Advanced Placement Chemistry Acid Base Equilibrium 2014
47.90 91.22 178.49 (261) 50.94 92.91 180.95 (262) 52.00 93.94 183.85 (263) 54.938 (98) 186.21 (262) 55.85 101.1 190.2 (265) 58.93 102.91 192.2 (266) H Li Na K Rb Cs Fr He Ne Ar Kr Xe Rn Ce Th Pr Pa Nd U Pm Np Sm Pu Eu Am Gd Cm Tb Bk Dy Cf Ho Es Er Fm Tm Md Yb No Lu Lr 1 3 11 19 37 55 87 2 10 18 36 54 86 Be Mg Ca Sr Ba Ra B Al Ga In Tl C Si Ge Sn Pb N P As Sb Bi O S Se Te Po F Cl Br I At Sc Y La Ac Ti Zr Hf Rf V Nb Ta Db Cr Mo W Sg Mn Tc Re Bh Fe Ru Os Hs Co Rh Ir Mt Ni Pd Pt Cu Ag Au Zn Cd Hg 1.0079 6.941 22.99 39.10 85.47 132.91 (223) 4.0026 20.179 39.948 83.80 131.29 (222) 140.12 232.04 140.91 231.04 144.24 238.03 (145) 237.05 150.4 (244) 151.97 (243) 157.25 (247) 158.93 (247) 162.50 (251) 164.93 (252) 167.26 (257) 168.93 (258) 173.04 (259) 174.97 (260) 9.012 24.30 40.08 87.62 137.33 226.02 10.811 26.98 69.72 114.82 204.38 12.011 28.09 72.59 118.71 207.2 14.007 30.974 74.92 121.75 208.98 16.00 32.06 78.96 127.60 (209) 19.00 35.453 79.90 126.91 (210) 44.96 88.91 138.91 227.03 58.69 106.42 195.08 (269) 63.55 107.87 196.97 (272) 65.39 112.41 200.59 (277) 4 12 20 38 56 88 5 13 31 49 81 6 14 32 50 82 7 15 33 51 83 8 16 34 52 84 9 17 35 53 85 21 39 57 89 58 90 *Lanthanide Series: Actinide Series: * 59 91 60 92 61 93 62 94 63 95 64 96 65 97 66 98 67 99 68 100 69 101 70 102 71 103 22 40 72 104 23 41 73 105 24 42 74 106 25 43 75 107 26 44 76 108 27 45 77 109 28 46 78 110 29 47 79 111 30 48 80 112 Periodic Table of the Elements Not yet named
Throughout the test the following symbols have the definitions specified unless otherwise noted. L, ml liter(s), milliliter(s) mm Hg millimeters of mercury g gram(s) J, kj joule(s), kilojoule(s) nm nanometer(s) V volt(s) atm atmosphere(s) mol mole(s) ATOMIC STRUCTURE E hν c λν E energy ν frequency λ wavelength Planck s constant, h 6.626 10 34 J s Speed of light, c 2.998 10 8 ms 1 Avogadro s number 6.022 10 23 mol 1 Electron charge, e 1.602 10 19 coulomb EQUILIBRIUM K c K p K a K b c d [C] [D] a b, where a A + b B c C + d D [A] [B] c d ( PC )( PD ) a b ( PA) ( PB) + - [H ][A ] [HA] - + [OH ][HB ] [B] K w [H + ][OH ] 1.0 10 14 at 25 C Equilibrium Constants K c (molar concentrations) K p (gas pressures) K a (weak acid) K b (weak base) K w (water) K a K b ph log[h + ],, poh log[oh ] 14 ph + poh ph pk a + log [A - ] [HA] pk a logk a, pk b logk b KINETICS ln[a] t ln[a] 0 kt 1-1 kt [ A] [ A] 0 t t ½ 0.693 k k rate constant t time t ½ half-life
GASES, LIQUIDS, AND SOLUTIONS PV nrt P A P total X A, where X A P total P A + P B + P C +... n m M K C + 273 D m V KE per molecule 1 2 mv2 moles A total moles Molarity, M moles of solute per liter of solution A abc P V T n m M D KE Ã A a b c pressure volume temperature number of moles mass molar mass density kinetic energy velocity absorbance molarabsorptivity path length concentration -1-1 Gas constant, R 8.314 J mol K 0.08206 L atm mol K -1-1 62.36 L torr mol K 1 atm 760 mm Hg 760 torr STP 0.00 C and 1.000 atm -1-1 THERMOCHEMISTRY/ ELECTROCHEMISTRY q mcdt DS ÂS products -ÂS reactants DH ÂDH products -ÂDH reactants f DG ÂDG products -ÂDG reactants DG DH - TDS -RT ln K -nfe q I t f f f q heat m mass c specific heat capacity T temperature S standard entropy H standard enthalpy G standard free energy n number of moles E I q t Faraday s constant, F standard reduction potential current (amperes) charge (coulombs) time (seconds) 96,485 coulombs per mole of electrons 1 joule 1volt 1coulomb
ACID BASE EQUILIBRIUM K a, K b, and Salt Hydrolysis What I Absolutely Have to Know to Survive the AP Exam The following might indicate the question deals with acid base equilibrium: ph, poh, [H 3 O + ], [OH ], strong and weak, salt hydrolysis, solubility product, K a, K b, acid or base dissociation constant, percent ionized, Bronsted-Lowry, Arrhenius, hydronium ion, etc Arrhenius Acids: Hydrogen ion, H +, donors Bases: Hydroxide ion, OH donors Bronsted-Lowry Acids: proton donors (lose H + ) Bases: proton acceptors (gain H + ) Acids and Bases: By Definition! HCl H + + Cl NaOH Na + + OH HNO 3 + H 2 O H 3 O + + NO 3 NH 3 + H 2 O NH 4 + + OH Hydrogen Ion, Hydronium Ion, Which is it!!!? Hydronium H + riding piggy-back on a water molecule; water is polar and the + charge of the naked proton is greatly attracted to Mickey's chin (i.e. the oxygen atom) H 3 O + Anthony H + Tony Often used interchangeably in problems; if H 3 O + is used be sure water is in the equation! Bronsted Lowry and Conjugate Acids and Bases: What a Pair! Acid and conjugate base pairs differ by the presence of one H + ion. HC 2 H 3 O 2 + H 2 O H 3 O + + C 2 H 3 O 2 HC 2 H 3 O 2 is the acid; thus C 2 H 3 O 2 is its conjugate base (what remains after the H + has been donated to the H 2 O molecule) H 2 O behaves as a base in this reaction. The hydronium ion is its conjugate acid (what is formed after the H 2 O accepts the H + ion) NH 3 + H 2 O NH 4 + + OH NH 3 is the base; thus NH 4 + is it s conjugate acid (what is formed after the NH 3 accepts the H + ion) H 2 O behaves as an acid in this reaction. The hydroxide ion is its conjugate base (what remains after the H + has been donated to NH 3 ) Understanding conjugate acid/base pairs is very important in understanding acid-base chemistry; this concepts allows for the understanding of many complex situations (buffers, titrations, etc ) Important Notes Amphiprotic/amphoteric--molecules or ions that can behave as EITHER acids or bases; water, some anions of weak acids, etc fit this bill. Monoprotic acids donating one H + Diprotic acids donating two H + Polyprotic acids donating 3+ H + Regardless, always remember: ACIDS ONLY DONATE ONE PROTON AT A TIME!!! Copyright 2014 National Math + Science Initiative, Dallas, Texas. All Rights Reserved. Visit us online at www.nms.org Page 1
Acid Base Equilibrium Ionization: That s What it s All About! Relative Strengths A strong acid or base ionizes completely in aqueous solution (100% ionized [or very darn close to it!]) The equilibrium position lies far, far to the right (favors products) Since a strong acid/base dissociates into the ions, the concentration of the H 3 O + /OH ion is equal to the original concentration of the acid/base respectively. They are strong electrolytes. Do Not confuse concentration (M or mol/l) with strength! Strong Acids Hydrohalic acids: HCl, HBr, HI; Nitric: HNO 3 ; Sulfuric: H 2 SO 4 ; Perchloric: HClO 4 Oxyacids! More oxygen atoms present, the stronger the acid WITHIN that group. The H + that is donated is bonded to an oxygen atom. The oxygen atoms are highly electronegative and are pulling the bonded pair of electrons AWAY from the site where the H + is bonded, polarizing it, which makes it easier [i.e. requires less energy] to remove thus the stronger the acid! O H O Br < H O Br O < H O Br Strong Bases Group IA and IIA (1 and 2) metal hydroxides; be cautious as the poor solubility of Be(OH) 2 and Mg(OH) 2 limits the effectiveness of these 2 strong bases. IT S A 2 for 1 SALE with the Group 2 (IIA) ions), i.e. 0.10M Ca(OH) 2 is 0.20M OH O Where the fun begins A weak acid or base does not completely ionize (usually < 10%) They are weak electrolytes. The equilibrium position lies far to the left (favors reactants) [H + ] is much less than the acid concentration thus to calculate this amount and the resulting ph you must return to the world of EQUILIBRIUM Chemistry! The vast majority of acid/bases are weak. Remember, ionization not concentration!!!! Acids and Bases ionize one proton (or H + ) at a time! For weak acid reactions: HA + H 2 O H 3 O + + A K a [H 3 O+ ][A - ] [HA] For weak base reactions: B + H 2 O HB + + OH K b [HB+ ][OH - ] [B] where K a is typically much less than 1 where K b is typically much less than 1 Copyright 2014 National Math + Science Initiative, Dallas, Texas. All Rights Reserved. Visit us online at www.nms.org Page 2
Acid Base Equilibrium Ionization: That s What it s All About! con t. Working it Out! Calculating the ph of weak acids simply requires some quick problem solving MONUMENTAL CONCEPT On the AP Exam the weak acid will be significantly weak so that the initial [HA] o is mathematically the same as the equilibrium concentration i.e. there is no need to subtracting the amount of weak acid ionized as it is mathematically insignificant. If given the concentration of the weak acid, HA and the ionization constant for the acid, K a, plug both into the equilibrium expression and solve. Both H 3 O + and A are equal (remember it's 1:1 always) so call them x and solve. K a [H 3 O+ ][A - ] [HA] Thus all you need to know is Never forget K a K a [x][x] [M o ] where x [H 3 O+ ] or K b [x][x] [M o ] where x [OH - ] K 1 10 b 14 Very important when you are given the ionization constant for the acid but need the conjugate base and vice-versa. Percent Ionization Often will be asked to determine how ionized the weak acid or base is % [x] [M o ] 100 where x [H 3 O+ ]or[oh ] Salts and ph: It s all about Hydrolysis A salt is the PRODUCT of an acid base reaction; SOME salts affect the ph of the solution. So an easy way to tell, you ask? Ask yourself, which acid and which base are needed to make that salt?.were they strong or weak? 1. A salt such as NaNO 3 does not affect the ph of the solution; if it could hydrolyze water the following would happen: Na + + 2 H 2 O NaOH + H 3 O + NO 3 + H 2 O HNO 3 + OH Neither of the salt s ions can hydrolyze water because it would result in the formation of a strong acid (HNO 3 ) or a strong base (NaOH), which would ionize 100% back into the ions. 2. K 2 S would make the solution more basic SB (KOH) & WA (H 2 S) NOTE: Only the ion that forms a weak acid or base through water hydrolysis can actaully hydrolyze water. In this case: S 2 + H 2 O H 2 S + OH The increase in OH ions means the anion of this salt makes the solution more basic 3. NH 4 Cl would make the solution more acidic since WB (NH 4 OH or NH 3 ) & SA (HCl) In this case: NH 4 + + H 2 O NH 3 + H 3 O + The increase in H 3 O + ion means the solution is more acidic Copyright 2014 National Math + Science Initiative, Dallas, Texas. All Rights Reserved. Visit us online at www.nms.org Page 3
Acid Base Equilibrium Salts and ph: It s all about Hydrolysis con t Working it Out! Calculating the ph of a salt solution A salt in solution is either behaving as a weak acid or a weak base. Remember, this means they do not ionize much. So treat them just like any other weak acid or base. The MAJOR DIFFERENCE; no K a or K b value will be provided. The salt is the conjugate acid or base to some weak acid or base that is provided - that K will be given you need to convert it. Example, the salt NaC 2 H 3 O 2 ; the K a for HC 2 H 3 O 2 is typically provided. The K b needs to be calculated. K a K b 1 10 14 Very important! Then solve as shown below to find the ph K b [x][x] [M o ] where x [OH - ] Copyright 2014 National Math + Science Initiative, Dallas, Texas. All Rights Reserved. Visit us online at www.nms.org Page 4
Acid Base Equilibrium Acid Base Equilibrium Cheat Sheet Relationships Equilibrium Expression K a [H 3 O+ ][A - ] [HA] K a [x][x] where x [H + 3 O ] [M o] [x] + % 100 where x [H3O ]or[oh ] [M o] K b [HB+ ][OH - ] [B] [x][x] where x [OH - K b ] [M ] o K 14 a Kb 1 10 ph log[h + ] ph + poh 14 poh log[oh ] K w [H 3 O + ][OH ] 1.0 10 14 @ 25 C Acidic ph < 7 [H 3 O + ] > [OH ] Neutral ph 7 [H 3 O + ] [OH ] Basic ph > 7 [H 3 O + ] < [OH ] Conjugate acids and bases K sp [M + ] x [A ] y MA(s) xm + (aq)+ ya (aq) Connections Equilibrium Buffers and Titrations Precipitation and Qualitative Analysis K a or K b with salt ph Potential Pitfalls Bonding and Lewis Structures justify oxyacid strengths Weak acids and bases be sure you know what you are using, acid or base K a or K b ; solving a K b problem gives [OH ] thus you are finding the poh! Weak is about IONIZATION not CONCENTRATION Copyright 2014 National Math + Science Initiative, Dallas, Texas. All Rights Reserved. Visit us online at www.nms.org Page 5
Acid Base Equilibrium NMSI SUPER PROBLEM NH 3 (aq) + H 2 O( ) NH + 4 (aq) + OH (aq) K b 1.80 10-5 1. Ammonia reacts with water as indicated in the reaction above. (a) Write the equilibrium constant expression for the reaction represented above. (b) Calculate the ph of a 0.150 M solution of NH 3 (c) Determine the percent ionization of the weak base NH 3. (d) Calculate the hydronium ion, H 3 O +, concentration in the above solution. Be sure to include units with your answer. When a specified amount of ammonium nitrate (NH 4 NO 3 ) is dissolved in water, the ammonium ions hydrolyze the water according to the partial reaction shown below. The resulting solution has a ph of 4.827. (e) Complete the reaction above by drawing the complete Lewis structures for both products of the hydrolysis reaction. (f) Determine the (i) molarity (M) of the ammonium ions in this solution (ii) number of moles ammonium ions in 250 ml of the above solution. Copyright 2014 National Math + Science Initiative, Dallas, Texas. All Rights Reserved. Visit us online at www.nms.orgpage 6