Chapter 16 Acid-Base Equilibria CHEMISTRY The Central Science 9th Edition Chapter 16 Acid-Base Equilibria Chapter 16

16.1: Acids and Bases: A Brief Review Acids: taste sour cause dyes to change color Bases: taste bitter feel soapy Arrhenius: acids increase [H+], bases increase [OH-] in AQUEOUS solution Arrhenius: acid + base  salt + water Problem: the definition confines us to aqueous solutions Chapter 16

16.2: Brønstead-Lowry Acids and Bases The H+ Ion in Water The H+(aq) ion is a proton with no electrons In water, the H+(aq) form clusters The simplest cluster is H3O+(aq) A hydrated proton Larger clusters are H5O2+ and H9O4+ Generally H+(aq) and H3O+(aq) are used interchangeably Chapter 16

Proton Transfer Reactions Brønsted-Lowry theory: Acid donates H+ and base accepts H+ Brønsted-Lowry base does not need to contain OH- HCl(aq) + H2O(l)  H3O+(aq) + Cl-(aq) HCl donates a proton to water (HCl is an acid) H2O accepts a proton from HCl (H2O is a base) Water can behave as either an acid or a base Amphoteric substances can behave as acids and bases With a stronger base, it is an acid (and vice versa) Chapter 16

Conjugate Acid-Base Pairs Whatever is left of the acid after the proton is donated is called its conjugate base Whatever remains of the base after it accepts a proton is called a conjugate acid After HA (acid) loses its proton it is converted into A- (base) HA and A- are conjugate acid-base pairs After H2O (base) gains a proton it is converted into H3O+ (acid) H2O and H3O+ are conjugate acid-base pairs Conjugate acid-base pairs differ by only one proton Chapter 16

Relative Strengths of Acids and Bases The stronger the acid, the weaker the conjugate base Chapter 16 Text, P. 618

Generalizations about Acid and Base Behavior Strong acids completely transfer their protons to water The conjugate base of a strong acid has negligible acid-base properties The conjugate acid of a strong base has negligible acid-base properties Weak acids partly dissociate in aqueous solution The conjugate bases of weak acids are weak bases Substances with negligible acidity (CH4) contain H but don’t behave as acids in water Their conjugate bases are strong bases (form (OH)- in water) Chapter 16

Proton Transfer Reactions Strong acids and bases react with water to liberate H+(aq) and (OH)-(aq) ions The leveling effect of water: one can’t distinguish among the strengths of the strong acids or strong bases Place HCl, HBr or HI in dilute acetic acid and there is a noticeable difference in their strengths H+ is the strongest acid that can exist in equilibrium in aqueous solution OH- is the strongest base that can exist in equilibrium in aqueous solution Chapter 16

Use chart ( P. 618) to determine which base is stronger (CO3-2) Proton Transfer Reactions In acid-base reactions, the equilibrium favors transfer of the proton from the stronger acid to the stronger base Strong acid in water: Equilibrium favors the right side HSO4-(aq) + CO3-2(aq) SO4-2(aq) + HCO3-(aq) Compare bases: CO3-2 is from HCO3- SO4-2 is from HSO4- Use chart ( P. 618) to determine which base is stronger (CO3-2) The stronger base and acid are consumed Proton is transferred to the stronger base Production of the weaker electrolytes is favored Chapter 16

The reaction favors the consumption of the stronger acid and stronger base and the formation of the weaker acid and weaker base (the weaker electrolytes) Chapter 16

Use chart ( P. 618) to determine which base is stronger (C2H3O2-) Proton Transfer Reactions Weak acid in water: Equilibrium favors the left side The anion of the acid is a stronger base, so it accepts a proton from the H3O+ HC2H3O2(aq) + H2O(l) H3O+(aq) + C2H3O2-(aq) Use chart ( P. 618) to determine which base is stronger (C2H3O2-) The stronger base and acid are consumed Proton is transferred to the stronger base Chapter 16

Complete Sample Problems: # 5, 7, 9, 11, 15 The reaction favors the consumption of the stronger acid and stronger base and the formation of the weaker acid and weaker base (the weaker electrolytes) Complete Sample Problems: # 5, 7, 9, 11, 15 Chapter 16

16.3: The Autoionization of Water In pure water the following equilibrium is established: at 25 C The above represents the autoionization of water The ion-product constant of water Chapter 16

16.4: The pH Scale In most solutions [H+(aq)] is quite small We define In neutral water at 25 C, pH = pOH = 7.00 In acidic solutions, [H+] > 1.0  10-7, so pH < 7.00 In basic solutions, [H+] < 1.0  10-7, so pH > 7.00 The higher the pH, the lower the pOH, the more basic the solution (range for pH is 0-14) Chapter 16

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In general for a number X, Other “p” Scales In general for a number X, For example, pKw = -log Kw pH, logs and Sig Figs: The number of places after the decimal of a pH, pOH, etc. is the same as the number of sig figs in the concentration you are taking the log of Chapter 16

Most accurate method to measure pH is to use a pH meter Measuring pH Most accurate method to measure pH is to use a pH meter Indicators change color as pH changes Indicators are less precise than pH meters Many indicators do not have a sharp color change as a function of pH Chapter 16

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