular structure Acid strength is measured by its tendency to ionize o Which is why HF although being more diverse in terms of electronegativity than HCl HI and HBr is considered a weak acid It s ability to form hydrogen bonds a strong bond serves it greater intermolecular forces thereby reducing its ability to ionize Essentially HF has a greater bond enthalpy than its hydrohalic acid counterparts HF HCl HBr HI o A high degree of polarity characterizes a stronger acid Oxoacids contain hydrogen oxygen and one other element Z which occupies a central position o If Z is highly electronegative or is in a high oxidation state it will attract more electrons making the Z O bond more covalent and the O H bond more polar Consequently the tendency for the hydrogen to be donated increases o As for oxoacids having different central atoms from the same group of the periodic table and that have the same oxidation number acid strength increases with increasing electronegativity HClO3 HBrO3 As for oxoacids having same central atoms but different number of attached groups the acid strength increases as the oxidation number of the central atom increases o Oxidation number of Cl increases HClO4 7 HClO3 5 HClO2 3 HClO 1 o In this case chlorine s ability to draw electrons away from the OH group increases thus making the O H bond more polar Thus HClO4 is the strongest because it is attached to the largest number of O atoms 15 12 Lewis Acids and Bases A Lewis base is a substance that can donate a pair of electrons A Lewis acid is a substance that can accept a pair of electrons When referring to acids and bases the Br nsted definition is more commonly used o The term Lewis acid is typically reserved for substances that can accept a pair of electrons but do not contain ionizable hydrogen atoms CHAPTER 16 ACID BASE EQUILIBRIA AND SOLUBILITY EQUILIBRIA 16 2 The Common Ion Effect According to Le Ch telier s Principle the addition of CH3COO ions from CH3COONa to a solution of CH3COOH will suppress the ionization of CH3COOH thereby decreasing the hydrogen ion concentration The common ion effect a shift in equilibrium caused by the addition of a compound having an ion in common with the dissolved substance 16 3 Buffer Solutions A buffer solution is a solution of o 1 a weak acid or a weak base o 2 its salt Such a solution has the ability to resist changes in pH upon the addition of small amounts of either acid or base o If an acid is added to a solution in the buffer zone of a titration curve the H ions will be consumed by the conjugate base in the buffer Buffer capacity depends on amount o The greater amount of acid and conjugate base in the buffer the To prepare a buffer solution with a specific pH we choose an acid whose greater capacity pKa is closed to the desired pH 16 4 Acid Base Titrations Strong acid strong base o Equivalence point at pH 7 Pages 733 736 in the textbook provide three sample calculations for measuring pH at different points of the titration There are four stages overall and I suggest you memorize the steps on pages 737 739 Weak acid strong base o Equivalence point at pH 7 o OH ions are in excess o Steps for measuring pH on pages 737 739 in the textbook Strong acid weak base o Equivalence point at pH 7 o H ions are in excess o Steps for measuring pH on pages 737 739 in the textbook 16 6 Solubility Equilibria Ksp is known as the solubility product o The product of molar concentrations of the constituent ions each raised to the power of its stoichiometric coefficient in the equilibrium equation o The smaller the Ksp the less soluble the compound is in water o Two ways to express a substance s solubility of moles of solute in …
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