Acid/Base Definition. Acid/Base Reactions. Major vs. Minor Species. Terms/Items you Need to Know. you need to memorize these!!

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1 Acid/Base Reactions some covalent compounds have weakly bound H atoms and can lose them to water (acids) some compounds produce OH in water solutions when they dissolve (bases) acid/base reaction are very important to biochemistry and environmental chemistry Acid/Base Definition Arrhenius definition: anything that produces H 3 O + in solution (or H + ) anything that produces OH in solution BronstedLowry definition: an acid is a proton (H + ) donor (HCN, HCl, etc.) a base is a proton (H + ) acceptor (CN, OH, etc.) you need to memorize these!! Terms/Items you Need to Know Monoprotic, diprotic, polyprotic acids Amphoteric The difference between a strong acid/base and a weak acid/base Major vs. Minor Species For a strong acid, a large K a means no reactant is left, so products are the major species. HNO 3 (aq) + H 2 O (l) NO x 3 (aq) + H 3 O + (aq) For a weak acid, a small K a means only a small amount of product forms, so reactants are the major species and products are the minor species HNO 2 (aq) + H 2 O (l) NO 2 (aq) + H 3 O + (aq) K a =

2 The ph Scale What s the ph of neutral water? = log[10 ph<7 : solution is acidic ph>7 : solution is basic Same thing for bases. Define poh = log[oh ] 10 4 less Since K w = [H 3 O + ][OH ] = 10 log[h 3 O + ] log[oh ] = log(10 ) or ph + poh = 14 Equilibrium Calculations Calculate the ph of a solution of M HClO 4. (perchloric acid) Step #1: identify the acid If strong acid = complete dissociation HClO 4 (aq) H + (aq) + ClO 4 (aq) M 0 0 init equil Step #2: ph = log [H + ] log ( ) = 1.60 Equilibrium Calculations Calculate the ph of a solution of M HClO? (hypochlorous acid) Step #1: identify the acid if weak acid = look up K a = HClO (aq) H + (aq) + ClO (aq) M 0 0 init x x x change ( x) x x equil Step #2: find x, then ph = log [H+] log ( ) = 4.53 Weak Base Example Calculate the ph of a solution of M trimethylamine (CH 3 ) 3 N? Step #1: identify it as a weak base (pk b =4.19) (CH 3 ) 3 N (aq) H 2 O OH (aq)+(ch 3 ) 3 NH + (aq) M 0 0 init x +x +x change ( x) x x equil Step #2: find x, poh = log [OH ] log ( ) = 2.89 Step #3: 14 poh = ph = Buffers Buffering demo Watch ph of water + acid/base Watch ph of buffer + acid/base Definition: A buffer is usually a mixture of conjugate acidbase pairs in solution. A buffer resists strong changes in ph.

3 How do you make a buffer? add a weak acid and its conjugate base to a solution A weak acid only partially breaks up: CH 3 COOH + H 2 O CH 3 COO + H 3 O + A weak base only pulls some H off H 2 O: CH 3 COO + H 2 O CH 3 COOH + OH You end up with mostly this in solution How do you make a buffer? add some NaOH to a weak acid CH 3 COOH + OH CH 3 COO + H 2 O don t add so much that you react all of the weak acid get as close to 50/50 mixture as possible add some HCl to a weak base CH 3 COO + HCl CH 3 COOH + Cl don t add so much that you react all of the weak base get as close to 50/50 mixture as possible Buffers Buffer capacity refers to how much of the buffer is there and how much acid or base it can absorb before it no longer works The more buffer you add to the solution, the higher the buffer capacity I could add: 1 mole of HOAc and 1 mole of NaOAc OR 10 mol of HOAc and 10 mol of NaOAc Which has the largest buffer capacity? 10 mol of HOAc and 10 mol of NaOAc ( 10!) tries to maintain this ph usually ± 1 ph unit Acid/Base Titrations titration : a way of determining the concentration of one solution by using another solution of known concentration The molar mass of the chemical can be determined from the equivalence point The identity of the acid/base can be determined from the midpoint Terms equivalence point (or stoichiometric point) the calculated point when enough titrant has been added to react with all of the unknown physical change = color, turbidity, temperature, conductivity, voltage midpoint the point in a weak acid titration where ph = pk a

4 weak acid Buffer region Stoichiometric (or equivalence) point Regions in a Titration 1. Starting ph 2. Buffer region calc 3. Equivalence point 4. Postequivalence point see AcidBaseTitrations worksheet midpoint, ph = pk a of OH added What type of K eq problem is it? Well, what is in the solution? weak acid, so ICE weak acid & weak base, so buffer weak base, so ICE What is being titrated by what? Are you titrating starting with a an weak acid or strong base? base? base weak acid strong base, so ph=14poh acid Dissolved Ionic Compounds some ionic compounds do not dissolve in water (at least not very much) PbSO 4 (s) Pb 2+ (aq) + SO 2 4 (aq) Reaction Types Precipitation when solutions of ions mix, sometimes an insoluble salt forms Pb(NO 3 ) 2 (aq) + 2 NaI (aq) PbI 2(s) + 2 NaNO 3(aq) soluble soluble insoluble soluble the net ionic reaction is: Pb 2+ (aq) + 2 I (aq) PbI 2(s) spectator ions: NO 3 and Na +

5 Barium Sulfate Ba 2+ is a mildly toxic heavy metal when it is soluble in water linked to elevated blood pressure ingestion can cause nausea, vomiting, diarrhea, and crampy abdominal pain within minutes of consuming the meal BaSO 4 is so insoluble (pk sp = 9.96) that it is ingested as a contrast agent in Xrays and CAT scans Olmsted and Williams 3 rd ed pg. 161 K sp Example What mass of AgCl will dissolve in a 500 ml sol n of M NaCl? AgCl (s) Ag + (aq) + Cl (aq) pk sp =9.74 initial(m) change(m) x +x eq. (M) x x = = (x)( x) assume: x<< x = / = M find the mass of AgCl: ( M)(0.5L)(143g/mol) = g Another K sp problem Mix 500 ml of M magnesium chloride with 200 ml of M NaOH, what mass of precipitate Mg(OH) 2 forms? Another K sp problem (cont.) (0.500L)(0.200 mol/l) = mol MgCl 2 (0.200L)(0.400 mol/l) = mol NaOH L L = L 0.100/0.700 = M MgCl /0.700 = M NaOH What is the reaction of interest? Mg 2+ Cl Na + OH Mg(OH) 2(s) Mg OH Another K sp problem (cont.) Mg(OH) 2(s) Mg 2+ (aq) + 2 OH (aq) pk sp =11.25 I(M) K sp says? NI(M) C(M) x +x +2x E(M) x x 2x K sp = =(2x) 2 ( x) assume: x<<0.0859check x=[( )/(0.0859*4)] ½ = M [Mg 2+ ] = x = M [OH ] = 2x = 8.1E6 M g Mg(OH) 2 = (0.057M)(.7L)(58.3g/mol)=2.3 g