Acid-Base EquilibriumAcid-Base Equilibrium

Dr. Ron RusayDr. Ron Rusay

Introduction to Aqueous AcidsIntroduction to Aqueous Acids

Acids: taste sour and cause certain dyes Acids: taste sour and cause certain dyes to change color.to change color.

Introduction to Aqueous BasesIntroduction to Aqueous Bases

Bases: taste bitter, feel soapy and cause certain dyes to turn color.Bases: taste bitter, feel soapy and cause certain dyes to turn color.

Models of Acids and BasesModels of Acids and Bases

ArrheniusArrhenius: Acids produce H: Acids produce H++ & bases & bases produce OHproduce OH ion in aqueous solutions . ion in aqueous solutions .Brønsted-LowryBrønsted-Lowry: Acids are H: Acids are H++ donors & donors & bases are proton acceptors.bases are proton acceptors.

HCl + HHCl + H22O O Cl Cl + H + H33OO++

acid baseacid base

Lewis Acids and BasesLewis Acids and Bases

Lewis AcidLewis Acid: electron pair : electron pair acceptoracceptor Lewis BaseLewis Base: electron pair : electron pair donordonor Example:Example:Al3+ + 6HOHAlOHH63+

Lewis Acids and BasesLewis Acids and Bases

The Mg2+ Ion is a Lewis Acid in the Chlorophyll Molecule; Fe2+ Ion is a Lewis Acid in normal

Hemoglobin

Conjugate Acid/Base PairsConjugate Acid/Base Pairs

HA(aq) + HHA(aq) + H22O(l) O(l) H H33OO++(aq) + A(aq) + A(aq)(aq)conj conjconj conj

acid 1 base 2 acid 2 base 1acid 1 base 2 acid 2 base 1conjugate acidconjugate acid: formed when the proton : formed when the proton is transferred to the base.is transferred to the base.conjugate baseconjugate base: everything that remains : everything that remains of the acid molecule after a proton is lost.of the acid molecule after a proton is lost.

http://chemconnections.org/general/movies/ConjugateAcidBaseActivity.swf

Strong & Weak Acids: Strong & Weak Acids: Dissociation Constant (Dissociation Constant (KKaa))

HA(aq) + HHA(aq) + H22O(l) O(l) H H33OO++(aq) + A(aq) + A(aq)(aq)

HA(aq) HA(aq) H H++(aq) + A(aq) + A(aq)(aq)

Ka3HOAHAHAHA==+−+−

Acid StrengthAcid Strength

Equilibrium position lies far to the right. Equilibrium position lies far to the right. (HNO(HNO33); Ka >> 1); Ka >> 1

Produces a conjugate base. Produces a conjugate base. (NO(NO33) and a ) and a

conjugate acid which are weaker than conjugate acid which are weaker than the starting acid and base (Hthe starting acid and base (H22O).O).

Strong Acid:Strong Acid:

Acid StrengthAcid Strength

Strong Acids:Strong Acids:

Acid StrengthAcid Strength(continued)(continued)

Equilibrium lies far to the left. Equilibrium lies far to the left. (CH(CH33COOH); Ka < 1COOH); Ka < 1

Yields a stronger (relatively strong) Yields a stronger (relatively strong) conjugate base than water. conjugate base than water. (CH(CH33COOCOO))

Weak Acid:Weak Acid:

Weak AcidsWeak Acids

Weak acids are only partially ionized in solution.Weak acids are only partially ionized in solution.

or or

KKaa is the acid dissociation constant. is the acid dissociation constant.

HA(aq) + H2O(l) H3O+(aq) + A-(aq)

HA(aq) H+(aq) + A-(aq)

[HA]

]][AO[H -3

+=aK [HA]

]][A[H -+=aK

Percent IonizationPercent Ionization Percent ionization is a way to assess relative acid Percent ionization is a way to assess relative acid

strengths.strengths. For the reactionFor the reaction

Percent ionization relates the Percent ionization relates the HH33OO++ ((aqaq)) equilibrium equilibrium

concentration, concentration, [H[H++]]eqmeqm, to the initial , to the initial HA(HA(aqaq))

concentration, concentration, [HA][HA]00..

100]HA[

][Hionization %

0×=

+eqm

HA(aq) + H2O(l) H3O+(aq) + A-(aq)

The Extent of The Extent of Dissociation Dissociation for Strong for Strong and Weak and Weak

AcidsAcids

Weak AcidsWeak Acids

The higher percent ionization, the The higher percent ionization, the stronger the acid.stronger the acid.

Percent ionization of a weak acid Percent ionization of a weak acid decreases as the molarity of the decreases as the molarity of the solution increases.solution increases.

For acetic acid, 0.05 For acetic acid, 0.05 MM solution is 2.0 % solution is 2.0 % ionized whereas a 0.15 ionized whereas a 0.15 MM solution is 1.0 solution is 1.0 % ionized.% ionized.

Weak AcidsWeak AcidsPercent IonizationPercent Ionization

QUESTIONQUESTIONNitric acid, HNO3, is considered to be a strong acid whereas nitrous acid, HNO2, is considered to be a weak acid. Which of the statements here is fully correct?

A. Nitric acid has an aqueous equilibrium that lies far to theright and NO3

– is considered a weak conjugate base.B. Nitric acid has a stronger conjugate base than nitrous acid.C. The dissociation of nitrous acid compared to an equal

concentration of nitric acid produces more H+.D. The equilibrium of nitrous acid lies far to the left and the

conjugate base is weaker than the conjugate base of nitricacid.

ANSWERANSWERA) correctly compares equilibrium and conjugate base

characteristics. The conjugate base of a strong acid is considered to be weak. The stronger the acid, the more reaction in water. Therefore, a weak acid’s equilibrium is favored to the left.

BasesBases

““StrongStrong” and “” and “weakweak” are used in the ” are used in the same sense for bases as for acids.same sense for bases as for acids.StrongStrong = = complete dissociation, Kcomplete dissociation, Kbb >> 1 >> 1 (concentration of hydroxide ion in solution)(concentration of hydroxide ion in solution)

NaOHNaOH(s)(s) Na Na++(aq)(aq) + OH + OH(aq)(aq)NaOHNaOH(s)(s) + H + H22OO(l)(l)

NaNa++(aq)(aq) + OH + OH(aq)(aq)

BasesBases(continued)(continued)

WeakWeak bases have very bases have very little dissociation, Klittle dissociation, Kbb < 1 < 1 ( little ionization with water)( little ionization with water)

CHCH33NHNH22(aq)(aq) + H + H22OO(l) (l)

CHCH33NHNH33++(aq)(aq) + OH + OH(aq)(aq)

How conductive is NaOHHow conductive is NaOH(aq)(aq) vs morphine, vs morphine, CC1717HH1919NONO3 3 (aq)(aq)??

QUESTIONQUESTIONAniline, C6H5NH2, was isolated in the 1800s and began immediate use in the dye industry. What is the formula of the conjugate acid of this base?

A. C6H5NH2+

B. C6H5NH3+

C. C6H5NH–

D. C6H5NH+

ANSWERANSWERB) correctly represents the result of aniline accepting a H+ ion as bases typically do. The conjugate acid of a base is represented as the base with the addition of a H+.

Acid-Base StrengthsAcid-Base Strengths

Strong Acid:

Strong Base:

Weak Base:

Weak Acid:

Water as an Acid and a BaseWater as an Acid and a Base Self-ionization Self-ionization

Water as an Acid and a BaseWater as an Acid and a Base

Water is Water is amphotericamphoteric (it can behave (it can behave either as an acid or a base).either as an acid or a base).

HH22O + HO + H22O O H H33OO++ + OH + OH

conj conjconj conj acid 1 base 2 acid 2 base 1acid 1 base 2 acid 2 base 1

KKww = 1 = 1 10 101414 at 25°C at 25°C

H2O(l) + H2O(l) H3O+(aq) + OH-(aq)

Water as an Acid and a BaseWater as an Acid and a Base Self-ionization Self-ionization

http://chemconnections.org/general/movies/KwActivity.swf

The pH ScaleThe pH Scale pH pH log[Hlog[H++] ] log[Hlog[H33OO++] ]

pH in water ranges from 0 to 14.pH in water ranges from 0 to 14.

KKww = 1.00 = 1.00 10 101414 = [H = [H++] [OH] [OH]]

pKpKww = = 14.00 = pH + pOH14.00 = pH + pOH As pH rises, pOH falls (sum = 14.00).As pH rises, pOH falls (sum = 14.00). There are no theoretical limits on the values There are no theoretical limits on the values

of pH or pOH. of pH or pOH. (e.g. pH of 2.0 (e.g. pH of 2.0 MM HCl is -0.301) HCl is -0.301)

The pH Values of Some Familiar

Aqueous Solutions

[H3O+]

[OH-]

[OH-] = KW

[H3O+]

neutralsolution

acidicsolution

basicsolution

[H3O+]> [OH-]

[H3O+]< [OH-]

[H3O+] = [OH-]

QUESTIONQUESTIONIn a solution of water at a particular temperature the [H+] may be 1.2 10–6 M. What is the [OH–] in the same solution? Is the solution acidic, basic, or neutral?

A. 1.2 10–20 M; acidicB. 1.2 10–20 M; basicC. 8.3 10–9 M; basicD. 8.3 10–9 M; acidic

ANSWERANSWERD. correctly shows the OH– molarity and classifies the solution as acidic. Kw = [H+][OH–] = 1.0 10–14 at 25°C. The H+ molarity is approximately 1,000 times greater than the OH–

concentration. Solutions with higher H+ concentrations than OH– are acidic.

pH Estimation

http://chemconnections.org/general/movies/pHEstimation.swf

QUESTIONQUESTIONAn environmental chemist obtains a sample of rainwater near a large industrial city. The [H+] was determined to be 3.5 10–6

M. What is the pH, pOH, and [OH–] of the solution?

A. pH = 5.46 ; pOH = 8.54; [OH–] = 7.0 10–6 MB. pH = 5.46 ; pOH = 8.54; [OH–] = 2.9 10–9 MC. pH = 12.56 ; pOH =1.44 ; [OH–] = 3.6 10–2 M D. pH = 8.54; pOH = 5.46; [OH–] = 2.9 10–9 M

ANSWERANSWERB. provides all three correct responses. The expression pH = –log[H+] can be used to find the pH then: 14.00 = pH + pOH

can be used to obtain the pOH. Finally, [OH–] = 10–pOH.

The pH ScaleThe pH Scale

Name: ____________________ Partner (if any): ____________________

The pH ScaleThe pH Scale

1.3 x10 1.3 x10 -12 -12 2.12.1 11.911.9

2.8 x10 2.8 x10 -5 -5 4.64.6

5.62 x10 5.62 x10 - 9 - 9

9.49.4

5.755.75

8.308.305.00 x10 5.00 x10 - 9 - 9

1.78 x10 1.78 x10 -6-6

2.00 x10 2.00 x10 - 6- 6

Acid Acid

Acid Acid

Base Base

Base Base

IndicatorsIndicators

Acid-Base IndicatorsAcid-Base Indicators

Titrations: Indicators & (pH) Titrations: Indicators & (pH) CurvesCurves

pH Curve is a plot of pH of the solution pH Curve is a plot of pH of the solution being analyzed as a function of the being analyzed as a function of the

amount of titrant added.amount of titrant added.Equivalence (stoichiometric) pointEquivalence (stoichiometric) point: :

Enough titrant has been added to Enough titrant has been added to react react exactly exactly with the solution being analyzed. with the solution being analyzed.

An indicator provides a visible color An indicator provides a visible color change to determine an (end point) change to determine an (end point)

volume of titrant.volume of titrant.

QUESTIONQUESTIONMost acid-base indicators are weak acids. In a titration of 0.50 M acetic acid (at 25°C, Ka = 1.8 10–5) with KOH, which indicator would best indicate the pH at the equivalence point? The approximate Ka for each choice is provided.

A. Bromophenol blue; Ka ~ 1 10–4

B. Methyl red; Ka ~ 1 10–5

C. Bromothymol blue; Ka ~ 1 10–7

D. Alizarin yellow; Ka ~ 1 10–10

ANSWERANSWERD. provides the best choice although there may also be better choices available than these four. The equivalence point pH should be as close as possible to the pKa of the indicator. As acetic acid is a fairly weak acid and NaOH is a strong base, the pH at the equivalence point will be above 7. The only choice above 7 in the list was Alizarin yellow. Without a more detailed calculation, this would be the best choice.

Methods for Measuring the pH of an Aqueous Solution

(a) pH paper (b) Electrodes of a pH meter

QUESTIONQUESTIONThe acid-base indicator bromocresol purple has an interesting yellow-to-purple color change. If the approximate Ka of this indicator is 1.0 10–6, what would be the ratio of purple [A–] to yellow [HA] at a pH of 4.0?

A. 100:1B. 1:100C. 1:1D. This choice indicates that I don’t know.

ANSWERANSWERB. shows the [A–]/[HA] ratio at pH 4.0 for bromocresol purple. The pH can be converted to [H+] and divided into the Ka value to reveal the [A–]/[HA] ratio at pH 4.0.Ka/[H+] = [A–]/[HA].

ConjugatesConjugates

KKaa x K x Kbb = ? = ?KKaa x K x Kbb = K = Kww

ConjugatesConjugates

KKaa x K x Kbb = K = Kww

What do pKWhat do pKa a and pKand pKb b refer to?refer to?

pKpKaa + pK + pKbb = ? = ?pKpKaa + pK + pKbb = pK = pKww

http://chemconnections.org/general/movies/Ka-KbActivity.swf

QUESTIONQUESTION

Use information on this table to determine which of the following bases would have the weakest conjugate acid:

OC6H5–; C2H3O2

–; OCl–; NH3

A. OC6H5–

B. C2H3O2–

C. OCl– D. NH3

ANSWERANSWERA. correctly identifies the base, among these four, with the

weakest conjugate acid. The Ka’s in the table can be used to compare conjugate acid strength. The higher the Ka value, the stronger the acid.

What are the respective equilibrium concentrations of What are the respective equilibrium concentrations of HH++ ((HH33OO++)?)?

pH is calculated from the equilibrium concentration of pH is calculated from the equilibrium concentration of HH++ ((HH33O)O)

Using Using KKaa, and the starting molarity of acid, the , and the starting molarity of acid, the equilibrium concentration of equilibrium concentration of HH++ ((HH33OO++)) can be can be estimated and then pHestimated and then pH); ); Strong acids 100%, Strong acids 100%, pH=1.00pH=1.00, Weak: less than 100%, Weak: less than 100%

Strong vs.Weak AcidsStrong vs.Weak AcidspH Estimations/ Calculations

What are the respective pH values for a 0.100M solution of HCl (Ka = ) and a 0.100M solution of HF (Ka = 3.53 x 10-4)?

Using Using KKaa, and the starting molarity of the weak acid, , and the starting molarity of the weak acid, the equilibrium concentration of the equilibrium concentration of HH++ ((HH33OO++)) can be can be estimated using an ICE approach and then the pH.estimated using an ICE approach and then the pH.

Ka = [H+][A–] / [HA – x] = x2/(0.100 M – x)

3.53 10–4 = x2/0.100; estimate @ x (10–5)1/2 representing the [H+], taking –log yields a pHpH >2 and <3.

Strong vs.Weak AcidsStrong vs.Weak AcidspH Estimations/ Calculations

What are the respective pH values for a 0.100M solution of HCl (Ka = ) and a 0.100M solution of HF (Ka = 3.53 x 10-4)?

QUESTIONQUESTIONWhich of the following correctly compares strength of acids, pH, and concentrations?

A. A weak acid, at the same concentration of a strong acid,will have a lower pH.

B. A weak acid, at the same concentration of a strong acid,will have the same pH.

C. A weak acid, at a high enough concentration more than astrong acid, could have a lower pH than the strong acid.

D. A weak acid, at a concentration below a strong acid,could have a lower pH than a strong acid.

ANSWERANSWERC. correctly predicts that it is possible to have a high enough concentration of the weak acid compared to a strong acid, and that the pH of the weaker acid would be lower (more acidic) than the more dilute stronger acid. Strength of an acid refers to its dissociation. The pH of a solution depends on the concentration, regardless of source, of the H+ ion.

Weak AcidsWeak AcidsKKaa and Calculating pH and Calculating pH

Write the balanced chemical equation clearly Write the balanced chemical equation clearly showing the equilibrium.showing the equilibrium.

Write the equilibrium expression. Use the Write the equilibrium expression. Use the value for value for KKaa

Let Let x =x = [[HH++];]; substitute into the equilibrium substitute into the equilibrium constant expression and solve. constant expression and solve.

Convert Convert [[HH++]] to pH. to pH.

Equilibrium Concentration Calculations Equilibrium Concentration Calculations pH from Initial Concentrations and KpH from Initial Concentrations and Kaa

What is the pH value for a 0.100M solution of HF (Ka = 3.53 x 10-4)?

HF(aq) H +

(aq) + F - (aq)

Ka =[H

+] [F -]

[HF]

Equilibrium Concentration CalculationsEquilibrium Concentration Calculations

Concentration (M) HF H +

F -

__________________________________________Initial 0.100 0 0Change 0.100-x +x +xFinal 0.100-x x x

Kc = = 3.53 x 10 -4 = [H +][F -]

[HF]x2

(0.100 - x)

Quadratic:0 = x2 + 3.53 x 10 -4 x - 3.53 x 10 -5

3.53 x 10 -4 (0.100 - x) = x2

x=[H +] = 0.00594 M; pH= 2.23

HF(aq) H +

(aq) + F - (aq)

Simplified:3.53 x 10 -4 =

x2

(0.100 )

3.53 x 10 -4 (0.100 ) = x2

x= [3.53 x 10 -4 (0.100 ) ]1/2x=[H

+] = 0.00805 M; pH= 2.09

QUESTIONQUESTIONButyric acid is a weak acid that can be found in spoiled butter. The compound has many uses in synthesizing other flavors. The Ka of HC4H7O2 at typical room temperatures is 1.5 10–5. What is the pH of a 0.20 M solution of the acid?

A. 5.52B. 4.82C. 2.76D. –0.70

ANSWERANSWERC. is correct assuming that the amount of dissociation of this weak acid is negligible when compared to its molarity.

Ka = [H+][A–] / [HA – x] = x2/(0.20 M – x)

becomes 1.5 10–5 = x2/0.20; once x is found, representing the [H+], taking –log of that yields the pH.

QUESTIONQUESTIONA 0.35 M solution of an unknown acid is brought into a lab. The pH of the solution is found to be 2.67. From this data, what is the Ka value of the acid?

A. 6.1 10–3

B. 1.3 10–5

C. 7.5 10–4

D. 2.1 10–3

ANSWERANSWERB. shows the unknown acid’s Ka value. The pH could be used to find the [H+] concentration , 10–2.67, then the Ka expression only has one unknown:

Ka = [0.00214][0.00214]/(0.35–0.00214)

Ka Values of Some Hydrated Metal Ions at 25oC

Ion Ka

Fe3+ (aq) 6 x 10-3

Sn2+ (aq) 4 x 10-4

Cr3+ (aq) 1 x 10-4

Al3+ (aq) 1 x 10-5

Be2+ (aq) 4 x 10-6

Cu2+ (aq) 3 x 10-8

Pb2+ (aq) 3 x 10-8

Zn2+ (aq) 1 x 10-9

Co2+ (aq) 2 x 10-10

Ni2+ (aq) 1 x 10-10

OxidesOxides

Acidic Oxides (Acid Anhydrides):Acidic Oxides (Acid Anhydrides):

OOX bond is strong and covalent.X bond is strong and covalent.

SOSO22, NO, NO22, CrO, CrO33

Basic Oxides (Basic Anhydrides):Basic Oxides (Basic Anhydrides):

OOX bond is ionic.X bond is ionic.

KK22O, CaOO, CaO

Structure and Acid-Base Structure and Acid-Base PropertiesProperties

Two important factors that effect acidity in Two important factors that effect acidity in binary binary compounds, eg. HCl (aq):compounds, eg. HCl (aq):

Bond Polarity Bond Polarity (smaller e.n. differences favor higher (smaller e.n. differences favor higher acidities)acidities)

Bond Strength Bond Strength (weak bonds favor higher acidity: more (weak bonds favor higher acidity: more protons [hydronium ions] in solution)protons [hydronium ions] in solution)

Select & explain which is the stronger acid: HBr vs. Select & explain which is the stronger acid: HBr vs. HF.HF.

http://chemconnections.org/general/movies/pHofSaltSolutions.swf

QUESTIONQUESTIONThe following salts were all placed in separate solutions at the same temperature so that their concentrations were all equal. Arrange them in order from lowest pH to highest pH.

NaCl; NH4NO3; Ca(C2H3O2)2; AlCl3

Additional information: Kb for NH3 = 1.8 10–5; Ka for HC2H3O2 = 1.8 10–5; Ka for Al(H2O)3+ = 1.4 10–5.

A. NaCl; NH4NO3; Ca(C2H3O2)2; AlCl3

B. AlCl3; NaCl; NH4NO3; Ca(C2H3O2)2

C. AlCl3; NH4NO3; NaCl; Ca(C2H3O2)2

D. NH4NO3; AlCl3; NaCl; Ca(C2H3O2)2

ANSWERANSWERC. correctly ranks the salt solutions from lowest pH (most acidic solution) to highest pH. The ranking is based on production of H+ from the salt ions interacting with water. Highly charged small metal ions such as Al3+ can produce H+ as can NH4

+. However, the Ka of the aluminum’s reaction is larger than the Ka for NH4

+. NaCl is neutral and the acetate ion undergoes a reaction that produces OH–, so it has a high pH.

Strength of OxyacidsStrength of Oxyacids

Strength of OxyacidsStrength of Oxyacids

Name the acids:Name the acids:

HBrO , HBrO , KKa a = 2.1 x 10= 2.1 x 10-8-8

HIO , HIO , KKa a = 2.3 x 10= 2.3 x 10-11-11

HClO , HClO , KKa a = 3.0 x 10= 3.0 x 10-8-8

HClOHClO2 2 , , KKa a = 1.2 x 10= 1.2 x 10-2-2

Is Is HBrOHBrO3 3 stronger or weaker than stronger or weaker than HClOHClO33??

A) stronger or B) weakerA) stronger or B) weaker

Strength of OxyacidsStrength of Oxyacids

Name the acids:Name the acids:

HBrO , HBrO , KKa a = 2.1 x 10= 2.1 x 10-8-8

HIO , HIO , KKa a = 2.3 x 10= 2.3 x 10-11-11

HClO , HClO , KKa a = 3.0 x 10= 3.0 x 10-8-8

HClOHClO2 2 , , KKa a = 1.2 x 10= 1.2 x 10-2-2

Is Is HBrOHBrO3 3 stronger or weaker than stronger or weaker than HClOHClO33??

A) stronger or A) stronger or B) weakerB) weaker

QUESTIONQUESTION

Rank 1.0M solutions of Rank 1.0M solutions of HBrO,HBrO, HIOHIO and and HClO HClO in order in order of increasing acidity.of increasing acidity.

HBrO , HBrO , KKa a = 2.1 x 10= 2.1 x 10-8-8

HIO , HIO , KKa a = 2.3 x 10= 2.3 x 10-11-11

HClO , HClO , KKa a = 3.0 x 10= 3.0 x 10-8-8

A) HBrO <A) HBrO < HIOHIO < < HClOHClO B) HIO <B) HIO < HBrOHBrO < < HClOHClO

C) HClO <C) HClO < HBrOHBrO < < HIOHIO D) HIO <D) HIO < HClOHClO < < HBrOHBrOANSWER: B) HIO <ANSWER: B) HIO < HBrOHBrO < < HClO HClO

(Increasing Ka values)(Increasing Ka values)

QUESTIONQUESTION

Rank 1.0M solutions of Rank 1.0M solutions of HBrO,HBrO, HIOHIO and and HClO HClO in in order of increasing pH.order of increasing pH.

HBrO , HBrO , KKa a = 2.1 x 10= 2.1 x 10-8-8

HIO , HIO , KKa a = 2.3 x 10= 2.3 x 10-11-11

HClO , HClO , KKa a = 3.0 x 10= 3.0 x 10-8-8

A) HBrO <A) HBrO < HIOHIO < < HClOHClO B) HIO <B) HIO < HBrOHBrO < < HClOHClO

C) HClO <C) HClO < HBrOHBrO < < HIOHIO D) HIO <D) HIO < HClOHClO < < HBrOHBrOANSWER: C) HClO <ANSWER: C) HClO < HBrOHBrO < < HIO HIO

(pH ~ -log Ka values)(pH ~ -log Ka values)

Strength of AcidsStrength of Acids

Strength of AcidsStrength of Acids

Br-CHBr-CH22COOH, I-CHCOOH, I-CH22COOH, CHCOOH, CH33COOHCOOH

pKa=2.69 pKa=3.12pKa=2.69 pKa=3.12 pKa=4.75pKa=4.75

1)1) Is chloroacetic acid more or less acidic than bromoacetic acid?Is chloroacetic acid more or less acidic than bromoacetic acid?2)2) Will its pKa be higher or lower than bromoacetic acid?Will its pKa be higher or lower than bromoacetic acid?

A) 1.more 2.higher B) 1.less 2.lower C) 1.less 2.higher D) 1.more 2.lower

Strength of AcidsStrength of Acids

Br-CHBr-CH22COOH, I-CHCOOH, I-CH22COOH, CHCOOH, CH33COOHCOOH

pKa=2.69 pKa=3.12pKa=2.69 pKa=3.12 pKa=4.75pKa=4.75

1)1) Is chloroacetic acid more or less acidic than bromoacetic acid?Is chloroacetic acid more or less acidic than bromoacetic acid?2)2) Will its pKa be higher or lower than bromoacetic acid?Will its pKa be higher or lower than bromoacetic acid?

D) 1.more 2.lower

QUESTIONQUESTION

Rank the following acids in order of Rank the following acids in order of decreasing acidity.decreasing acidity.

1) Br-CH1) Br-CH22COOH, 2) I-CHCOOH, 2) I-CH33COOH, 3) CHCOOH, 3) CH33COOHCOOH

pKa= 2.69pKa= 2.69 pKa=3.12 pKa=3.12 pKa=4.75 pKa=4.75

A) 1 > 2 > 3A) 1 > 2 > 3 B) 3 > 2 > 1B) 3 > 2 > 1 C) 2 > 3 > 1C) 2 > 3 > 1

ANSWER: B) 3 > 2 > 1; LOWER pKa HIGHER Acidity ANSWER: B) 3 > 2 > 1; LOWER pKa HIGHER Acidity

QUESTIONQUESTIONAscorbic acid, also known as vitamin C, has two hydrogen atoms that ionize from the acid. Ka1 = 7.9 10–5; Ka2 = 1.6 10–12. What is the pH, and C6H6O6

2– concentration of a 0.10 M solution of H2C6H6O6?

•2.55; [C6H6O62–] = 0.050 M

•2.55; [C6H6O62–] = 1.6 10–12 M

•1.00; [C6H6O62–] = 1.6 10–12 M

•5.10; [C6H6O62–] = 0.050 M

ANSWERANSWERB. shows both correct answers. In a diprotic acid with two small, widely separated Ka values the pH of a solution can be obtained from using the first Ka and the molarity. The concentration of the dianion can be closely approximated by assuming very little dissociation of the second acidic hydrogen, so that Ka2 is very close to the molarity.

Amino AcidsAmino Acids

More than 700 amino acids occur naturally, but 20 More than 700 amino acids occur naturally, but 20 (22?) of them are especially important.(22?) of them are especially important.

These 20 amino acids are the building blocks of These 20 amino acids are the building blocks of proteins in humans and developed organismsproteins in humans and developed organisms

They differ in respect to the group attached to the They differ in respect to the group attached to the carbon. Why do you suppose they are written with + carbon. Why do you suppose they are written with + and – charges?and – charges?

CC CC

OO

OO––

RRRR

HH

HH33NN++

Amino AcidsAmino Acids

•Our bodies can synthesize about 10 amino acids.Our bodies can synthesize about 10 amino acids.

•Essential amino acids are the other 10 amino Essential amino acids are the other 10 amino acids, which have to be ingested.acids, which have to be ingested.

•The The -carbon in all amino acids except glycine is -carbon in all amino acids except glycine is chiral (has 4 different groups attached to it).chiral (has 4 different groups attached to it).

•Chiral molecules exist as two non-superimposable Chiral molecules exist as two non-superimposable mirror images called enantiomers.mirror images called enantiomers.

•L-amino acids are the common natural L-amino acids are the common natural enantiomers.enantiomers.

Amino AcidsAmino Acids

•L-amino acids are the common natural L-amino acids are the common natural enantiomers, eg. Alanine above.enantiomers, eg. Alanine above.

Sickle Cell AnemiaSickle Cell AnemiaNormal hemoglobin vs sickle cell hemoglobinNormal hemoglobin vs sickle cell hemoglobin

Valine replaces GlutamateValine replaces Glutamate

Sickle Cell AnemiaSickle Cell AnemiaNormal hemoglobin vs sickle cell hemoglobinNormal hemoglobin vs sickle cell hemoglobin

http://chemconnections.org/Presentations/Columbia/slide9-3.html

Neutralization ReactionsNeutralization Reactions

Would there be a difference in the reaction of HF versus HCl?

Neutralizations / TitrationsNeutralizations / Titrations

Are there differences in the titration of HF versus HCl? ….1) mass wise? …. 2) pH wise?

A) 1.NO 2.NO B) 1.YES 2.YES C) 1.YES 2.NO D) 1.NO 2.YES

Neutralizations / TitrationsNeutralizations / Titrations

Are there differences in the titration of HF versus HCl? ….1) mass wise? …. 2) pH wise?

D) 1.NO 2.YES

What is the pH of a solution made from adding 500. mL of What is the pH of a solution made from adding 500. mL of 2.00 M HOAc2.00 M HOAc(aq)(aq) (Ka = 1,8 x 10 (Ka = 1,8 x 10 -5-5) to 100. mL of 5.100M ) to 100. mL of 5.100M

NaOH NaOH (aq)(aq) ? ?

(This question relates to the titration of acetic acid.)(This question relates to the titration of acetic acid.)

QUESTIONQUESTION

A) 4.74A) 4.74 B) 4.76B) 4.76 C) 9.24C) 9.24 D) 9.26D) 9.26

What is the pH of a solution made from adding What is the pH of a solution made from adding 500. mL of 2.00 M HOAc500. mL of 2.00 M HOAc(aq)(aq) (Ka = 1,8 x 10 (Ka = 1,8 x 10 -5-5) to ) to

100. mL of 5.100M NaOH 100. mL of 5.100M NaOH (aq)(aq) ? ?

QUESTIONQUESTION

A) 4.74A) 4.74 B) 4.76B) 4.76 C) 9.24C) 9.24 D) 9.26D) 9.26

QUESTIONQUESTIONMost acid-base indicators are weak acids. In a titration of 0.50 M acetic acid (at 25°C, Ka = 1.8 10–5) with KOH, which indicator would best indicate the pH at the equivalence point? The approximate Ka for each choice is provided.

A. Bromophenol blue; Ka ~ 1 10–4

B. Methyl red; Ka ~ 1 10–5

C. Bromothymol blue; Ka ~ 1 10–7

D. Alizarin yellow; Ka ~ 1 10–10

ANSWERANSWERD. provides the best choice although there may also be better choices available than these four. The equivalence point pH should be as close as possible to the pKa of the indicator. As acetic acid is a fairly weak acid and NaOH is a strong base, the pH at the equivalence point will be above 7. The only choice above 7 in the list was Alizarin yellow. Without a more detailed calculation, this would be the best choice.

QUESTIONQUESTIONThe acid-base indicator bromocresol purple has an interesting yellow-to-purple color change. If the approximate Ka of this indicator is 1.0 10–6, what would be the ratio of purple [A–] to yellow [HA] at a pH of 4.0?

A. 100:1B. 1:100C. 1:1D. This choice indicates that I don’t know.

ANSWERANSWERB. shows the [A–]/[HA] ratio at pH 4.0 for bromocresol purple. The pH can be converted to [H+] and divided into the Ka value to reveal the [A–]/[HA] ratio at pH 4.0.Ka/[H+] = [A–]/[HA].