acidsandbasespresentation20111206.notebook Properties of cids cids release hydrogen ion(s) into (aqueous) solution cids neutralize bases in a neutralization reaction. cids and ases cids corrode active metals. cids turn blue litmus to red. cids taste sour. Properties of ases ases release a hydroxide ion(s) into a water solution. ases neutralize acids in a neutralization reaction. ases denature protein. ases turn red litmus to blue. ases taste bitter. rrhenius cids and ases definition of acids and bases from the 1800's onsidered obsolete now since it only relates to reactions in water, aqueous solutions. He defined acids and bases this way: n acid is a substance that, when dissolved in water, increases the concentration of hydrogen ions. base is a substance that, when dissolved in water, increases the concentration of hydroxide ions. rønstedlowry cids and ases more modern definition: formulated in the early 1900's More general since it works for all reactions; not just in water n acid is a proton, H +, donor. base is a proton, H +, acceptor. rønstedlowry cids and ases rønstedlowry acid: must have a removable (acidic) proton or must transfer a proton to another substance rønstedlowry base: must have a pair of nonbonding electrons or must accept a proton 1
acidsandbasespresentation20111206.notebook 1 ronstedlowry base is defined as a substance that. 2 ronstedlowry acid is defined as a substance that. increases [H+] when placed in H2O decreases [H+] when placed in H 2O increases [OH] when placed in H2O acts as a proton acceptor acts as a proton donor increases K a when placed in H 2O decreases [H + ] when placed in H2O increases [OH ] when placed in H 2O acts as a proton acceptor acts as a proton donor rønstedlowry cids and ases _ + Hl + H 2 O l + H 3 O + + Hl donates the proton and acts as a rønstedlowry acid H 2 O accepts the proton and acts as a rønstedlowry base H 3 O + is called a hydrated proton or a hydronium ion. Lewis cids rønstedlowry acids replaced rrhenius acids because the former were more general: rrhenius acids could only be defined in aqueous (water) solutions. rønstedlowry acids don't have that limitation. Similarly, rønstedlowry acids are limited to substances that gain or lose hydrogen. ut there are acids and bases that don't. The most general approach is that of Lewis acids; which don't require an aqueous environment or an exchange of hydrogen. It is also written as H + Lewis cids Lewis ases Lewis bases are defined as electronpair donors. Lewis acids are defined as electronpair acceptors. toms with an empty valence orbital can be Lewis acids. nything that could be a rønstedlowry base is a Lewis base. Lewis bases can interact with things other than protons, however. Therefore, this definition is the broadest of the three. 2
acidsandbasespresentation20111206.notebook 3 Which of the following compounds could never act as an acid? SO 4 2 HSO 4 H 2 SO 4 NH 3 H 3 OOH 4 ccording to the following reaction model, which reactant is acting like an acid? H 2 O + H 2 SO 4 H 3 O + + HSO 4 H 2 SO 4 H 2 O H 3 O + HSO 4 None of the above 5 ccording to the following reaction, which reactant is acting as a base? H 3 O + + HSO 4 H 2 O + H 2 SO 4 H 2 SO 4 H 2 O H 3 O + HSO 4 None of the above 6 For the following reaction, identify whether the compound in bold is behaving as an acid or a base. H 3 PO 4 + H 2 O H 2 PO 4 + H 3 O + cid ase Neither oth None of the above 7 For the following reaction, identify whether the compound in bold is behaving as an acid or a base. H 3 PO 4 + H 2 O H 2 PO 4 + H 3 O + 8 For the following reaction, identify whether the compound in bold is behaving as an acid or a base. H 3 PO 4 + H 2 O H 2 PO 4 + H 3 O + cid ase oth cid ase oth Neither Neither None of the above None of the above 3
acidsandbasespresentation20111206.notebook cids in Water onjugate cids and ases What Happens When an cid issolves in Water? _ + + Which is the acid? Which is the base? Water acts as a rønstedlowry base and takes a proton (H+) from the acid. s a result, the conjugate base of the acid and a hydronium ion are formed. The term conjugate comes from the Latin word conjugare, meaning to join together. Reactions between acids and bases always yield their conjugate bases and acids. remove H + HNO2(aq) + H2O(l) NO2 (aq) + H3O (aq) cid ase onjugate conjugate base acid add H+ onjugate cids and ases remove H + HNO2(aq) + H2O(l) NO2 (aq) + H3O (aq) cid ase onjugate conjugate base acid add H+ fter the acid donates a proton, the result is called its conjugate base. fter the base accepts a proton, the result is called its conjugate acid. mphoteric Substances If a substance can act both as an acid and base, it is known as amphoteric. For example, water can act as a base or acid depending on the situation. Hl + H 2 O l + H 3 O + bove, water accepts a proton, thus acting as a base. NH 3 +H 2 O NH 4 + + OH bove, water donates a proton, thus acting as an acid. 9 substance that is capable of acting as both an acid and as a base is. autosomal conjugated amphoteric saturated miscible nother term for amphoteric is amphiprotic. For each of the following substances, write two equations, one showing it as a ronstedlowry acid and another showing it as a ronstedlowry base. HO 3 HSO 4 mphoteric Substances H 2 O 4
Strong Weak Negligible Strong Weak Negligible Strong Weak Negligible acidsandbasespresentation20111206.notebook 10 What is the conjugate acid of NH 3? NH3 11 What are the conjugate bases of HlO 4, H 2 S, PH 4 +, HO 3? + NH2 + NH3 + NH4 lo 4 +, HS, PH 3, O 3 lo 4, HS, PH 3, O 3 2 NH4OH lo 4 2, HS 2, PH 3 3, O 3 2 cid and ase Strength cid and ase Strength 100% ionized in H2O cid strength increases Strong Weak Negligible cid ase Hl l H2SO4 HSO4 HNO3 NO3 H3O + H2O HSO4 SO4 2 H3PO4 H2PO4 HF F H2H3O2 2H3O2 H2O3 HO3 H2S HS H2PO4 NH4 + HO3 2 HPO4 HPO4 NH3 2 H2O OH 2 O3 3 PO4 OH O 2 H2 H H4 H3 ase strength increases 100% protonated in H2O Strong acids are completely dissociated in water. Their conjugate bases are quite weak. 100% ionized in H2O cid strength increases Strong Weak Negligible cid ase Hl l H2SO4 HSO4 HNO3 NO3 H3O + H2O 2 SO4 H3PO4 H2PO4 HSO4 HF F H2H3O2 2H3O2 H2O3 HO3 H2S HS H2PO4 NH4 + HO3 HPO4 2 HPO4 NH3 2 O3 PO4 H2O OH OH O 2 H2 H H4 H3 2 3 ase strength increases 100% protonated in H2O Weak acids only dissociate partially in water. Their conjugate bases are weak bases. cid and ase Strength Strong cids 100% ionized in H2O cid strength increases Strong Weak Negligible cid ase Hl l H2SO4 HSO4 HNO3 NO3 H3O + H2O HSO4 SO4 2 H3PO4 H2PO4 HF F H2H3O2 2H3O2 H2O3 HO3 H2S HS H2PO4 NH4 + HO3 2 HPO4 HPO4 NH3 2 2 O3 3 PO4 H2O OH OH O 2 H2 H H4 H3 ase strength increases 100% protonated in H2O Substances with negligible acidity do not dissociate in water. They will not readily give up protons. Their conjugate bases are exceedingly strong. The seven strong acids are: Hl hydrochloric acid Hr hydrobromic acid HI hydroiodic acid HNO 3 nitric acid H 2 SO 4 sulfuric acid HlO 3 chloric acid HlO 4 perchloric acid Memorize this list. 5
acidsandbasespresentation20111206.notebook Strong cids The seven strong acids are strong electrolytes because they are 100% ionized. In other words, these compounds exist totally as ions in aqueous solution. For the monoprotic strong acids (acids that donates only one proton per molecule of the acid), the hydronium ion concentration equals the acid concentration. Strong ases ll strong bases are group of compounds called "metal hydroxides." ll alkali metals in Group I form hydroxides that are strong bases: LiOH, NaOH, KOH, etc. Only the heavier alkaline earth metals in Group II form strong bases: a(oh) 2, Sr(OH) 2, and a(oh) 2. gain, these substances dissociate completely in aqueous solution. In other words, NaOH exists entirely as Na + ions and OH ions in water. [acid] = [H 3 O + ] So, if you have a solution of 0.5 M Hl, then [H 3 O + ] = 0.5 M 12 What is the hydroxide ion concentration of a 0.22 M calcium hydroxide solution? 13 What is the concentration of a 25ml solution of 0.05M Hl when diluted to final volume of 100ml? 0.11 0.22 0.44 0.88 Not enough information. 14 What is the concentration of a 50 ml solution of 0.025M H 2 SO 4 diluted with 150 ml of water? 15 What is the [H + ] ion concentration of a 50 ml solution of 0.025M H 2 SO 4, when diluted with 150 ml of water? 6
Strong Weak Negligible 2 2 3 cid strength increases cid strength increases Strong Weak Negligible ase strength increases Strong Weak Negligible Strong Weak Negligible ase strength increases cid strength increases + 2 2 2 2 3 Strong Weak Negligible ase strength increases acidsandbasespresentation20111206.notebook 16 solution of 25 ml of 0.1M Hl and 50 ml of 0.5M HNO 3 are mixed together. What is the [H+] ion concentration of the resulting solution? 17 solution of 25 ml of 0.1M Hl and 50 ml of 0.5M HNO 3 are mixed together. What is the molarity of the resulting solution? cid and ase Strength In any acidbase reaction, equilibrium will favor the reaction in which the proton moves toward the stronger base. In other words, a stronger base will "hold onto" its proton whereas a strong acid easily releases its proton(s). 100% ionized in H2O Hl (aq) + H 2O (l) H 3O + (aq) + l (aq) acid base conj. acid conj. base n alternative way to consider equilibrium is that it will favor the reaction WY from the stronger acid. cid ase Hl l H2SO4 HSO4 HNO3 NO3 H3O + H2O HSO4 2 SO4 H3PO4 H2PO4 HF F H2H3O2 2H3O2 H2O3 HO3 H2S HS H2PO4 HPO4 NH4 + HO3 HPO4 NH3 O3 PO4 H2O OH OH O 2 H2 H H4 H3 2 100% protonated in H2O cid and ase Strength Hl (aq) + H 2 O (l) H 3 O + (aq) + l (aq) acid base conj. acid conj. base 100% ionized in H2O cid ase Hl l H2SO4 HSO4 HNO3 NO3 H3O + H2O HSO4 SO4 H3PO4 H2PO4 HF F H2H3O2 2H3O2 H2O3 HO3 H2S HS H2PO4 NH4 HO3 HPO4 HPO4 NH3 O3 H2O OH PO4 OH O 2 H2 H H4 H3 100% protonated In this example, H 2 O is a much stronger base than l, so the proton moves from Hl to H 2 O onversely, Hl is a much stronger acid than the hydronium ion, so equilibrium lies very far to the right K >>1 in H2O cid and ase Strength onsider this equilibrium between acetic acid and acetate ion: H 3 OOH (aq) + H 2 O (l) H 3 O + (aq) + H 3 OO (aq) oes equilibrium lie to the left (K<1) or to the right (K>1)? If you look for the stronger acid: quilib lies away from the stronger acid. If you look for the stronger base: quilib favors this base accepting a proton. cid and ase Strength H 3 OOH (aq) + H 2 O (l) H 3 O + (aq) + H 3 OO (aq) 100% ionized in H2O Strong Weak Negligible cid ase Hl l H2SO4 HSO4 HNO3 NO3 H3O + H2O HSO4 SO4 2 H3PO4 H2PO4 HF F H2H3O2 2H3O2 H2O3 HO3 H2S HS H2PO4 HPO4 2 NH4 + NH3 HO3 O3 2 HPO4 2 PO4 3 H2O OH OH O 2 H2 H protonated H4 H3 100% in H2O Since the hydronium ion is a stronger acid than acetic acid, equilibrium lies to the left (K<1). 7
acidsandbasespresentation20111206.notebook cid and ase Strength cetic acid is a weak acid. This means that only a small percent of the acid will dissociate. The double headed arrow is used only in weak acid or weak base dissociation equations. H 3 O 2 H (aq) + H 2 O(l) H 3 O + (aq) + H 3 O 2 (aq) 18 Strong acids have conjugate bases. strong weak neutral negative single arrow is used for strong acid or strong bases which dissociate completely. NaOH Na + (aq) + OH (aq) 19 Hr, hydrobromic acid is a strong acid. This means that. aqueous solutions of Hr contain equal concentrations of H + and OH does not dissociate at all when it is dissolved in water cannot be neutralized by a base dissociates completely to H + and r when it dissolves in water H utoionization of Water s we have seen, water is amphoteric, meaning that it can act as either an acid or a base. In pure water, a few molecules act as bases and a few act as acids, in a process referred to as autoionization. O + H O H O H + O H H H H The double headed arrow indicates that both the forward and reverse reactions occur simultaneously. H 2 O (l) + H 2 O (l) H 3 O + (aq) + OH (aq) utoionization of Water H 2 O (l) + H 2 O (l) H 3 O + (aq) + OH (aq) When there is an equilibrium state, the ratio of products to reactants yields a constant. This value is known as the equilibrium constant, K and will be discussed in more depth later in this unit. ll concentrations are in M, molarity, as designated by brackets, [ ]. IonProduct onstant In most dilute acid and base solutions, the concentration of undissociated water, remains more or less a constant. We can thus disregard the denominator in the equilibrium expression. So, K = [H3O+ ] x [OH ] [H2O] x [H2O] becomes Kw = [H3O + ] x [OH ] K = [H3O + ] x [OH ] [H2O] x [H2O] 8
acidsandbasespresentation20111206.notebook IonProduct onstant 20 The magnitude of K w indicates that K w = [H 3 O + ] x [OH ] This special equilibrium constant, K w is referred to as the ionproduct constant for water. t 25, K w = 1.0 x 10 14. Since this is such a small number, we conclude that pure water contains relatively very few ions. water ionizes to a very small extent the autoionization of water is exothermic water ionizes very quickly water ionizes very slowly 21 The ionproduct constant for water, K w is represented by [H2O] 2 [H3O + ] x [OH ] [H3O + ] + [OH ] [H3O + ] [OH ] ph It is a measure of hydrogen ion concentration, [H + ] in a solution, where the concentration is measured in moles H + per liter, or molarity. The ph scale ranges from 014. ph is defined as the negative base10 logarithm of the concentration of hydronium ion. ph = log [H 3 O + ] ph ph is defined as the negative base10 logarithm of the concentration of hydronium ion. ph = log [H3O + ] Hydrogen ion concentration, [H+] in moles/liter ph 1.0 x 10 1 1 1.0 x 10 2 2 1.0 x 10 10 10 Is the relationship between [H + ] and ph a direct or an inverse one? ph ecause of the base10 logarithm, each 1.0point value on the ph scale differs by a value of ten. solution with ph = 9 has a hydrogen ion concentration, [H + ], that is ten times more than a ph = 10 solution. solution with ph = 8 has a hydrogen ion concentration, [H + ], that is 10 2 or 100 times more than a ph = 10 solution. solution with ph = 7 has a hydrogen ion concentration, [H + ], that is 10 3 or 1000 times more than a ph = 10 solution. 9
acidsandbasespresentation20111206.notebook ph Recall that in pure water, the ionproduct is K w = 1.0 10 14 In pure water, the hydronium ion concentration and hydroxide ion concentrations are equal: [H 3 O + ] = [OH ] Therefore, in pure water, [H 3 O + ] = 1.0 10 14 = 1.0 x 10 7 M 22 The molar concentration of hydronium ion, [H 3 O + ], in pure water at 25 is. 0 1 7 10 7 10 14 and [OH ] = 1.0 10 14 = 1.0 x 10 7 M 23 solution with ph = 3 has a hydrogen ion concentration that is than a solution with ph = 5. 24 solution with ph = 14 has a hydrogen ion concentration that is than a solution with ph = 11. 2x more 2x less 100x more 100x less 3x more 3x less 1000x more 1000x less ph Therefore, in pure water, ph = log [H 3 O + ] = 7.00 ph = log (1.0 10 7 ) = 7.00 n acid has a higher [H 3 O + ] than pure water, so its ph is <7. I [H+] > [OH] There are excess hydrogen ions in solution. ph S [H+] < [OH] There are excess hydroxide ions in solution. base has a lower [H 3 O + ] than pure water, so its ph is >7. Solution type [H +](M) [OH] (M) ph value cidic > 1.0x10 7 <1.0x10 7 <7.00 Neutral =1.0x10 7 =1.0x10 7 =7.00 asic <1.0x10 7 > 1.0x10 7 >7.00 Solution type [H +](M) [OH] (M) ph value cidic > 1.0x10 7 <1.0x10 7 <7.00 Neutral =1.0x10 7 =1.0x10 7 =7.00 asic <1.0x10 7 > 1.0x10 7 >7.00 10
acidsandbasespresentation20111206.notebook 25 For a basic solution, the hydrogen ion concentration is than the hydroxide ion concentration. 26 For an acidic solution, the hydroxide ion concentration is than the hydrogen ion concentration. greater than less than equal to greater than less than equal to Not enough information. Not enough information. 27 Which solution below has the highest concentration of hydroxide ions? 28 Which solution below has the lowest concentration of hydrogen ions? ph = 3.21 ph = 7.00 ph = 8.93 ph = 12.6 ph = 11.4 ph = 8.53 ph = 5.91 ph =1.98 ph These are the ph values for several common substances. More acidic attery acid lemon juice pure rain or water distilled water 0 0 90 1 2 1 3 4 2 5 6 gastric fluid carbonated beverages vinegar orange juice beer coffee egg yolks milk 29 For a 1.0M solution of a weak base, a reasonable ph would be. 2 6 7 9 13 sea water baking soda 3 7 8 blood 9 4 10 milk of magnesia household ammonia 11 More basic household bleach household lye 5 12 13 14 11
acidsandbasespresentation20111206.notebook 30 For a 1.0M solution of a strong acid, a reasonable ph would be. 2 6 7 9 13 31 solution with the ph of 5.0 is basic has a hydrogen ion concentration of 5.0M is neutral has a hydroxideion concentration of 1x10 9 32 The ph of a solution with a concentration of 0.01M hydrochloric acid is 10 2 12 2 10 12 How o We Measure ph? For less accurate measurements, one can use Litmus paper Red litmus paper turns blue above ~ph = 8 lue litmus paper turns red below ~ph = 5 Or an indicator (usually an organic dye) such as one of the following: Methyl violet ph range for color change 0 2 4 6 8 10 12 14 Thymol blue Methyl orange Methyl red romothymol blue Phenolphthalein lizarin yellow R How o We Measure ph? For more accurate measurements, one uses a ph meter, which measures the voltage in the solution. How o We alculate ph? Recall that ph is defined as the negative base10 logarithm of the concentration of hydronium ion (or hydrogen ion). ph = log [H 3 O + ] or ph = log [H + ] 12
acidsandbasespresentation20111206.notebook How o We alculate ph? What is the ph of the solution with hydrogen ion concentration of 5.67 x 10 8 M (molar)? ph = log [H + ] First, take the log of 5.67 x 10 8 = 7.246 Now, change the sign from to + nswer: ph = 7.246 33 What is the ph of a solution with a hydrogen ion concentration of 1 x 10 5 M? 1 x 10 5 5 5 9 If you take the log of 5.67 x 10 8, you will end up with an incorrect answer. The order of operations: 1. Take the log 2. Switch the sign 34 What is the ph of a solution with a hydrogen ion concentration of 1 x 10 12 M? 1 x 10 12 12 2 12 35 What is the ph of an aqueous solution at 25.0 in which [H + ] is 0.0025 M? 3.4 2.6 2.6 3.4 2.25 36 What is the ph of an aqueous solution at 25.0 in which [H + ] is 0.025 M? +1.60 1.60 +12.4 12.4 1.25 dditional ph calculations If you are given the ph and asked to find the [H + ] (or [H 3 O + ]) in a solution, use the inverse log. Since ph = log [H + ], then [H + ] = 10 ph What is the hydrogen ion concentration (M) in a solution of Milk of Magnesia whose ph = 9.8? [H + ] = 10 9.8 [H + ] = 1.58 x 10 10 M or mol/liter 13
acidsandbasespresentation20111206.notebook 37 What is the ph of a solution whose hydronium ion concentration is 7.14 x 10 3 M? 38 What is the ph of a solution whose hydronium ion concentration is 1.92 x 10 9 M? 39 What is the hydronium ion concentration in a solution whose ph = 4.29? 40 What is the hydroxide ion concentration in a solution whose ph = 4.29? 41 The ph of an aqueous solution is 11.58. What is the molarity of the hydrogen ion, H +? Other p Scales The p in ph tells us to take the negative base10 logarithm of the quantity (in this case, hydronium ions). 2.63 nswer x 10 12 M Some similar examples are poh = log [OH] pk w = log K w pk a = log K a 14
acidsandbasespresentation20111206.notebook Relationship between ph and poh ecause [H 3 O + ] [OH ] = Kw = 1.0 x 10 14, we know that log [H 3 O + ] + log [OH ] = log K w = 14.00 or, in other words, ph + poh = pk w = 14.00 42 n aqueous solution of an acid has a hydrogen ion concentration of 2.5x10 4. What is the OH ion concentration of this solution? 4.0x10 11 5.0x10 11 4.5x10 8 7.5x10 11 4.0x10 7 43 n aqueous solution has a hydrogen ion concentration of 1.5x10 12. What is the ph of this solution? Report your answer to 3 significant figures. cid issociation onstants H (aq) + H 2 O (l) (aq) + H 3 O + (aq) For a generalized acid dissociation, the equilibrium expression would be K c, K K a = [H 3 O + ][ ] [H] This equilibrium constant is called the aciddissociation constant, Ka. K a = [H 3O + ][ ] [H] cid issociation onstants The greater the value of K a, the stronger is the acid. 44 The acid dissociation constant (K a ) of HF is 6.7 x 10 4. Which of the following is true of a 0.1M solution of HF? [HF] is greater than [H + ][F ] [HF] is less than [H + ][F ] [HF] is equal to [H + ][F ] [HF] is equal to [H ][F + ] K a = [H 3O + ][ ] [H] 15
acidsandbasespresentation20111206.notebook alculating K a from the ph The ph of a 0.10 M solution of formic acid, HOOH, at 25 is 2.38. alculate K a for formic acid at this temperature. The dissociation equation for formic acid may be written as a reaction with water HOOH + H 2 O HOO + H 3 O + alculating K a from the ph The ph of a 0.10 M solution of formic acid, HOOH, HOOH at 25 + H is 2 2.38. O HOO alculate + H Ka 3 O + for formic acid at this temperature. From this dissociation equation, we We know can that obtain the K a expression: K a = [H 3 O + ][HOO ] [HOOH] or, without water HOOH HOO + H + Weak cids: alculating K a from the ph The ph of a 0.10 M solution of formic acid, HOOH, at 25 is 2.38. alculate K a for formic acid at this temperature. To calculate K a, we need the equilibrium concentrations of all three things. We can find [H 3 O + ], which is the same as [HOO ], from the ph. Weak cids: alculating K a from the ph ph = log [H 3 O + ] 2.38 = log [H 3 O + ] 2.38 = log [H 3 O + ] 10 2.38 = 10 log [H3O+] = [H 3 O + ] 4.2 x 10 3 M = [H 3 O + ] = [HOO ] Note that this is a monoprotic acid, so [acid] = [conjugate base] Weak cids: alculating K a from the ph Now, we substitute values into the Ka expression and solve: K a = K a = [H 3 O + ][HOO ] [HOOH] [4.2 10 3 ] [4.2 10 3 ] [0.10] alculating Percent Ionization One way to compare the strength of two acids is by the extent to which each one ionizes. This is done by calculating percent ionization, or the ratio of [H + ] ions that are produced, compared to the original acid concentration. [H 3 O + ] eq Percent Ionization = 100 [H] initial K a = 1.8 10 4 16
acidsandbasespresentation20111206.notebook alculating Percent Ionization [H 3 O + ] eq Percent Ionization = 100 [H] initial In this example [H 3 O + ] eq = 4.2 10 3 M [HOOH] initial = 0.10 M alculating Percent Ionization What would be the analogous formula to calculate percent ionization for a base? [OH ] eq Percent Ionization = 100 [ase] initial 4.2 x 10 3 Percent Ionization = 100 0.10 = 4.2 % alculating ph from K a alculate the ph of a 0.30 M solution of acetic acid, H 2 H 3 O 2, at 25. K a for acetic acid at 25 is 1.8 x 10 5. First, we write the dissociation equation for acetic acid H 2 H 3 O 2 (aq) + H 2 O (l) H 3 O + (aq) + 2 H 3 O 2 (aq) alculating ph from K a alculate the ph of a 0.30 M solution of acetic acid, H2H3O2, at 25. Ka for acetic acid at 25 is 1.8 x 10 5. From the dissociation equation, we obtain the equilibrium constant expression: K a = [H 3O + ][ 2 H 3 O 2 ] [H 2 H 3 O 2 ] alculating ph from K a alculate the ph of a 0.30 M solution of acetic acid, H2H3O2, at 25. Ka for acetic acid at 25 is 1.8 x 10 5. We next set up an I chart... [H 2H 3O 2], M [H 3O + ], M [2H3O2], M Initial 0.30 M 0 0 alculating ph from K a alculate the ph of a 0.30 M solution of acetic acid, H 2 H 3 O 2, at 25. K a for acetic acid at 25 is 1.8 x 10 5. Now, substituting values from the I chart into the K a expression yields 1.8 10 5 = (x) 2 (0.30) hange x + x + x quilibrium about 0.30 M x x We are assuming that x will be very small compared to 0.30 and can, therefore, be ignored. (1.8 10 5 ) (0.30) = x 2 5.4 x 10 6 = x 2 2.3 x 10 3 = x Remember what your "x" is! In this case, it's [H 3O + ], but other times it's [OH ]. 17
acidsandbasespresentation20111206.notebook alculating ph from K a alculate the ph of a 0.30 M solution of acetic acid, H 2 H 3 O 2, at 25. K a for acetic acid at 25 is 1.8 x 10 5. ph = log [H 3 O + ] ph = log (2.3 x 10 3 ) ph = 2.64 Significant figure rules for ph on the P exam: The calculated ph value should have as many IML places as the [H + ] has sig figs. So if the [H + ] has 2 sig figs, report the ph to the 0.01 place value. If the [H + ] has 3 sig figs, report the ph to the 0.001 place value. 45 What is the hydrogen ion concentration if the acid dissociation constant is 1 x 10 6 and the acid concentration is 0.01M? 10 6 10 5 10 4 10 3 K a = [H 3O + ][ ] [H] 46 What is the concentration of the acid if the ph is 4 and the K a is 1 x 10 7? 10 7 10 5 10 3 10 1 K a = [H 3O + ][ ] [H] Polyprotic cids Polyprotic acids are characterized by having more than one acidic proton. Here are some examples: Sulfuric acid H 2SO 4 Phosphoric acid H 3PO 4 arbonic acid H 2O 3 Oxalic acid H 2 2O 4 Polyprotic cids Polyprotic acids have a K a value for each proton that can be removed. For example, consider carbonic acid, H 2 O 3. The first ionization equation for carbonic acid is H 2 O 3 + H 2 O < > HO 3 + H 3 O + Write the Ka expression for this equation. This is referred to as K a1. Polyprotic cids The second ionization equation for carbonic acid is HO 3 + H 2 O < > O 3 2 + H 3 O + Write the Ka expression for this equation. This is referred to as K a2. 18
acidsandbasespresentation20111206.notebook Polyprotic cids Now, we examine both dissociation constants. Polyprotic cids Notice that the bicarbonate ion, HO 3, appears in each expression; in the numerator for K a1 and in the denominator for K a2. H 2 O 3 + H 2 O < > HO 3 + H 3 O + HO 3 + H 2 O < > O 3 2 + H 3 O + K a1 = [HO 3 ][H 3 O + ] [H 2 O 3 ] [O 2 3 ][H 3 O + ] K a2 = [HO 3 ] [HO 3 ][H 3 O + ][O 2 3 ][H 3 O + ] K a1 x K a2 = [H 2 O 3 ] [HO 3 ] [HO 3 ][H 3 O + ][O 2 3 ][H 3 O + ] K a1 x K a2 = [H 2 O 3 ] [HO 3 ] Polyprotic cids The equation for the complete ionization of carbonic acid is H 2 O 3 + H 2 O < > O 2 3 + 2 H 3 O + and the K a expression for this reaction is [O 2 3 ][H 3 O + ] 2 K = [H 2 O 3 ] Polyprotic cids Generally, the ph of polyprotic acids depends only on the removal of the first proton. This holds true when the difference between the K a1 and K a2 values is at least 10 3. [HO 3 ][H 3 O + ][O 2 3 ][H 3 O + ] K a1 x K a2 = [H 2 O 3 ] [HO 3 ] K a1 x K a2 = overall K So, the product of K a1 x K a2 for a diprotic acid yields the overall K for complete dissociation. 47 The Ka of carbonic acid is 4.3 x 10 7 H 2 O 3 <> H + + HO 3 This means that H 2 O 3 is a. 48 diprotic acid, H 2 X, has the following dissdiprotic ociation constants: K a1 = 2.0 x 10 4 K a2 = 3.0 x 10 6 What is the overall K value for this acid? good hydrogenion acceptor good hydrogenion donor poor hydrogenion acceptor poor hydrogenion donor 5.0 X 10 10 6.0 X 10 10 5.0 X 10 24 6.0 x 10 24 6.0 x 10 24 19
acidsandbasespresentation20111206.notebook Weak ases ases react with water to produce hydroxide ion. Weak ases + H 2 O H + OH The equilibrium constant expression for this reaction is K b = [H][OH ] [ ] where K b is the basedissociation constant. ven though NH 3 does not have the hydroxide ion, OH, in its formula, note that it is a base according to both the rrenhius and ronstedlowry definitions. Just as for K a, the stronger a base is, it will have a higher K b value. In fact, since the strong bases dissociate 100%, their K b values are referred to as "very large". Weak ases K b can be used to find [OH ] and, ultimately, ph. 49 Which base has the smallest base dissociation constant, K b? potassium hydroxide sodium hydroxide calcium hydroxide ammonia 50 base has a dissociation constant, K b = 2.5 x 10 11. Which of the following statements is true? 51 substance with a K a of 1x10 5 would be classified as a. This is a concentrated base. This base ionizes slightly in aqueous solution. This is a strong base. n aqueous solution of this would be acidic. strong acid weak acid strong base weak base 20
acidsandbasespresentation20111206.notebook Weak ases Most problems involving weak bases require you to calculate either ph or the basedissociation constant, K b. ph of asic Solutions What is the ph of a 0.15 M solution of NH 3? NH 3 (aq) + H 2 O (l) NH 4 + (aq) + OH (aq) s with weak acids, first write the equilibrium expression for the dissociation equation. Obtain the K b value from an earlier page. K b = [NH 4 + ][OH ] [NH 3 ] =1.8 x 10 5 ph of asic Solutions ph of asic Solutions What is the ph of a 0.15 M solution of NH 3? 1.8 10 5 = (x) 2 (0.15) Tabulate the data [NH 3 ], M [NH 4 + ], M [OH ], M Initially 0.15 0 0 hange x +x +x t quilibrium NH 3 (aq) + H 2 O (l) NH 4 + (aq) + OH (aq) 015 x 0.15 x x (1.8 10 5 ) (0.15) = x 2 2.7 x 10 6 = x 2 1.6 x 10 3 = x gain, remember what your "x" is! ph of asic Solutions In this case, "x" is [OH ]. Therefore, [OH ] = 1.6 x 10 3 M poh = log (1.6 x 10 3 ) poh = 2.80 K a and K b For a conjugate acidbase pair, K a and K b are related in a special way. Write the ionization equations for the following: 1) reaction of ammonia (NH 3 ) and water So, now solving for ph: ph = 14.00 2.80 ph = 11.20 2) reaction of ammonium ion (NH 4 + ) and water 21
acidsandbasespresentation20111206.notebook K a and K b K a and K b NH 3 + H 2 O NH 4 + + OH NH 4 + + H 2 O NH 3 + H 3 O + NH 3 + H 2 O NH 4 + + OH NH 4 + + H 2 O NH 3 + H 3 O + Write the corresponding equilibrium constant expression for each of these equations. K b = [NH 4 + ][OH ] [NH 3 ] K a = [NH 3][H 3 O + ] [NH 4 + ] What do these expressions have in common? Which of these expressions is referred to as "K a"? Which is referred to as "K b"? K b x K a = K b x K a = K a and K b [NH + 4 ][OH ][NH 3 ][H 3 O + ] [NH 3 ] [NH + 4 ] [NH + 4 ][OH ] [NH 3 ][H 3 O + ] [NH 3 ] [NH + 4 ] K [OH ][H 3 O + b x K a = ] = K w K a and K b For a specific conjugate acidbase pair, K a and K b are related in this way: K a x K b = K w Therefore, if you know the value of one of them, you can calculate the other. So, the product of K a x K b for any conjugate acidbase pair yields the ionproduct constant, K w. 52 For the acid HN, what is the equation on which the K a expression is based? HN <> OH + HN HN <> OH + N HN + H 2 O <> N H 3 O + HN + H 2 O <> OH + N 53 The K a for an acid, HX, is 2.0 x 10 4. What is the K b for its conjugate base, X? No calculator. 5 X 10 11 2 X 10 10 5 X 10 10 8 x 10 10 2 x 10 4 K a x K b = K w 22
acidsandbasespresentation20111206.notebook 54 The K a for HN is 4.0 x 10 10. What is the K b for the cyanide ion, N? No calculator. 2.5 X 10 3 2.5 X 10 4 2.5 X 10 5 4 x 10 4 4 x 10 4 K a x K b = K w 55 The K b for a base,, is 5.0 x 10 8. What is the K a for its conjugate acid, H +? No calculator. 5.0 X 10 8 5.0 X 10 6 2.0 X 10 5 2.0 X 10 6 2.0 X 10 7 K a x K b = K w 56 Which of the substances below is the strongest acid? HlO HO 3 H 2S NH 3H 3 + Neutralization Reactions Neutralization reactions are a special class of doublereplacement reactions that occur between an acid and a base. Recall from last year that the general formula for a doublereplacement reaction is: H 2S and HlO + > + oublereplacement reactions are also known as ionexchange or precipitation reactions. Neutralization Reactions The general formula for any acidbase neutralization reaction is: acid + base > salt + water cidase Properties of Salt Solutions acid + base > salt + water When an acid and base react together, the resulting solution is not always neutral. The ph of the resulting mixture depends on the relative strengths of the acid and of the base. The ph of a salt solution depends on the ability of the salt's ions to hydrolyze. (Hydrolyze means to interact with water molecule and dissociate.) Note that the term "salt" refers to any ionic compound that does not include H + or OH. 23
acidsandbasespresentation20111206.notebook cidase Properties of Salt Solutions The ph of a salt solution depends on the ability of the salt's ions to hydrolyze. onsider the anion, X, which may be formed as the conjugate base of an acid. This anion may undergo hydrolysis as shown below: X + H 2 O <> Predict the products. o you think the resulting solution will be acidic, basic or neutral? Justify your response. cidase Properties of Salt Solutions The ph of a salt solution depends on the ability of the salt's ions to hydrolyze. onsider the anions chloride ion, l and cyanide ion, N. Which one has the ability to hydrolyze? l + H 2O <> Hl + OH <> H + + l + OH ecause Hl is a strong acid, it will dissociate completely in solution. Therefore, since the products are H + + l + OH, there is no effect on ph. We conclude that l OS NOT HYROLYZ. N + H 2O <> HN + OH ecause HN is a weak acid, it will not dissociate completely in solution. s a result, the excess hydroxide ions cause the solution to become. cidase Properties of Salt Solutions The ph of a salt solution depends on the ability of the salt's ions to hydrolyze. s we have seen, the conjugate base (l ) of a strong acid (Hl) will not hydrolyze. In fact, none of the anions from the strong seven acids will hydrolyze. In other words, these anions will not affect ph. List here the conjugate bases of the strong seven acids. gain, none of these anions will undergo hydrolysis to affect ph. cidase Properties of Salt Solutions The ph of a salt solution depends on the ability of the salt's ions to hydrolyze. Metal cations can also hydrolyze to affect ph. onsider the ability of the aluminum and potassium cations l 3+ and K + to hydrolyze: l 3+ + H 2O <> l(oh) 3 + H + ecause l(oh) 3 is a weak base, it will not dissociate completely in solution. The excess hydrogen ions the ph. K + + H 2 O <> KOH + H + <> K + + OH + H + ecause KOH is a strong base it will dissociate completely into K + and OH. There is no effect on ph due to the balance of the hydroxide ions and hydrogen ions. cidase Properties of Salt Solutions cidase Properties of Salt Solutions The ph of a salt solution depends on the ability of the salt's ions to hydrolyze. s we have seen, the metal cation (K + ) of a strong base (KOH) will not hydrolyze. In fact, none of the cations from the strong bases will hydrolyze. In other words, these metal cations in solution will not affect ph. List here the metal cations from Groups 1 and 2 that form strong bases. Summary: nions from weak acids will raise ph by creating excess OH ions. ations from weak bases will lower ph by creating excess H + ions. nions from strong acids will have NO effect on ph. ations from strong bases will have NO effect on ph. gain, none of these cations will undergo hydrolysis to affect ph. 24
acidsandbasespresentation20111206.notebook cidase Properties of Salt Solutions We have observed that metal cations (hydrated in solution) lower the ph of a solution. Lone pairs on oxygen are attracted to the cation. The shift of electron density in water makes the OH bond more polar. Hydrogen atoms become more acidic, and thus more likely to become H + ions in solution. cidase Properties of Salt Solutions We have observed that metal cations (hydrated in solution) lower the ph of a solution. attraction between a 1+ cation and H2O molecule. ompare the following: attraction between a 3+ cation and H2O molecule. Shift in electron density away from hydrogen atoms. The cation with the greater charge will make the solution more acidic. Summary of cidase Properties of Salts When a salt is formed between Strong acid Strong base Strong acid Weak base xample Nal l(no3)3 Salt anion comes from this acid HNO3 Salt cation comes from this base (rown, LeMay & ursten Section 16.9) the ph of the resulting solution is Hydrolysis I Salts from strong acid and strong base Neither cation nor anion will undergo hydrolysis ph will not change Weak acid Strong base H2H3O2 a(oh)2 nions from strong acids: l, r, I lo3, lo4, NO3, SO4 2 Weak acid Weak base NH4N HN NH3 asic if K b>k a cidic if K a>k b ations from strong bases: K +, Li +, Na +, K +, Rb +, s +, a 2+, a 2+. Sr 2+ Hydrolysis II Salts from strong acid and weak base xamples: NH 4 l, NH 4 NO 3, Mgl 2, Mg(NO 3 ) 2 The anions will have no effect on ph. Only the cation will undergo hydrolysis: Mg 2+ + H 2 O < > Mg(OH) 2 + H + Since Mg(OH) 2 is a weak base, it will not dissociate completely. So the ph will decrease due to excess H + ions. Similarly with ammonium ion: NH 4 + + H 2 O < > NH 3 + H 3 O + Hydrolysis III Salts from weak acid and strong base xamples: LiF, a( 2 H 3 O 2 ) 2, NaOl, Na 2 H 3 O 2, NaF The cations will have no effect on ph. Only the anion will undergo hydrolysis: F + H 2 O < > HF +OH Since HF is a weak acid, it will not dissociate completely. So the ph will increase due to excess OH ions. Similarly with the hypochlorite ion: Ol + H 2 O < > HOl +OH 25
acidsandbasespresentation20111206.notebook Hydrolysis IV Salts from weak Hydrolysis acid and I weak base Salts from strong acid and strong base xample: NH 4 N onsider the ions that make up the salt and write hydrolysis equations for each: NH 4 + + H 2 O < > NH 3 + H 3 O + N + H 2 O < > HN +OH You must compare the K a of the cation, NH 4 + with the K b of the anion, N. However, you will not normally find these values in common tables. Instead, you will find the K b of NH 3 and the K a of HN. Use the formula K a x K b = K w to obtain the values you need. If the K a > K b, then the solution is acidic. If the K a < K b, then the solution is basic. Hydrolysis IV Salts from weak Hydrolysis acid and I weak base Salts Will an from aqueous strong solution acid of NH 4 and N be strong acidic or base basic? What is the K a of the cation, NH 4 +? Look up the K b value for NH 3. Use the formula K a = K w / K b What is the K b of the anion, N? Look up the K a value for HN. Use the formula K b = K w / K a If the Ka > Kb, then the solution is acidic. If the Ka < Kb, then the solution is basic. cidase Properties of Salt Solutions Revised Summary: nions from weak acids will raise ph by creating excess OH ions. ations from weak bases will lower ph by creating excess H + ions. nions from strong acids will have NO effect on ph. ations from strong bases will have NO effect on ph. Salts from STRONG I + STRONG S = ph of 7 Salts from STRONG I + WK S = ph below 7 57 0.1M aqueous solution of will have a ph of 7.0 at 25 o. NaOl Kl NH 4 l a( 2 H 3 O 2 ) 2 none of these Salts from WK I + STRONG S = ph above 7 If both cation and anion come from a weak base and weak acid, then the solution will be basic if Kb > Ka the solution will be acidic if Ka > Kb 58 0.1M aqueous solution of has a ph of 7.0 Na 2 S KF NaNO 3 NH 4 l NaF 59 n aqueous solution of will produce a basic solution. NH 4 lo 4 Kr Nal MgO 3 NaHO 3 26
acidsandbasespresentation20111206.notebook 60 n aqueous solution of will produce an acidic solution. NH 4 lo 4 Kr Nal SrO 3 NaHO 3 61 n aqueous solution of will produce a neutral solution. NH 4 lo 4 lr 3 Fel 3 SrO 3 al 2 62 Of the following substances, an aqueous solution of will form basic solutions. Factors ffecting cid Strength NH 4 l, u(no 3 ) 2, K 2 O 3, NaF NH 4 l and u(no 3 ) 2 NH 4 l and K 2 O 3 K 2 O 3 and NaF NaF only NH 4 l only The more polar the HX bond and/or the weaker the HX bond, the more acidic the compound. So acidity increases from left to right across a row and from top to bottom down a group. Factors ffecting cid Strength In oxyacids, in which an OH is bonded to another atom, Y, the more electronegative Y is, the more acidic the acid. K a = 3.0x10 8 K a = 2.3x10 11 Factors ffecting cid Strength mong oxyacids, the more oxygen atoms in the molecule, the stronger the acid would be. The electron density will be more towards the oxygen atoms and the oxidation number increases, the strength of the acid increases. In HOl, the electron density will be shifted to the more electronegative l atom weakening the OH bond. The strength of the acid will be in the order HOl > HOr > HOI 27
acidsandbasespresentation20111206.notebook Factors ffecting cid Strength Factors ffecting cid Strength Resonance in the conjugate bases of carboxylic acids stabilizes the base and makes the conjugate acid more acidic. For a series of oxyacids, acidity increases with the number of oxygen atoms. Lewis cids Lewis cids rønstedlowry acids replaced rrhenius acids because they were more general: rrhenius acids could only be defined in aqueous (water) solutions. rønstedlowry acids don't have that limitation. Similarly, rønstedlowry acids are limited to substances that gain or lose hydrogen. ut there are acids and bases that don't. The most general approach is that of Lewis acids; which don't require an aqueous environment or an exchange of hydrogen. Lewis acids are defined as electronpair acceptors. toms with an empty valence orbital can be Lewis acids. Lewis ases 63 Which of the following cannot act as a Lewis base? l NH 3 Lewis bases are defined as electronpair donors. nything that could be a rønstedlowry base is a Lewis base. F 3 N H 2 O Lewis bases can interact with things other than protons, however. 28
acidsandbasespresentation20111206.notebook 64 In the reaction F 3 + F <> F 4 F 3 acts as a/an acid. rrhenius ronstedlowry Lewis rrhenius, ronstedlowry, and Lewis rrhenius and ronstedlowry N lso print slide 164 as a full page Summary of cidase Properties of Salts (Section 16.9) Summary of cidase Properties of Salts (Section 16.9) When a salt is formed between Strong acid Strong base xample Nal Salt anion comes from this acid Salt cation comes from this base the ph of the resulting solution is When a salt is formed between Strong acid Strong base xample Salt anion comes from this acid Salt cation comes from this base the ph of the resulting solution is Nal Hl NaOH ph=7, neutral Strong acid Weak base l(no3)3 HNO3 Strong acid Weak base l(no3)3 HNO3 l(oh)3 ph below 7, acidic Weak acid Strong base H2H3O2 a(oh)2 Weak acid Strong base a(2h3o2)2 H2H3O2 a(oh)2 ph above 7, basic Weak acid Weak base NH4N HN NH3 asic if K b>k a cidic if K a>k b Weak acid Weak base NH4N HN NH3 asic if Kb>Ka cidic if Ka>Kb Show the hydrolysis reactions for N and for NH4 +. Show how to use Ka of HN to get the Kb for N = 2.0 x 10 5 Show how to use the Kb of NH3 to get the Ka for NH4 + = 5.6 x 10 10 salts and hydrolysis Salt cid ase Hydrolysis ph Nal Hl NaOH None neutral, 7 KNO3 HNO3 KOH None neutral, 7 NH4l Hl NH3 NH4 + +H2O > NH3 + H3O + acidic KN HN KOH N + H2O > HN + OH basic KF HF KOH F + H2O > HF + OH basic NH4H3OO H3OOH NH3 NH4+ + H2O > NH3 + H3O+ acidic? H3OO+ H2O > H3OOH + OH basic? compare ka of H3OOH and Kb of NH3 whichever larger will have the effect on the solution H3NH3l Hl H3NH2 H3NH3 + + H2O > H3NH2 + H3O acidic NH4F HF NH3 F + H2O > HF + OH basic? NH4 + + H2O > NH3 + H3O+ acidic? Ka HF = 6.8 104 Kb NH3 = 1.8 10 5 it is acidic! NH4N HN NH3 N + H2O > HN + OH basic NH4+ + H2O > NH3 + H3O+ acidic? Mg(O3) H2O3 Mg(OH)2 O3 2 + H2O > HO3 1 + OH basic NaOl HOl NaOH Ol + H2O > HOL + OH basic Kr Hr KOH none neutral,7 ny weak acid/base combination of salt origin, the Ka or Kb values are needed to predict the outcome of the solution. 29