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Acids and Bases: Understanding Definitions, Conjugate Pairs, and Strength, Lecture notes of Acting

University System of New HampshireActing

The Arrhenius and Lewis concepts of acids and bases, the identification of conjugate acid-base pairs, and the strengths of acids and bases. It includes examples, exercises, and problem-solving tips.

Typology: Lecture notes

2021/2022

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AcidBaseNotesNotes.notebook
1
March31,2015
Mar108:30AM
Therearethreedefinitionsforacidsandbaseswewillneedtounderstand.
ArrheniusConcept
:anacidsuppliesH
+
toanaqueoussolution.AbasesuppliesOH

toanaqueoussolution.Thisistheoldest
definitionbutmostlimiting.
BronstedLowryConcept
:anacidisaproton(H
+
)donor.Abaseisaproton(H
+
)acceptor.Whenanaciddonatesaproton,it
becomesabase(actinginthereversedirection)͞whenabaseacceptsaproton,itbecomesanacid(actinginthereversedirection).
Youwillneedtoidentifyconjugateacidbasepairs.
Example:Formicacid,HCOOH:(IUPACname:methanoicacid)
Mar108:31AM
LewisConcept
:anacidisanelectronpairacceptor.Abaseisanelectronpairdonor.Thisisthemostwiderangingofthethree
(i.e.itworksforeverything).ExamplesofLewisacidsincludeAl
3+
,H
+
,BF
3
.ExamplesofLewisbasesincludeNO
2

,NH
3
,and
H
2
O.
IdentifytheLewisacidandbaseineachofthefollowingreactionsandnametoproductionthatforms:

Cu
2+
(aq)+4NH
3
(aq)=Cu(NH
3
)
4
2+
(aq)


I

(aq)+I
2
(aq)=I
3

(aq)
Fe
3+
(aq)+6H
2
O(l)=Fe(H
2
O)
6
3+
(aq)
IndicatetheBLacidbaseconjugatepairsandidentifytheLewisacidandbaseinthehydratediron(II)ion,[Fe(H
2
O)
6
]
3+
Mar108:31AM
AcidBaseStrength
Thestrengthofanacidisindicatedbytheequilibriumpositionofthedissociationreaction.Iftheequilibriumliesfartotheleft
(asindicatedbythesmallvalueofK
a
),theaciddoesnotdissociatetoanygreatextentandisweak:
HA=
H
+
+
A

i
1000
0
0
c
20
+20
+20
e
980
20
20
Iftheequilibriumliesfartotheright,theacidstronglydissociatesandisstrong:
HA=H
+
+A

i
1000
00
c
999+999+999
e
1
999999
Notethat:
thestrongertheacid,theweakeritsconjugatebase(BLacids)
thestrongerthebase,theweakeritsconjugateacid(BLbases)
Youneedtomemorizethestrongacidsandbasesgivenbelow.Allotherscanbeconsideredweak.
Mar108:32AM
Strongacids
HCl
hydrochloricacid
HClO
3
chloricacid
HBr
hydrobromicacid
HClO
4
perchloricacid
HI
hydroiodicacid
H
2
SO
4
sulfuricacid
HNO
3
nitricacid
Mar108:32AM
StrongBases
Metal

Hydroxides
:thesearegroup1andgroup2metalsbondedtoahydroxylgroup.ForexampleNaOH,Mg(OH)
2
,etc.
Metal

Oxides
:thesearegroup1andgroup2metalsbondedwithoxygen.Inwaterthey“attack”thewatermoleculeand
breakituptoformhydroxideion,OH

.
K
2
O+H
2
O=2K
+
+2OH

Mar108:32AM
AcidBaseBehaviorandChemicalStructure
Sometimesyouwillneedtocomparethestrengthsofacids.Thiscanbedonebyconsideringthestructureoftheacids.Two
generaltrendsareusefulinexplainingdifferences:
Foracidhydrides(HCl,HBr,etc.)theacidityincreaseswithinagroupasthesizeofthecentralatomincreases.Thisisbecause
bondstrengthdecreasesassizeincreases.HFisaweakacidbecausetheHFbondisstrong.Anotherfactortoconsideristhe
abilityoftheanionformed(X

)–thegreaterthestabilityoftheconjugatebase,thestrongertheacidis.Forexample,
HI>HBr>HCl
ForoxyacidsthathavethesamenumberofOHgroupsandthesamenumberofOatoms,acidstrengthincreaseswithincreasing
electronegativityofthecentralatom.Forexample,
HClO>HBrO>HIO
HClO
3
isstrongerthanHBrO
3
pf3
pf4
pf5

Partial preview of the text

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Mar 108:30 AM

There are three definitions for acids and bases we will need to understand. Arrhenius definition but Concept most limiting.: an acid supplies H+^ to an aqueous solution. A base supplies OH to an aqueous solution. This is the oldest BronstedLowry becomes a base (acting Concept in the : an reverse acid is direction); a proton (H when+ ) donor. a base Aaccepts base isa proton,a proton it (H becomes+ ) acceptor. an acid When (acting an (^) inacid the donates reverse a direction). proton, it You will need to identify conjugate acidbase pairs. Example: Formic acid, HCOOH: (IUPAC name: methanoic acid)

Mar 108:31 AM

Lewis Concept : an acid is an electron pair acceptor. A base is an electron pair donor. This is the most wideranging of the three (i.e. H it works for everything). Examples of Lewis acids include Al3+ , H+ , BF 3. Examples of Lewis bases include NO 2 ,^ NH 3 ,^ and 2 O. Identify the Lewis acid and base in each of the following reactions and name to product ion that forms: Cu2+ (aq) + 4NH 3 (aq) = Cu(NH 3 ) 4 2+ (aq) I (aq) + I 2 (aq) = I 3 (aq) Fe3+ (aq) + 6H 2 O(l) = Fe(H 2 O) 6 3+ (aq) Indicate the BL acidbase conjugate pairs and identify the Lewis acid and base in the hydrated iron(II) ion, [Fe(H 2 O) 6 ]3+

Mar 108:31 AM

AcidBase Strength The strength of an acid is indicated by the equilibrium position of the dissociation reaction. If the equilibrium lies far to the left (as indicated by the small value of Ka), the acid does not dissociate to any great extent and is weak: HA = H^ +^ + A i c (^100020 0) +20 (^0) + e 980 20 20 If the equilibrium lies far to the right, the acid strongly dissociates and is strong: HA = H+^ + A i c (^1000999) +999 (^0) +999 0 e 1 999 999 Note that: the the strongerstronger thethe acid,base, the the weakerweaker itsits conjugateconjugate base acid (BL(BL acids)bases) You need to memorize the strong acids and bases given below. All others can be considered weak.

Mar 108:32 AM

Strong acids HCl hydrochloric acid HClO 3 chloric acid HBr HI hydrobromichydroiodic acid acid HHClO 4 perchloric acid HNO^2 SO^4 sulfuric^ acid 3 nitric^ acid

Mar 108:32 AM

Strong Bases Metal Hydroxides: these are group 1 and group 2 metals bonded to a hydroxyl group. For example NaOH, Mg(OH) 2 , etc. Metal Oxides: these are group 1 and group 2 metals bonded with oxygen. In water they “attack” the water molecule and break it up to form hydroxide ion, OH. K 2 O^ +^ H 2 O^ =^ 2K+^ +^ 2OH

Mar 108:32 AM

AcidBase Behavior and Chemical Structure general Sometimestrends are useful^ you^ willin explaining^ need^ to^ compare differences:^ the^ strengths^ of^ acids.^ This^ can^ be^ done^ by^ considering^ the^ structure^ of^ the^ acids.^ Two For acid hydrides (HCl, HBr, etc.) the acidity increases within a group as the size of the central atom increases. This is because bond strength decreases as size increases. HF is a weak acid because the HF bond is strong. Another factor to consider is the ability of the anion formed (X ) – the greater the stability of the conjugate base, the stronger the acid is. For example, HI > HBr > HCl For oxyacids that have the same number of OH groups and the same number of O atoms, acid strength increases with increasing electronegativity of the central atom. For example, HClO > HBrO > HIO HClO 3 is stronger than HBrO 3

Mar 108:32 AM

  1. the For number oxyacids of oxygen that have atoms the attacheda central to atom, the central acid strength atom increases. increases as H 2 SO 3 For^ example,^ H^2 SO^4 is^ stronger^ than

Mar 108:33 AM

AcidBase Properties of Oxides An acidic oxide is a nonmetal oxide. Nonmetal oxides (also referred to as acid anhydrides ) react in water to produce acidic solutions. For example, NO 2 (g) + H 2 O(l) = HNO 3 (aq) ‡ H+ (aq) + NO 3 (aq) A basic oxide is a metal oxide. Metal oxides (also referred to as basic anhydrides ) react with water to produce a basic solution. For example, Na 2 O(s) + H 2 O(l) = 2NaOH(aq) ‡ Na+ (aq) + OH (aq)

Mar 108:33 AM

Autoionization of Water Water is an amphoteric substance (it can act as a BL acid or base). For the autoionization reaction, 2 H 2 O(l) = H 3 O+ (aq) + OH (aq) and Kw = 1.0 X 1014 at 25 o C (the simplified reaction can be expressed as: H 2 O(l) = H+ (aq) + OH (aq) but H+^ immediately attacks a water to form H 3 O+ ) Note: H+^ and H 3 O+^ are used interchangeably

Mar 108:33 AM

CRITICAL POINT : in aqueous solutions, the ion product [H 3 O+ ][OH ] always must equal 1.0 X 1014 at 25 o C. To “p” something in chemistry means to take the negative log base 10. So, pH = log[H + ] or log[H 3 O+ ] psoup = log[soup] pH Scale^ pOH^ =^ log[OH^ ]^ pbrain^ =^ log[brain] [H+ ] 1 107 1014 pH 0 7 14 acid base

Mar 108:33 AM

Again note: Kw = [H 3 O+ ][OH ] = 1 X 1014 pKw = pH + pOH pH + pOH = 14 Also note: pX = log[X] [X] = 10 px Problem: Fill in pH = 6. pOH = [H+ ] = [OH ] = acidic, basic, or neutral?

Mar 108:33 AM

Calculating the pH of Acidic Solutions In order to properly assess acidbase problems in aqueous solution, you must recognize that autoionization of water is ALWAYS occurring in aqueous solutions, and be able to determine whether autoionization will contribute significantly to the acidbase character of a solution. Strong Acid Solutions to the right^ Calculating that is, thethe acidpH^ of completely^ strong^ acid dissociates.^ solutions^ is The^ in^ general autoionization^ fairly^ straightforward, of water is negligible^ because as^ the a contributordissociation ofequilibrium H +^ to the^ liessolution^ so^ far (via can contributeLe Chatelier). a relatively The rare large exception proportion to this of is H (^) +when (^) to the your solution. concentrated acid is exceptionally dilute (< 106 M). In that case water The bottom line is the [H+ ] at equilibrium = [strong acid]o, except in very dilute solutions. Problem pairs.: Calculate the pH and [OH ] of a 5.0 X 103 M perchloric acid solution. Indicate BronstedLowry acidbase conjugate

Mar 108:35 AM

Problem : Calculate Ka1 = 6.5 the X pH 102 of (^) , aK (^) a21.40 = 6.1 M oxalicX 105 acid. solution and the equilibrium concentrations of H 2 C 2 O 4 , HC 2 O 4 , C 2 O 42 , and OH. Note:

Mar 108:35 AM

Calculating the pH of Basic Solutions The key to understanding the pH of basic solutions is to recognize that, in an equilibrium sense, bases work in the same way acids Using do.lithium Just hydroxideas there are in (^) waterboth strong as an example,and weak acids, there are both strong and weak bases. Strong bases completely dissociate. LiOH(s) (^) ‡ Li+ (aq) + OH (aq) Therefore, one can consider that [OH ] = [LiOH]o. Once you know [OH ], you can use Kw to calculate [H+ ] and pH. Recall hydroxides. that all alkali hydroxides are strongly basic. Alkaline earth hydroxides are strongly basic, but somewhat less soluble than alkali Weak bases react with water (we use the term “undergo hydrolysis”) as described in the following reaction: B(aq) + H 2 O(l) = BH+ (aq) + OH (aq) base1 acid2 acid1 base

Mar 108:36 AM

Problem : Calculate the pH of a 0.350 M solution of methylamine, CH 3 NH 2. The Kb = 4.38 X 104.

Mar 108:36 AM

AcidBase Properties of Salts Salts are ionic compounds. They dissociate in water and may exhibit acidbase behavior. The key question in deciding whether each component a salt will (^) act of (^) the as an salt? acidic,” basic, or neutral species in solution is “ What are the acidbase properties, and strengths, of Salts that consist of the cations of strong bases and the anions of strong acids have no effect on [H+ ]. The ions of these salts do not “react” with water; they have no acidbase properties. Example: KCl Ions having no acid base properties: Li Na+^ + ClI K+^ Br Rb+^ NO 3

Mar 108:41 AM

  1. basic Salts solution. whose cation has neutral properties (i.e. Na+ , K+ ) and whose anion is the conjugate base of a weak acid will produce a Example: NaC 2 H 3 O 2
  2. Salts in which the cation is the conjugate acid of a weak base will produce an acidic solution. Example: NH 4 Cl

Mar 108:41 AM

  1. For a salt where both cation and anion exhibit acidbase behavior, the overall pH is determined by comparing K (^) a with Kb. NOTE: Kw = Ka X Kb (for conjugate pairs) Example: NH 4 C 2 H 3 O 2

Mar 108:42 AM

Problem : Using tables of constants, predict whether each of the following will create an acidic, basic, or neutral solution. Na 3 PO 4 KI NH 4 F

Mar 108:42 AM

Problem : Calculate the pH of a 0.500 M NaNO 2 solution. (Ka for HNO 2 = 4.0 X 104 ).

Mar 108:42 AM

Problem : Calculate the pH of a 0.800 M NH 4 CN solution. The Ka for HCN = 6.2 X 1010 , the Kb for NH 3 = 1.8 X 105.