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acids _bases

This document provides an overview of acid-base theories and chemistry. It defines acids and bases according to three main theories: 1) Arrhenius theory defines acids as substances that donate H+ ions in water and bases as those that donate OH- ions. 2) Bronsted-Lowry theory defines acids as proton donors and bases as proton acceptors, providing a better explanation of substances like ammonia. 3) Lewis theory defines acids as electron pair acceptors and bases as electron pair donors, explaining both traditional acids/bases and coordination compounds. The document then discusses acid/base reactions and equilibrium expressions, relative acid/base strengths, and the differences between strong vs. weak acids and

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CHAPTER 14CHAPTER 14 THE CHEMISTRY OFTHE CHEMISTRY OF ACIDS AND BASESACIDS AND BASES
"ACID""ACID" Latin word acidus, meaning sour.Latin word acidus, meaning sour. (lemon)(lemon)
"ALKALI""ALKALI" Water solutions feel slippery andWater solutions feel slippery and taste bitter (soap).taste bitter (soap).
ACID-BASEACID-BASE THEORIESTHEORIES
Arrhenius Definition acidacid----donates a hydrogen ion (Hdonates a hydrogen ion (H++ )) in waterin water basebase--donates a hydroxide ion in--donates a hydroxide ion in water (OHwater (OH-- )) This theory was limited toThis theory was limited to substances with those "parts";substances with those "parts"; ammonia is a MAJOR exception!ammonia is a MAJOR exception!
Bronsted-Lowry DefinitionBronsted-Lowry Definition acidacid----donates a proton in waterdonates a proton in water basebase----accepts a proton in wateraccepts a proton in water This theory is better; it explainsThis theory is better; it explains ammonia as a base! This is the mainammonia as a base! This is the main theory that we will use for ourtheory that we will use for our acid/base discussion.acid/base discussion.
Lewis DefinitionLewis Definition acidacid----accepts an electron pairaccepts an electron pair basebase--donates an electron pair--donates an electron pair This theory explainsThis theory explains allall traditionaltraditional acids and bases + a host ofacids and bases + a host of coordination compounds and is usedcoordination compounds and is used widely in organic chemistry. Useswidely in organic chemistry. Uses coordinate covalent bondscoordinate covalent bonds
The Bronsted-Lowry ConceptThe Bronsted-Lowry Concept of Acids and Basesof Acids and Bases Using this theory, you should beUsing this theory, you should be able to write weak acid/baseable to write weak acid/base dissociation equations and identifydissociation equations and identify acid, base, conjugate acid andacid, base, conjugate acid and conjugate base.conjugate base.
Conjugate Acid-Base PairConjugate Acid-Base Pair A pair of compounds that differA pair of compounds that differ by the presence of one Hby the presence of one H++ unit.unit. This idea is critical when it comesThis idea is critical when it comes to understanding buffer systems.to understanding buffer systems.
AcidsAcids donate a proton (Hdonate a proton (H++ ))
Neutral CompoundNeutral Compound HNOHNO33 + H+ H22OO  HH33OO++ + NO+ NO33 -- acid base CA CBacid base CA CB
CationCation NHNH44 ++ + H+ H22OO  HH33OO++ + NH+ NH33 acid base CA CBacid base CA CB
AnionAnion HH22POPO44 -- + H+ H2200  HH33OO++ + HPO+ HPO44 2-2- acid base CA CBacid base CA CB
In each of the acid examples---noticeIn each of the acid examples---notice the formation ofthe formation of HH33 OO++ This species is named theThis species is named the hydroniumhydronium ionion.. It lets you know that the solution isIt lets you know that the solution is acidicacidic!!
Hydronium, HHydronium, H33 OO++ --H--H++ riding piggy-back on a waterriding piggy-back on a water molecule.molecule. Water is polar and the + charge of theWater is polar and the + charge of the naked proton is greatly attracted tonaked proton is greatly attracted to Mickey's chin!)Mickey's chin!)
BasesBases accept a proton (Haccept a proton (H++ ))
Neutral CompoundNeutral Compound NHNH33 + H+ H22OO  NHNH44 ++ + OH+ OH-- base acid CA CBbase acid CA CB
AnionAnion COCO33 2-2- + H+ H22OO  HCOHCO33 -- + OH+ OH-- base acid CA CBbase acid CA CB
AnionAnion POPO44 3-3- + H+ H22OO  HPOHPO44 2-2- + OH+ OH-- base acid CA CBbase acid CA CB
In each of the basic examples--In each of the basic examples-- notice the formation ofnotice the formation of OH-OH- -- this-- this species is named thespecies is named the hydroxidehydroxide ionion. It lets you know that the. It lets you know that the solution issolution is basicbasic!!
Exercise 1Exercise 1 In the following reaction, identifyIn the following reaction, identify the acid on the left and its CB onthe acid on the left and its CB on the right. Similarly identify the basethe right. Similarly identify the base on the left and its CA on the right.on the left and its CA on the right. HBr + NHHBr + NH33  NHNH44 ++ + Br+ Br--
What is the conjugate base of HWhat is the conjugate base of H22S?S? What is the conjugate acid of NOWhat is the conjugate acid of NO33 -- ??
ACIDS ONLYACIDS ONLY DONATEDONATE ONE PROTON AT AONE PROTON AT A TIME!!!TIME!!!
– monoproticmonoprotic--acids donating one H--acids donating one H++ (ex. HC(ex. HC22HH33OO22)) – diproticdiprotic--acids donating two H--acids donating two H++ 's (ex.'s (ex. HH22CC22OO44)) – polyproticpolyprotic--acids donating many--acids donating many H++ 's (ex. H PO )
Polyprotic BasesPolyprotic Bases accept more than one Haccept more than one H++ anions with -2 and -3 chargesanions with -2 and -3 charges (example: PO(example: PO44 3-3- ; HPO; HPO44 2-2- ))
Amphiprotic or AmphotericAmphiprotic or Amphoteric molecules or ions that can behave asmolecules or ions that can behave as EITHER acids or bases:EITHER acids or bases: water, anions of weak acidswater, anions of weak acids (look at the examples above—sometimes(look at the examples above—sometimes water was an acid, sometimes it acted aswater was an acid, sometimes it acted as a base)a base)
Exercise 2Exercise 2 AcidAcid Dissociation (Ionization)Dissociation (Ionization) ReactionsReactions Write the simple dissociationWrite the simple dissociation (ionization) reaction (omitting(ionization) reaction (omitting water) for each of the followingwater) for each of the following acids.acids.
a. Hydrochloric acid (HCl)a. Hydrochloric acid (HCl) b. Acetic acid (HCb. Acetic acid (HC22HH33OO22)) c. The ammonium ion (NHc. The ammonium ion (NH44 ++ )) d. The anilinium ion (Cd. The anilinium ion (C66HH55NHNH33 ++ )) e. The hydrated aluminum(III) ione. The hydrated aluminum(III) ion [Al(H[Al(H O)O) ]]3+3+
SolutionSolution A: HCl(aq) -> HA: HCl(aq) -> H++ (aq) + Cl(aq) + Cl-- (aq)(aq) B: HCB: HC22HH33OO22(aq)(aq)  HH++ (aq) + C(aq) + C22HH33OO22 -- (aq)(aq) C: NHC: NH44 ++ (aq)(aq)  HH++ (aq) + NH(aq) + NH33(aq)(aq)
Solution, cont.Solution, cont. D: CD: C66HH55NHNH33 ++ (aq)(aq)  HH++ (aq) + C(aq) + C66HH55NHNH22(aq)(aq) E: Al(HE: Al(H22O)O)66 3+3+ (aq)(aq)  HH++ (aq) + Al(H(aq) + Al(H22O)O)55OHOH22 ++ (aq)(aq)
Relative Strengths of AcidsRelative Strengths of Acids and Basesand Bases Strength is determined by theStrength is determined by the position of the "dissociation"position of the "dissociation" equilibrium.equilibrium.
Strong acids/Strong basesStrong acids/Strong bases dissociate completely in waterdissociate completely in water have very large K valueshave very large K values
Weak acids/Weak basesWeak acids/Weak bases dissociate only to a slight extent indissociate only to a slight extent in waterwater dissociation constant is very smalldissociation constant is very small
Strong Weak
Do NotDo Not confuse concentrationconfuse concentration with strength!with strength!
Strong AcidsStrong Acids Hydrohalic acids:Hydrohalic acids: HCl, HBr, HIHCl, HBr, HI Nitric: HNONitric: HNO33 Sulfuric: HSulfuric: H22SOSO44 Perchloric: HClOPerchloric: HClO
The more oxygenThe more oxygen present in thepresent in the polyatomic ion,polyatomic ion, the stronger itsthe stronger its acid WITHIN thatacid WITHIN that group.group.
Strong BasesStrong Bases Hydroxides OR oxides of IA andHydroxides OR oxides of IA and IIA metalsIIA metals Solubility plays a role (those thatSolubility plays a role (those that are very soluble are strong!)are very soluble are strong!)
The strongerThe stronger the acid, thethe acid, the weaker its CB.weaker its CB. The converse isThe converse is also true.also true.
Weak Acids and Bases -Weak Acids and Bases - EquilibriumEquilibrium expressionsexpressions The vast majority of acid/bases areThe vast majority of acid/bases are weak.weak. Remember, this means they do notRemember, this means they do not ionize much.ionize much.
The equilibrium expression for acidsThe equilibrium expression for acids is known as theis known as the KKaa (the acid(the acid dissociation constantdissociation constant ).). It is set up the same way as inIt is set up the same way as in general equilibrium.general equilibrium.
Many common weak acids areMany common weak acids are oxyacids,oxyacids, like phosphoric acid andlike phosphoric acid and nitrous acid.nitrous acid.
Other common weakOther common weak acids are organicacids are organic acids,acids, those that contain athose that contain a carboxyl groupcarboxyl group COOH groupCOOH group like acetic acid andlike acetic acid and benzoic acid.benzoic acid.
For Weak Acid Reactions:For Weak Acid Reactions: HA + HHA + H22OO  HH33OO++ + A+ A-- KKaa == [H[H33OO++ ][A][A-- ]] < 1< 1 [HA][HA]
Write the KWrite the Kaa expression for aceticexpression for acetic acid using Bronsted-Lowry.acid using Bronsted-Lowry. (Note: Water is a pure liquid and(Note: Water is a pure liquid and is thus, left out of the equilibriumis thus, left out of the equilibrium expression.)expression.)
Weak bases (bases without OHWeak bases (bases without OH-- )) react with water to produce areact with water to produce a hydroxide ion.hydroxide ion.
Common examples of weak bases areCommon examples of weak bases are ammonia (NHammonia (NH33), methylamine), methylamine (CH(CH33NHNH22), and ethylamine (C), and ethylamine (C22HH55NHNH22).). The lone pair on N forms a bond withThe lone pair on N forms a bond with an Han H++ . Most weak bases involve N.. Most weak bases involve N.
The equilibrium expression forThe equilibrium expression for basesbases is known as theis known as the KKbb ..
For Weak Base Reactions:For Weak Base Reactions: B + HB + H22OO  HBHB++ + OH+ OH-- KKbb == [H[H33OO++ ][OH][OH-- ]] <1<1 [B][B]
Set up the KSet up the Kbb expression forexpression for ammonia using Bronsted-Lowry.ammonia using Bronsted-Lowry.
Notice that KNotice that Kaa and Kand Kbb expressionsexpressions look very similar.look very similar. The difference is that a baseThe difference is that a base produces the hydroxide ion inproduces the hydroxide ion in solution, while the acid produces thesolution, while the acid produces the hydronium ion in solution.hydronium ion in solution.
Another note on this point:Another note on this point: HH++ and Hand H33OO++ are both equivalentare both equivalent terms here. Often water is leftterms here. Often water is left completely out of the equation sincecompletely out of the equation since it does not appear in the equilibrium.it does not appear in the equilibrium. This has become an acceptedThis has become an accepted practice.practice. (*However, water is very important(*However, water is very important in causing the acid to dissociate.)in causing the acid to dissociate.)
Exercise 3Exercise 3 Relative Base StrengthRelative Base Strength Using table 14.2, arrange theUsing table 14.2, arrange the following species according to theirfollowing species according to their strength as bases:strength as bases: HH22O, FO, F-- , Cl, Cl-- , NO, NO22 -- , and CN, and CN--
SolutionSolution ClCl-- < H< H22O < FO < F-- < NO< NO22 -- < CN< CN--
WATERWATER THE HYDRONIUM IONTHE HYDRONIUM ION AUTO-IONIZATIONAUTO-IONIZATION THE pH SCALETHE pH SCALE
Fredrich Kohlrausch, aroundFredrich Kohlrausch, around 1900, found that no matter how1900, found that no matter how pure water is, it still conducts apure water is, it still conducts a minute amount of electricminute amount of electric current. This proves that watercurrent. This proves that water self-ionizes.self-ionizes.
Since the water molecule isSince the water molecule is amphoteric, it may dissociateamphoteric, it may dissociate with itself to a slight extent.with itself to a slight extent. Only about 2 out of a billionOnly about 2 out of a billion water molecules are ionized atwater molecules are ionized at any instant!any instant!
HH22O(l) + HO(l) + H22O(l) <=> HO(l) <=> H33OO++ (aq) + OH(aq) + OH-- (aq)(aq) The equilibrium expression usedThe equilibrium expression used here is referred to as thehere is referred to as the KKww (ionization constant for water)(ionization constant for water) ..
In pure water or dilute aqueousIn pure water or dilute aqueous solutions, the concentration of watersolutions, the concentration of water can be considered to be a constantcan be considered to be a constant (55.4 M), so we include that with the(55.4 M), so we include that with the equilibrium constant and write theequilibrium constant and write the expression as:expression as: KKeqeq[H[H22O]O]22 = K= Kww = [H= [H33OO++ ][OH][OH-- ]]
KKww = 1.0 x 10= 1.0 x 10-14-14 (K(Kww = 1.008 x 10= 1.008 x 10-14-14 @ 25° Celsius)@ 25° Celsius) Knowing this value allows us toKnowing this value allows us to calculate the OHcalculate the OH-- and Hand H++ concentration forconcentration for various situationvarious situations.s.
[OH[OH-- ] = [H] = [H++ ] : solution is neutral (in] : solution is neutral (in pure water, each of these is 1.0 x 10pure water, each of these is 1.0 x 10-7-7 )) [OH[OH-- ] > [H] > [H++ ] : solution is basic] : solution is basic [OH[OH-- ] < [H] < [H++ ] : solution is acidic] : solution is acidic
KKww = K= Kaa x Kx Kbb another very beneficial equationanother very beneficial equation
Exercise 5Exercise 5 Autoionization of WaterAutoionization of Water At 60°C, the value of KAt 60°C, the value of Kww is 1 X 10is 1 X 10-13-13 .. a. Using Le Chatelier’s principle,a. Using Le Chatelier’s principle, predict whether the reactionpredict whether the reaction 2H2H22O(l)O(l)  HH33OO++ (aq) + OH(aq) + OH-- (aq)(aq) is exothermic or endothermic.is exothermic or endothermic.
Exercise 5, cont.Exercise 5, cont. b. Calculate [Hb. Calculate [H++ ] and [OH] and [OH-- ] in a] in a neutral solution at 60°C.neutral solution at 60°C.
SolutionSolution A: endothermicA: endothermic B: [HB: [H++ ] = [OH] = [OH-- ] = 3 X 10] = 3 X 10-7-7 MM
The pH ScaleThe pH Scale Used toUsed to designate thedesignate the [H[H++ ] in most] in most aqueousaqueous solutions wheresolutions where HH++ is small.is small.
pH = - log [HpH = - log [H++ ]] pOH = - log [OHpOH = - log [OH-- ]] pH + pOH = 14pH + pOH = 14 pH = 6.9 and lower (acidic)pH = 6.9 and lower (acidic) = 7.0 (neutral)= 7.0 (neutral) = 7.1 and greater (basic)= 7.1 and greater (basic)
Use as many decimal places asUse as many decimal places as there are sig.figs. in the problem!there are sig.figs. in the problem! The negative base 10 logarithm ofThe negative base 10 logarithm of the hydronium ion concentrationthe hydronium ion concentration becomes the whole number;becomes the whole number; therefore, only the decimals to thetherefore, only the decimals to the right are significant.right are significant.
Exercise 6Exercise 6 Calculating [HCalculating [H++ ] and [OH] and [OH-- ]] Calculate [HCalculate [H++ ] or [OH] or [OH-- ] as required for] as required for each of the following solutions ateach of the following solutions at 25°C, and state whether the solution25°C, and state whether the solution is neutral, acidic, or basic.is neutral, acidic, or basic. a. 1.0 X 10a. 1.0 X 10-5-5 MM OHOH-- b. 1.0 X 10b. 1.0 X 10-7-7 MM OHOH-- c. 10.0c. 10.0 MM HH++
SolutionSolution A: [HA: [H++ ] = 1.0 X 10] = 1.0 X 10-9-9 M,M, basicbasic B: [HB: [H++ ] = 1.0 X 10] = 1.0 X 10-7-7 M,M, neutralneutral C: [OHC: [OH-- ] = 1.0 X 10] = 1.0 X 10-15-15 M,M, acidicacidic
Exercise 7Exercise 7 Calculating pH andCalculating pH and pOHpOH Calculate pH and pOH for each ofCalculate pH and pOH for each of the following solutions at 25°C.the following solutions at 25°C. a. 1.0 X 10a. 1.0 X 10-3-3 MM OHOH-- b. 1.0b. 1.0 MM HH++
SolutionSolution A: pH = 11.00A: pH = 11.00 pOH = 3.00pOH = 3.00 B: pH = 0.00B: pH = 0.00 pOH = 14.00pOH = 14.00
ExampleExample Order the following from strongest baseOrder the following from strongest base to weakest base. Use table 14.2.to weakest base. Use table 14.2. HH22O NOO NO33 -1-1 OClOCl-1-1 NHNH33
Exercise 8Exercise 8 CalculatingCalculating pHpH The pH of a sample of human bloodThe pH of a sample of human blood was measured to be 7.41 at 25°C.was measured to be 7.41 at 25°C. Calculate pOH, [HCalculate pOH, [H++ ], and [OH], and [OH-- ] for] for the sample.the sample.
SolutionSolution pOH = 6.59pOH = 6.59 [H[H++ ] = 3.9 X 10] = 3.9 X 10-8-8 [OH[OH-- ] = 2.6 X 10] = 2.6 X 10-7-7 MM
Exercise 9Exercise 9 pH of Strong AcidspH of Strong Acids Calculate the pH of:Calculate the pH of: a. 0.10a. 0.10 MM HNOHNO33 b. 1.0 X 10b. 1.0 X 10-10-10 MM HClHCl
SolutionSolution A: pH = 1.00A: pH = 1.00 B: pH = 7.00B: pH = 7.00
Exercise 10Exercise 10 The pH of Strong BasesThe pH of Strong Bases Calculate the pH of a 5.0 X 10Calculate the pH of a 5.0 X 10-2-2 MM NaOH solution.NaOH solution.
SolutionSolution pH = 12.70pH = 12.70
Calculating pH of WeakCalculating pH of Weak Acid SolutionsAcid Solutions Calculating pH of weak acidsCalculating pH of weak acids involves setting up an equilibrium.involves setting up an equilibrium.
Always start by…Always start by… 1) writing the equation…1) writing the equation… 2) setting up the acid equilibrium2) setting up the acid equilibrium expression (Kexpression (Kaa)…)… 3) defining initial concentrations,3) defining initial concentrations, changes, and final concentrations inchanges, and final concentrations in terms of X …terms of X …
4) substituting values and variables4) substituting values and variables into the Kinto the Kaa expression…expression… 5) solving for X5) solving for X (use the(use the RICERICE diagram learned indiagram learned in general equilibrium!)general equilibrium!)
Example:Example: Calculate the pH of a 1.00 x 10Calculate the pH of a 1.00 x 10-4-4 MM solution of acetic acid.solution of acetic acid.
The KThe Kaa of acetic acid is 1.8 x 10of acetic acid is 1.8 x 10-5-5 HCHC22HH33OO22 ↔↔ HH++ + C+ C22HH33OO22 -- KKaa == [H[H++ ][C][C22HH33OO22 -- ]] = 1.8 x 10= 1.8 x 10-5-5 [HC[HC22HH33OO22]]
RReaction HCeaction HC22HH33OO22 ↔↔ H+ + CH+ + C22HH33OO22 -- IInitial 1.00 x 10nitial 1.00 x 10-4-4 0 00 0 CChange -x +x +xhange -x +x +x EEquilibrium 1.00 x 10quilibrium 1.00 x 10-4-4 - x x x- x x x
1.8 x 101.8 x 10-5-5 == (x)(x) _(x)(x) _ 1.00x101.00x10-4-4 -- xx 1.8 x 10-51.8 x 10-5 ≈≈ (x)(x) _(x)(x) _ 1.00 x 101.00 x 10-4-4 x = 4.2 x 10x = 4.2 x 10-5-5 Often, the -x in a KOften, the -x in a Kaa expressionexpression can be treated as negligible.can be treated as negligible.
When you assume that x isWhen you assume that x is negligible, you must check thenegligible, you must check the validity of this assumption.validity of this assumption.
To be valid, x must be less thanTo be valid, x must be less than 5% of the number that it was to be5% of the number that it was to be subtracted from.subtracted from. % dissociation =% dissociation = "x""x" x 100x 100 [original][original]
In this example, 4.2 x 10In this example, 4.2 x 10-5-5 is greateris greater than 5% of 1.00 x 10than 5% of 1.00 x 10-4-4 .. This means that the assumption thatThis means that the assumption that x was negligible was invalid and xx was negligible was invalid and x must be solved for using themust be solved for using the quadratic equation or the method ofquadratic equation or the method of successive approximation.successive approximation.
Use of theUse of the Quadratic EquationQuadratic Equation a acb bx 2 42 − ±−=
ax2 + bx + c = 0 0108.1108.1 952 =×−×+ −− xx
5 105.3 − ×=x 5 102.5 − ×−=x )1(2 )108.1)(1(4)108.1(108.1 9255 −−− ×−−×±×− =x and Using the values: a = 1, b = 1.8x10-5 , c= -1.8x10-9
Since a concentration canSince a concentration can not be negative…not be negative… x = 3.5 x 10x = 3.5 x 10-5-5 MM x = [Hx = [H++ ] = 3.5 x 10] = 3.5 x 10-5-5 pH = -log 3.5 x 10pH = -log 3.5 x 10-5-5 = 4.46= 4.46
Another method which some peopleAnother method which some people prefer is the method of successiveprefer is the method of successive approximations. In this method, youapproximations. In this method, you start out assuming that x isstart out assuming that x is negligible, solve for x, and repeatedlynegligible, solve for x, and repeatedly plug your value of x into theplug your value of x into the equation again until you get theequation again until you get the same value of x two successivesame value of x two successive times.times.
Exercise 11Exercise 11 The pH of Weak AcidsThe pH of Weak Acids The hypochlorite ion (OClThe hypochlorite ion (OCl-- ) is a) is a strong oxidizing agent often foundstrong oxidizing agent often found in household bleaches andin household bleaches and disinfectants. It is also the activedisinfectants. It is also the active ingredient that forms wheningredient that forms when swimming pool water is treated withswimming pool water is treated with chlorine.chlorine.
In addition to its oxidizing abilities,In addition to its oxidizing abilities, the hypochlorite ion has a relativelythe hypochlorite ion has a relatively high affinity for protons (it is ahigh affinity for protons (it is a much stronger base than Clmuch stronger base than Cl-- , for, for example) and forms the weaklyexample) and forms the weakly acidic hypochlorous acid (HOCl,acidic hypochlorous acid (HOCl, KKaa = 3.5 X 10= 3.5 X 10-8-8 ).).