Ch 18 Principles of Chemical Reactivity Other Aspects of Aqueous Equilibria Review of strong electrolytes Electrolytes are cpds that ionize or dissociate into their constituent ions to produce aqueous solutions that conduct an electric current Strong electrolytes are ionized or dissociated completely or almost completely in dilute aqueous solutions Strong electrolytes include strong acids and strong bases and most soluble salts 18 1 The Common Ion Effect Common ion effect when a solution of a weak electrolyte is altered by adding one of its ions from another source the ionization of the weak electrolyte is suppressed The common ion effect is a term used to describe the effect on a solution of two dissolved solutes that contain the same ion CH3COOH H CH3COO add NaCH3COO what happens to eq shift in eq position is known as common ion effect Examples Weak acid Weak base Conjugate acid base pairs H2PO4 H HPO42 buffer in internal cells H2CO3 H HCO3 blood plasma buffer NH3 H2O NH4 OHCH3COOH CH3COO H Calculate the concentration of H and the pH of a solution that is 0 15 M in acetic acid and 0 15 M in sodium acetate What is ionization 5 This is a comparison of the acidity of a pure acetic acid solution and the buffer described in the last example 6 Compare the acidity of a pure acetic acid solution and the buffer described in the example Solution 0 15 M CH3COOH H 1 6 x 10 3 pH 2 80 0 15 M CH3COOH 0 15 M NaCH3COO buffer 1 8 x 10 5 4 74 H is 89 times greater in pure acetic acid than in buffer solution 7 What is pH of the solution that results from adding 30 0 ml of 0 015 M KOH to 50 0 ml of 0 015 M benzoic acid Ka 6 3 x 10 5 18 2 Controlling pH Buffer Solutions Buffer solutions resist a change in pH upon addition of small amounts of acid or base A buffer mixture contains an aq solution of an acidbase conjugate pair prepared by mixing a weak acid or base with a salt of that acid or base The acidic component reacts with added strong bases and the basic component reacts with added strong acids Buffers are made from a weak acid or base and its conjugate base or acid There are two common kinds of buffer solutions 1 Solutions made from a weak acid plus a soluble ionic salt of the weak acid 2 Solutions made from a weak base plus a soluble ionic salt of the weak base 10 1 Solutions made of weak acids plus a soluble ionic salt of the weak acid One example of this type of buffer system is The weak acid acetic acid CH3COOH The soluble ionic salt sodium acetate NaCH3COO The weak acid reacts with bases CH 3COOH CH 3COO H 100 Na CH 3COO Na CH 3COO The salt anion a base reacts with acids 11 General Expressions for Buffer Solutions The general expression for the ionization of a weak monoprotic acid is H A HA The generalized ionization constant expression for a weak acid is Ka H A HA If we solve the expression for H this relationship results useful expression remember it or derive it HA H K A a acid salt Can use if conc is 0 050 M 12 The relationship developed is valid for buffers containing a weak monoprotic acid and a soluble ionic salt If the salt s cation is not univalent the relationship changes to acid H K n salt where n charge on cation a Take the log of both sides 13 Simple rearrangement of this equation and application of algebra yields the Henderson Hasselbalch equation acid log H log K a log salt multiply by 1 salt log H log K a log acid salt pH pK a log acid The Henderson Hasselbalch equation is one method to calculate the pH of a buffer given the concentrations of the salt and acid Also applicable for bases 14 Use the Henderson Hasselbalch eq to calculate the pH of this solution Ka 4 5 x 10 4 0 10 M in nitrous acid and 0 15 M sodium nitrite pH pKa log salt acid Solutions that contain a weak acid plus a salt of the weak acid are always less acidic than solutions that contain the same concentration of the weak acid alone WHY Solutions that contain a weak base plus a salt of the weak base are always less basic than solutions that contain the same concentration of the weak base alone 2 Buffers that contain a weak base plus the salt of a weak base One example of this buffer system is ammonia plus ammonium nitrate NH3 H 2 O NH 4 OH 100 4 NH 4 NO3 NH NO Kb NH OH 1 8 10 4 NH3 3 5 17 Calculate the pH of a 0 15 M NH3 solution Kb 1 8 x 10 5 Calculate the concentration of OH and the pH of the solution that is 0 15 M in aqueous ammonia NH3 and 0 30 M in ammonium nitrate NH4NO3 What is ionization Use Henderson Hasselbalch eq pOH pKb log salt base 19 Remember Henderson Hasselbalch eq also useful for bases just change pH to pOH and pKa to pKb pH pKa log conj base acid pOH pKb log conj acid base A comparison of the aqueous ammonia concentration to that of the buffer described above shows the buffering effect Solution 0 15 M NH3 OH 1 6 x 10 3 M pH 11 20 0 15 M NH3 0 15 M NH4NO3 buffer 9 0 x 10 6 M 8 95 The OH in aqueous ammonia is 180 times greater than in the buffer 21 We can derive a general relationship for buffer solutions that contain a weak base plus a salt of a weak base similar to the acid buffer relationship The general ionization equation for weak bases is B H 2 O BH OH where B represents a weak base 22 The general form of the ionization expression is Kb Solve for the OH BH OH B OH B base Kb BH salt For salts that have univalent ions OH base Kb salt For salts that have divalent or trivalent ions base OH K b n salt where n charge on anion 23 Simple rearrangement of this equation and application of algebra yields the Henderson Hasselbalch equation log OH base log K b log salt multiply by 1 log OH log K b log pOH pK b log salt salt base base 24 Buffering Action These movies show that buffer solutions resist changes in pH 25 Preparing Buffer Solutions A buffer must have 2 characteristics 1 pH control control pH at the desired value can use Henderson Hasselbalch equation choose acid or base that has pKa or pKb that is near the desired pH 2 Buffer capacity keep pH approximately constant after the addition of acid or base buffers are usually prepared as 0 10M to 1 0 M Any buffer will loose its ability to keep the pH …