Definitions Acid/Base Arrhenius Bronsted-Lowry HA + H2O B + H2O HA and A- are conjugate pairs (HA is the acid, A- is the conjugate base) B and HB+ are conjugate pairs H2O is amphoteric (revisit for pH scale)

Write a Ka expression HA (aq) + H2O (l) A- (aq) + H3O+ (aq) Write a Kbexpression B (aq) + H2O (l) HB+ (aq) + OH- (aq) What do you think about the magnitude of Ka/Kb? What does a large K indicate? What does a small K indicate?

What functional group do you see? What is the conjugate acid? Aniline is a base. Its formula is C6H5NH2. Its structure is shown. Write an equation to represent dissociation and write a Kb expression. What functional group do you see? What is the conjugate acid? Would you expect Kb to be big or small and why? What about behavior of conjugate acid? Conjugates show opposite behavior

Using Structure to Determine Strength Why is HF a WA, but HCl, HBr, and HI are SA? HF is highly polar. Does this strengthen or weaken the H-F bond? Based on that, would you expect HF to be weak or strong acid? It’s not what you expect-why? Orbital overlap

Using Structure ctd. Trend for oxyacids Ex. HClO, HClO2, HClO3, HClO4 acids whose H+ is attached to an O Ex. HClO, HClO2, HClO3, HClO4 As ↑ #O, ↑ acid strength. Why? Draw Lewis structures to find out…

Problem to Try Arrange Cl-, CN-, ClO2- in ordering of increasing base strength. Ka for HCl = large Ka for HCN = 6.2x10-10 Ka for HClO2 = 1.2x10-2

H2O is Amphoteric/pH Scale Water can react with itself (autoionization of water) H2O (l) + H2O (l) H3O+ (aq) + OH- (aq) Write a K expression for reaction (Kw) @25oC, Kw for pure water = 1x10-14 If pure water is neutral, what can you say about [H+] and [OH-]? What is true about pH if neutral? pH = -log [H3O+] p of anything = - log (anything)

pH Scale When pH = 7, neutral and [H+] = [OH-] What does it mean to have pH < 7? What about pH > 7?

Calculate pH of Acid/Base Solutions Must identify if it’s a strong acid/strong base or weak acid/weak base. If strong, treat as 100% dissociation and use pH = - log [H+] or pOH = -log [OH-] Note: pH + pOH = 14 If weak, need ICE box

Calc pH of Strong What is the pH of a 0.10 M HCl solution? What is the pH of 0.10 M H2SO4 solution? What is the pH of 0.10 M Ba(OH)2 solution?

Calc pH of Weak Acid Calculate the pH of 1.0 M HF solution. Ka for HF = 7.2x10-4

In previous problem, we assumed the 5% rule. assumed that HF does not really ionization all that much (1.0 M stays close to 1.0 M so we eliminated the –x factor) The 5% rule says that the % ionization is < 5% forward process is not very favorable % ionization = amount dissociated x 100 initial amount From previous example, amount dissociated is x (0.0268 M) and initial amount = 1.0 M. % ionization = 0.0268/1.0 x 100 = 2.68%

Side Note For WA, % ionization increases as solution becomes more dilute. Meaning, 1.0 M HF solution will ionize less than a 0.10 M HF solution. % ionization for 1.0 M solution = 2.68% % ionization for 0.10 M solution = 8.49 %

Calculate the pH of Weak Base Calculate the pH of 1.0 M NH3 solution. Kb for NH3 = 1.8x10-5 Confirm that 5% rule is valid.

Can use pH to determine K value A 0.020 M solution of niacin, vitamin B, has pH of 3.26. What is the Ka value? First, complete the equation and set up ICE.

Can use % ionization to calc K A 0.15 M solution of WA is 3.0% ionizes. Calculate Ka.

Find pH of mixture of acids or bases If Strong with Weak Assume the weak do not ionize enough to contribute significantly to pH of solution and therefore, the pH of the solution = pH of the strong If weak with weak. Two scenarios Scenario 1: If K values for each are similar, use ICE box to find [H+] from each. Add to get total [H+] or [OH-] and then take – log [total] to get pH or pOH Scenario 2: if K of one is >>> then K of the other, only need 1 ICE box for the stronger of the two (assume the other does not ionize enough to contribute to pH)

pH of mixture Find pH of solution that contains 0.10 M HCl with 0.10 M CH3CO2H. Find pH of solution that contains 0.10 M HF (Ka = 7.2 x10-4) with 0.10 M CH3CO2H (Ka = 1.8 x 10-5) Find pH of solution that contains 0.10 M HF with 0.10 M HClO (Ka = 2.9 x 10-8)

pH of conjugates/salts Recall SA has WCB; SB has WCA WA has SCB; WB has SCA What about acidity of salts, which are a combo of CA with CB? Split apart the ions and decide where they come from. If one is a weak conjugate and the other is strong, strong wins. If they are both weak, salt is neutral If they are both strong, need to find K values

pH of salts Split ions and decide where they come from Ex. NaCl cations (CA) come from bases and anions (CB) come from acids Ex. NaCl What about NaF? NH4Cl?

pH of NH4F? Decide what type of conjugates they are? Both strong, must find K values to determine which is stronger Kconj = Kw/Koriginal acid or base KCA = Kw/Kb KCB = Kw/Ka Kb NH3 = 1.8 x 10-5 Ka for HF = 7.2 x 10-4

More salts K2SO4 NH4CH3CO2 NH4CN Kb NH3 = 1.8 x 10-5 Ka CH3CO2H = 1.8 x 10-5 Ka HCN = 6.2 x 10-10