I Chapter 16 Acid Base Equilibria A Arrhenius concept of acids and bases 16 1 Acids and Bases A Brief Review o An acid is a substance that when dissolved in water increases the concentration of H ions o A base is a substance that when dissolved in water increase the concentration of O H ions 16 2 Br nsted Lowry Acids and Bases B Br nsted and Lowry proposed a more general definition of acids and bases acid base reactions involve the transfer of H ions from one substance to another 1 The H ion in Water Hydronium ion H 3 O One of the many substances formed when water interacts with a proton H 3 O aq 2 and H aq are interchangeable but the first more closely represent reality Proton Transfer Reactions Br nsted Lowry acid donates H in any solution Br nsted Lowry base accepts an H in any solution Amphiprotic the quality of a substance that can act as both an acid and a base 3 Conjugate Acid Base Pairs Conjugate acid base pair An acid and base that differ only in the presence or absence of a proton Conjugate base a compound formed by removing a proton from an acid Conjugate acid a compound formed by the addition of proton from a base 4 Relative Strengths of Acids and Bases As the strength of an acid increase the strength of its conjugate base decreases A strong acid completely transfers its protons to water The conjugate base of a strong acid shows negligible basicity A weak acid only partially dissociates in water The conjugate base of a weak acid is a weak base A substance with negligible acidity contains hydrogen but does not demonstrate any acidic behavior in water The conjugate base of a substance with negligible acidity is a strong base The leveling effect stronger acids react with water to produce H ions and stronger bases react with water to produce OH ions In every acid base reaction equilibrium favors transfer of the proton from the stronger acid to the stronger base to form the weaker acid and the weaker base 16 3 The Autoionization of Water C Autoionization When a molecule of a substance can donate a proton to a molecule of the same substance 1 The Ion Product of Water Ion product constant K w the equilibrium constant for the autoionization of water K w 16 4 The pH Scale D pH log pH decreases as H increases pOH log pH pOH 14 at 25 Measuring pH 1 16 5 Strong Acids and Bases E Strong acids and bases are strong electrolytes existing entirely as ions in aqueous solutions Strong Acids 1 2 Strong Bases 16 6 Weak Acids F Acid dissociation constant K a the equilibrium constant for the ionization of an acid As the strength of an acid increase its K a value increases 1 Page 668 2 Calculating K a from pH Percent Ionization percent ionization concentrationionized original concentration The percent ionization for an acid HA is 100 3 Using K a to calculate pH 1 Write the ionization equilibrium 2 Write the equilibrium constant expression and the value for the equilibrium constant 3 ICE table that shit 4 Substitute the equilibrium concentration into the equilibrium constant expression and solve for x 5 Use the concentration of H to find pH by log H You did it I believed in you The percent ionization decreases as the concentration increases 4 AY YO Polyprotic Acids in the hiz house Polyprotic acids when an acid has more than one ionizable H atom It s always easier to remove the first proton from a polyprotic acid than to remove the second G 16 7 Weak Bases B aq H 2 O l H B aq O H aq K b The base dissociation constant K b always refers to the equilibrium in which a base reaction with H 2O to form the corresponding conjugate acid and O H 1 Type of Weak Bases First type a neutral substance that has an atom with a nonbonding par of electron that accept a proton These substances contain ammonia and a related class of compounds known as the amines In organic amines at least one N H bond in NH3 is replace by an N C bond Like NH3 amines can abstract a proton from a water molecule by forming an N H bond Second type anions of weak acids H 16 8 Relationship between K a and K b K a K b K w 10 14 I 16 9 Acid Base Properties of Salt Solutions 1 An anion s ability to react with water X aq H 2O l HX aq OH aq If Ka Kb the ion cause the solution to be acidic If Kb Ka the solution is made basic by the ion 2 A cation s ability to react with water Polyatomic cations containing one or more protons can be considered the conjugate acids of weak bases 3 The Combined Effect of Cation and Anion in Solution To determine whether a salt forms an acidic a basic or a neutral solution when dissolved in water we must consider the action of both the cation and anion There are four possible combos o 1 If the salt contains an anion that does not react with water and a cation that does not react with water we expect the pH to be neutral o 2 If the salt contains an anion that reacts with water to produce hydroxide ions and a cation that does not react with water we expect the pH of the solutions to be basic o 3 If the salt contains a cation that reacts with water to produce hydronium ions and an anion that does not react with water we expect the pH of the solution to be acidic o 4 If the salt contains an anion and a cation both capable of reacting with water both hydroxide and hydronium ions are produced Whether the solution is basic neutral or acidic depends on the relative abilities of the ions to react with water 16 10 Acid Base Behavior and Chemical Structure 1 Factors that affect Acid Strength J 1 The polarity of a molecule containing hydrogen o H will act as a proton donor an acid if the H X bond is polarized in this way H X 2 The strength of the bond between H and X o As bond strength increases strength of acid decreases 3 The stability of the conjugate base X o As stability of the conjugate base increases strength of acid increases As element x increases in size the strength of an H X bond tends to decrease As the electronegativity of X increases the acidity increases 2 3 Binary Acids Oxyacids Oxyacids Acids in which Oh groups and possibly additional oxygen atoms are bound to a central Generally as the electronegativity of the central atom increase the acidity of the substance The strength of an acid increases as additional electronegative atoms bond the central atom In a series of oxyacids the acidity increases as the oxidation number of the central atom Carboxylic …