Formation of a salt (ionic compound): Neutralization reaction molecular Full ionic Eliminate spect ions to yield net ionic Hydrolysis/ reaction with water Anions of Weak Acids Consider the weak acid HF A solution where NaF(aq) is added to pure water is basic Cations of Weak Bases Consider the weak base CH3NH2 We are what we repeatedly do. Excellence, then, is not an act, but a habit. Aristotle Page 1 of 13
Determine [HF], [H3O + ], [F ] and ph of a 1.0 M solution of HF. KA of HF is 7.2 10 4 Determine [HF], [H3O + ], [F ] and ph of a 1.0 M solution of NaF. KA of HF is 7.2 10 4 [HF] = 1.0 M [H3O + ] = 0.027 M [F ] = 0.027 M ph = 1.57 [HF] = 2.7 10 9 M [H3O + ] = 2.7 10 9 M [F ] = 1.0 M ph = 8.57 We are what we repeatedly do. Excellence, then, is not an act, but a habit. Aristotle Page 2 of 13
KA and KB for a Conjugate Acid-Base Pair Describe the chemistry of an aqueous ammonium chloride solution Describe the chemistry of an aqueous sodium acetate (CH3COONa) solution Describe the chemistry of an aqueous sodium chloride solution Review Concept Check Benzoic acid, HC7H5O2, has an ionization constant with the value: KA = 6.4 10 5 at 25 o C. Calculate the value of pkb for the conjugate base of benzoic acid. a. 4.19 b. 4.35 c. 9.65 d. 9.81 e. 13.46 We are what we repeatedly do. Excellence, then, is not an act, but a habit. Aristotle Page 3 of 13
Sodium acetate (CH3COONa) has applications in photographic development and textile dyeing. What is the ph of 0.25 M sodium acetate? KA of CH3COOH is 1.8 10 5. ph = 9.07 Summary/ Review Cations from strong bases: Group 1A alkali metal cations (Li +, Na +, K +, etc) heavier alkaline earth cations of group 2A (Ca 2+, Sr 2+, Ba 2+, etc) These ions have no acid strength Cations from weak bases: NH3 and its derivatives NH4 +, CH3NH3 +, (CH3)2NH2 +, (CH3)3NH +, etc Hydrated cations of small, highly charged metal cations: Al 3+, Fe 2+, Fe 3+, Cr 3+, Zn 2+ Anions from strong monoprotic acids Cl, Br, I, NO3, ClO3, and ClO4 These ions have no base strength Anions from a weak acid: CN, NO2, F, CH3COO, CO3 2 etc.. These ions have some base strength These ions have some acid strength We are what we repeatedly do. Excellence, then, is not an act, but a habit. Aristotle Page 4 of 13
Salts that Yield Neutral Solutions the anion of a strong acid and the cation of a strong base yields a neutral solution Neither ion has any appreciable acid or base strength Salts that Yield Acidic Solutions The cation is the conjugate acid of a weak base and therefore has some acidic strength It will to react with water to yield H3O + ions The anion is the conjugate base of a strong acid and has no strength as a base It has no tendency to react with water Net result is an acidic solution Salts that Yield Basic Solutions The cation is the conjugate acid of a strong base and therefore has no acidic strength It has no tendency to react with water The anion is the conjugate base of a weak acid and has some strength as a base It will to react with water to yield OH ions Predict whether aqueous solutions of the following are acidic, basic, or neutral, and write an equation for the reaction of any ion that reacts with water. Potassium perchlorate, KClO4 Sodium benzoate, C6H5COONa neutral Salts of Weakly Acidic Cations and Weakly Basic Anions If a salt solution consists of the anion of a weak acid and the cation of a weak base, the ph of the solution will depend on the relative acid strength or base strength of the ions Compare the relative values of KA and KB of the ions If KA > KB, the solution will be acidic (phsoln < 7) If KA < KB, the solution will be basic (phsoln > 7) If KA for the cation and KB for the anion are comparable, the solution will contain approximately equal concentrations of H3O + and OH ions (phsoln 7) basic We are what we repeatedly do. Excellence, then, is not an act, but a habit. Aristotle Page 5 of 13
Concept Check If aqueous solutions of the following soluble ionic compounds are formed, the ph of each solution is LiCl CH3NH3Br Na3PO4 a. acidic basic neutral b. basic neutral acidic c. Neutral basic acidic d. Neutral acidic basic AlCl3 is an acidic salt since Al 3+ (aq) froms a complex that acts as an acid in solution. Calculate the ph of a 0.10 M AlCl3; solution. Hint: KA of Al(H2O)6 3+ = 1.4 10 5 The Leveling Effect HClO4 is a stronger than HClO3, but there is no difference between them when dissolved in water due to water s leveling effect All strong acids and bases are equally strong in water Practice Problems ph=2.93 Predict whether the following salt solutions are neutral, acidic, or basic, and calculate the ph of each. Hint: Write the equation for dissociation of the salt and any equilibrium established. 0.25 M NH4Br; KB of NH3 = 1.8 10 5 ph=4.93 We are what we repeatedly do. Excellence, then, is not an act, but a habit. Aristotle Page 6 of 13
0.10 M NaCN; KA of HCN = 4.9 10 10 0.20 M NaNO2; KA of HNO2 = 4.6 10 4 ph=11.16 ph= 8.32 We are what we repeatedly do. Excellence, then, is not an act, but a habit. Aristotle Page 7 of 13
Buffers: Acid-Base Equilibria An acid-base buffer is a solution that lessens the impact of ph from the addition of acid or base A 100-mL sample of dilute HCl is adjusted to ph 5.00. A 100-mL sample of an acetate buffer is adjusted to ph 5.00 We are what we repeatedly do. Excellence, then, is not an act, but a habit. Aristotle Page 8 of 13
How a Buffer Works A buffer works acts to absorb small amounts of added acid or base Concept Check NaOH is added to buffer solution which is 0.50 M NH3 and 0.50 M NH4Cl. The added NaOH (OH ) reacts with in the buffer and [NH3]. a. NH4 + ; increases b. NH4 + ; decreases c. NH3; increases d. NH3 decreases Relative Concentrations of Buffer Components & The Henderson-Hasselbalch Equation Consider a solution of acetic acid at equilibrium We are what we repeatedly do. Excellence, then, is not an act, but a habit. Aristotle Page 9 of 13
Calculate the ph of a solution consisting of 0.50 M CH3COOH and 0.50 M CH3COONa Note: KA of CH3COOH = 1.8 10 5. ph= 4.74 Calculate the ph of the solution resulting when 0.020 mole of NaOH(s) is added to 1.0 L of a buffer consisting of 0.50M CH3COOH and 0.50M CH3COONa. (Assume the addition caused a negligible change in volume.) ph = 4.78 We are what we repeatedly do. Excellence, then, is not an act, but a habit. Aristotle Page 10 of 13
Calculate the ph of the solution resulting when 0.020 mole of HCl(g) is added to 1.0 L of a buffer consisting of 0.50M CH3COOH and 0.50M CH3COONa. (Assume the addition caused a negligible change in volume.) ph = 4.71 Concept Check Which of the following gives a buffer solution when equal volumes of the two solutions are mixed? a. 0.65 M HNO3 and 0.65 M NaNO3 b. 0.65 M HNO2 and 0.65 M NaNO2 c. 0.65 M HCl and 0.65 M NaCl d. 0.65 M HNO2 and 0.65 M HNO3 e. 0.65 M HNO2 and 0.65 M NaF Buffer Capacity measure the ability of a buffer to maintain the ph with addition of an acid or base The greater the concentrations of the buffer components, the more acid or base it can absorb w/ little ph change We are what we repeatedly do. Excellence, then, is not an act, but a habit. Aristotle Page 11 of 13
Buffer Range [ HA ] The closer the ratio - [ A ] is to 1, the more effective the buffer If one component is more than 10 times the other, buffering action is poor Choose buffer system so that pka w/in one unit of desired ph Concept Check A buffer consists of 1.2 M N2H4 (Kb = 8.9 10 7 ) and 0.80 M N2H5Cl. What is the ph of this buffer solution? a. 6.23 b. 5.87 c. 8.13 d. 7.77 e. 7.95 Concept Check A buffer solution with a ph of 6.00 is needed in an experiment. Which of the following buffer systems would work best? a. H2PO4 / HPO4 2 KA of H2PO4 is 6.2 10 8 b. NH3 / NH4 + KB of NH3 is 1.8 10 5 c. N2H4 / N2H5 + KB of N2H4 is 9.6 10 7 d. H2CO3 / HCO3 KA of H2CO3 is 4.3 10 7 e. HNO2 / NO2 KA of is HNO2 is 7.1 10 4 A buffer consists of 3.0 M NH3 (KB = 1.8 10 5 ) and 2.0 M NH4Cl. What is the ph of this buffer after the addition of 1.5 moles of HNO3 to 1.0 L of this buffer? (Assume the volume does not change on addition of the HNO3) ph=8.89 We are what we repeatedly do. Excellence, then, is not an act, but a habit. Aristotle Page 12 of 13
Which of the following mixture WILL produce a buffer solution if 1.0 liters of each solution is added together? What s in the beaker? Consider the reaction(s) that occur upon mixing a. 1.5 M HNO3 and 0.8 M NaNO3 b. 1.4 M NH3 and 1.8 M HCl c. 0.80 M HF and 0.80 M NaOH d. 1.6 M HC2H3O2 and 1.4 M NaOH e. None of the above D We are what we repeatedly do. Excellence, then, is not an act, but a habit. Aristotle Page 13 of 13