CHEM 1312H 1st Edition Lecture 26 16 1 ACIDS AND BASES A BRIEF REVIEW acid Arrhenius substances that increase the concentration of H ions in water base Arrhenius substances that increase the concentration OH ions in water Arrhenius definition is rather limited because it is restricted to aqueous solutions 16 2 BR NSTED LOWRY ACIDS AND BASES acid a substance molecule or ion that donates a proton to another substance o a substance can only function as an acid IF something else acts as a base simultaneously base a substance that accepts a proton o a molecule or ion must have a nonbonding pair of electrons it can use to accept the H and bind to it amphiprotic a substance capable of acting as an acid or base H 2O is an ex o an amphiprotic substance is a base when combined with something more acidic than itself and acts as an acid when combined with something more strongly acidic than itself conjugate acid base pair an acid or base such as HA and A that differ only in the presence or absence of a proton o every acid has a conjugate base formed by removing a proton from the acid for instance OH is H2O s conjugate base and A is HA s conjugate base o every base has a conjugate acid formed by adding a proton to the base H3O is H2O s conjugate acid and HA is the conjugate acid of A RELATIVE STRENGTH OF ACIDS AND BASES the more easily a substance gives up a proton the less easily its conjugate base accepts a proton the more easily a base accepts a proton the harder it is for the conjugate acid to give it up the stronger an acid the weaker its conjugate base the stronger a base the weaker its conjugate acid strong acids completely transfers its protons to water dissociates pretty much completely weak acids only partially dissociates exists in the solution as a mix of the undissociated acid and its conjugate base 16 3 THE AUTOIONIZATION OF WATER when one water molecule donates a proton to another molecule so H2O H2O H3O OH no water remains ionized for very long because the forward and reverse reactions are very rapid Kw H3O OH 1 0 x 10 14 These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute 16 4 THE pH SCALE pH log H pOH log OH pH pOH 14 00 16 6 WEAK ACIDS most acidic substances are only partially ionized Kais the dissociation constant the magnitude indicates the tendency of acid to ionize in water The larger the value the stronger the acid Weak acids tend to have k values ranging from 10 2 to 10 10 Ka H3O A HA as the concentration of a weak acid increases the equilibrium concentration of H aq increases as expected o however the percent ionization decreases as concentration increases also so doubling the concentration of a weak acid does not double the concentration of H aq POLYPROTIC ACIDS acids that have more than one ionizable H atom ex H2SO3 H2SO3 H HSO3Ka1 1 7E 2 2HSO3 H SO3 Ka2 6 4E 8 Ka2 is much smaller than Ka1 because of electrostatic attractions positively charged protons can be lost more easily from a neutral H 2SO3 than a negatively charged HSO3 AKA IT S EASIER TO REMOVE THE FIRST PROTON FROM AN ACID THAN IT IS TO REMOVE THE SECOND 16 7 WEAK BASES Kb BH OH B Where Kb refers to the equilibrium in which a base reacts with water to form the corresponding conjugate acid and OH2 Types of weak bases 1 neutral substances that have an atom with a nonbonding pair of electrons that can accept a proton a lot of these contain a nitrogen atom includes ammonia and amines 2 anions of weak acids 16 8 RELATIONSHIP BETWEEN KaAND Kb For any conjugate acid base pair Ka Kb Kw Similarly pKa pKb pKw 14 00 where pKa logKa 16 9 ACID BASE PROPERTIES OF SALT SOLUTIONS because salts are good electrolytes any salt dissolved in water is basically completely dissociated acid base properties of salt solutions arise from cations and anions interacting with each other hydrolysis when ions react with water to generate hydronium or hydroxide ions 16 10 ACID BASE BEHAVIOR AND CHEMICAL STRUCTURE TRENDS a molecule with H will act as a proton donor only if the H A bond is polarized so that the H atom has a partial positive charge o nonpolar H A bonds like H C in CH4 produce neutral aq solutions o if the bond is polarized like it is in NaH when the H has a negative charge and behaves like a proton acceptor it s basic o bond strength also plays a role bond strength increases as you move up the HI HBrHCl HF group o lastly the more stable the conjugate base is the stronger the acid for BINARY ACIDS in which A represents members of the same group in the periodic table o H A bond strength decreases as element A increases in size aka down a period bond strength decreases and acidity increases o acidity increases as the electronegativity of A increases so increases as you move left to right across a period o for more trends see table below from pg 706 of Chemistry the Central Science 13 th edition Brown LeMayBursten Murphy Woodward Stoltzfus for OXYACIDS in which OH groups and possibly additional oxygen atoms are bound to a central atom the strength of an acid increases as additional electronegative atoms bond to the central atom for CARBOXYLIC ACIDS that are often written as COOH benzoic acid formic acid etc o the additional oxygen atom attached to the carbon of the carboxyl group draws electron density from the O H bond this increases its polarity also helps stabilize the conjugate base 16 11 LEWIS ACIDS AND BASES Recall a Lewis acid is an electron pair acceptor and a Lewis base is an electron pair donor