Chem 150 Unit 6 - Chemical Properties II Acids and Bases Water has played a major role in our discussions so far this semester; and for good reason, water

Chem 150Chem 150Unit 6 - Chemical Properties IIUnit 6 - Chemical Properties II

Acids and BasesAcids and Bases

Water has played a major role in our discussions so Water has played a major role in our discussions so far this semester; and for good reason, water far this semester; and for good reason, water

comprises up to 70% of the mass of most living cells. comprises up to 70% of the mass of most living cells. As a consequence, the influence of its chemical and As a consequence, the influence of its chemical and

physical properties on living systems cannot be physical properties on living systems cannot be ignored. In this unit we will look at another chemical ignored. In this unit we will look at another chemical property of water: its ability to split in half and form property of water: its ability to split in half and form

hydrogen ions (Hhydrogen ions (H++) and hydroxyl ions (OH) and hydroxyl ions (OH--).).

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IntroductionIntroduction

Goals for Unit 6 include:Goals for Unit 6 include:

• Be able to describe some the chemical and physical Be able to describe some the chemical and physical characteristics of acids and bases.characteristics of acids and bases.

• Be able to describe the different definitions of acids and Be able to describe the different definitions of acids and base, and in particular, the Brønsted-Lowry definition.base, and in particular, the Brønsted-Lowry definition.

• Beable to describe how Brønsted-Lowry acids and bases Beable to describe how Brønsted-Lowry acids and bases are related to one another as conjugate acid/base pairs.are related to one another as conjugate acid/base pairs.

• Be able to write an equilibrium expression for a reversible Be able to write an equilibrium expression for a reversible reaction and be able to use Le Châtelier's principle to reaction and be able to use Le Châtelier's principle to predict how an equilibrium will respond when it is predict how an equilibrium will respond when it is perturbed.perturbed.

33

IntroductionIntroduction

Goals for Unit 6 include (con’d):Goals for Unit 6 include (con’d):

• Be able to use quantities such as [HBe able to use quantities such as [H33OO++], [OH], [OH--], ], pHpH to to determine whether a solution is acidic, basic or neutral.determine whether a solution is acidic, basic or neutral.

• Be able to describe the processes of neutralization and Be able to describe the processes of neutralization and titration.titration.

• Be able to describe how the Be able to describe how the pHpH of a solution affects the of a solution affects the relative concentrations of the acid and its conjugate base in relative concentrations of the acid and its conjugate base in a solution and be able to relate this to a buffer solution.a solution and be able to relate this to a buffer solution.

44


Some common acids and bases:Some common acids and bases:

55


Let’s take a trip to the lab and test what acids and bases do Let’s take a trip to the lab and test what acids and bases do to the to the pHpH of water of water

http://www.chem.uwec.edu/Chem150_S07//overheads/unit6/vlab-big.html

66


So far we have been looking at operational ways of So far we have been looking at operational ways of distinguishing acids from bases:distinguishing acids from bases:

OperationOperation AcidAcid BaseBase

Taste itTaste it SourSour BitterBitter

Feel itFeel it Not SlipperyNot Slippery SlipperySlippery

Effect onEffect onLitmus PaperLitmus Paper Turns PinkTurns Pink Turns BlueTurns Blue

Measure the Measure the pHpHLower the pHLower the pH

of waterof water((pHpH < 7) < 7)

Raises the pHRaises the pHof waterof water((pHpH > 7) > 7)

77

Definitions of Acids and BasesDefinitions of Acids and Bases

These are all operational definitions of acids and bases:These are all operational definitions of acids and bases:

For a a more in depthFor a a more in depthdiscussion on the operational definitiondiscussion on the operational definitionof acids and basesof acids and basessee the Unit 6see the Unit 6Elaboration - Definitions of Acids and Bases-Elaboration - Definitions of Acids and Bases-Part I: Operational DefinitioPart I: Operational Definitionn

http://www.chem.uwec.edu/Chem150_S07/elaborations/unit6/unit6-a-operational.html



88


The operational definition of an acid and a baseThe operational definition of an acid and a base

• pHpH = -log([H = -log([H++])])• It is observed thatIt is observed that

• when [Hwhen [H++] ↓] ↓, pH, pH ↑ ↑• when [Hwhen [H++] ↑, ] ↑, pH pH ↓↓• when [OHwhen [OH--] ↑, ] ↑, pH pH ↑↑• when [OHwhen [OH--] ↓, ] ↓, pH pH ↓↓

AcidAcid BaseBase

OperationalOperationalLower the pHLower the pH



99


Arrhenius definitionArrhenius definition• Attempts to give a chemical explanation for whyAttempts to give a chemical explanation for why

• some substances cause the some substances cause the pHpH to go to go downdown when added to water when added to water(Acids)(Acids)

• some substances cause the some substances cause the pHpH to go to go upup when added to water when added to water(Bases)(Bases)

• The observations are,The observations are,

• Acids cause the [HAcids cause the [H++] to increase] to increase

• Bases cause the [OHBases cause the [OH--] to increase] to increase

Svante Arrhenius1859-19271859-1927SwedishSwedish

Svante Arrhenius1859-19271859-1927SwedishSwedish

http://en.wikipedia.org/wiki/Svante_Arrhenius

http://en.wikipedia.org/wiki/Svante_Arrhenius

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The Arrhenius definition explains these observations as The Arrhenius definition explains these observations as follows:follows:

• Arrhenius acid:Arrhenius acid:• Releases hydrogen ions when dissolved in water.Releases hydrogen ions when dissolved in water.

• Arrhenius base:Arrhenius base:• Releases hydroxyl ions when dissolved in water.Releases hydroxyl ions when dissolved in water.

H+ + Cl-H2O

HCl

NaOH Na+ + OH-H2O

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For a a more in depthFor a a more in depthdiscussion on the Arrhenius definitiondiscussion on the Arrhenius definitionof acids and basesof acids and basessee the Unit 6see the Unit 6Elaboration - Definitions of Acids and Bases-Elaboration - Definitions of Acids and Bases-Part II: Arrhenius DefinitioPart II: Arrhenius Definitionn

http://www.chem.uwec.edu/Chem150_S07/elaborations/unit6/unit6-b-Arrhenius.html



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Definitions of acids and bases now include:Definitions of acids and bases now include:

AcidAcid BaseBase




ArrhreniusArrhreniusReleases HReleases H++

when dissolvedwhen dissolvedin waterin water

Releases OHReleases OH--


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There are substances that fit the operational definition of an There are substances that fit the operational definition of an acid or base, but which do not fit the Arrhenius definitionacid or base, but which do not fit the Arrhenius definition• Example: 0.01 M NHExample: 0.01 M NH33 has a has a pHpH of 10.61 of 10.61

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Aqueous ammonia (NHAqueous ammonia (NH33) does not fit the Arrhenius definition ) does not fit the Arrhenius definition of a base because it has no OHof a base because it has no OH-- to release: to release:

NH3 + OH-H2O

?

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The solution to this mystery is that the water is acting not only The solution to this mystery is that the water is acting not only as a solvent, but also as a reactant:as a solvent, but also as a reactant:

• The ammonia molecule is splitting the water molecule, The ammonia molecule is splitting the water molecule, holding onto the proton (Hholding onto the proton (H++), and releasing the hydroxyl ion ), and releasing the hydroxyl ion (OH(OH--).).

NH3 (aq) + OH (aq)H2O (l)+ NH4 (aq)

1616


The Brønsted-Lowry definition of acids and bases accounts The Brønsted-Lowry definition of acids and bases accounts for the acid-base behavior of molecules like NHfor the acid-base behavior of molecules like NH33..

Johannes Nicolaus Brønsted1879-19471879-1947

DanishDanish

Johannes Nicolaus Brønsted1879-19471879-1947

DanishDanish

Thomas Martin Lowry1874-19361874-1936

EnglishEnglish

Thomas Martin Lowry1874-19361874-1936

EnglishEnglish

http://en.wikipedia.org/wiki/Johannes_Nicolaus_Br%C3%B8nsted

http://en.wikipedia.org/wiki/Johannes_Nicolaus_Br%C3%B8nsted

http://en.wikipedia.org/wiki/Martin_Lowry

http://en.wikipedia.org/wiki/Martin_Lowry

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The Brønsted-Lowry definition of acids and bases:The Brønsted-Lowry definition of acids and bases:

• An base is a proton (HAn base is a proton (H++) ) acceptoracceptor..

• An acid is a proton (HAn acid is a proton (H++) ) donordonor

or more specifically:or more specifically:

• A Brønsted-Lowry base A Brønsted-Lowry base acceptsaccepts a proton from a Brønsted- a proton from a Brønsted-Lowry acidLowry acid

• A Brønsted-Lowry acid A Brønsted-Lowry acid donatesdonates a proton to a Brønsted- a proton to a Brønsted-Lowry baseLowry base

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Ammonia is a base becauseAmmonia is a base because

• The NHThe NH33 acceptsaccepts a proton from water, so the NH a proton from water, so the NH33 is the is the basebase

• The water The water donatesdonates a proton to the NH a proton to the NH33, so the water is the , so the water is the acidacid..

NH3 (aq) + OH (aq)H2O (l)+ NH4 (aq)

base acid

1919


Acid-base reactions are reversible, meaning that they can go Acid-base reactions are reversible, meaning that they can go in either direction.in either direction.

In the reverse direction,In the reverse direction,• The NHThe NH44

++ donatesdonates a proton to the OH a proton to the OH-- ion, so the NH ion, so the NH44++ is is

the the acidacid• The OHThe OH-- ion ion acceptsaccepts a proton from the NH a proton from the NH44

++, so the OH, so the OH-- is is the the basebase..

AcidAcidAcidAcidBaseBaseBaseBase BaseBaseBaseBaseAcidAcidAcidAcid

NH3 + OHH2O+ NH4

2020


These definitions can also be applied to acids, such as HCl:,These definitions can also be applied to acids, such as HCl:,

In the forward direction:In the forward direction:

• The HCl The HCl donatesdonates a proton to the water, so the HCl is the a proton to the water, so the HCl is the acidacid

• The water The water acceptsaccepts a proton from the HCl, so the water is the a proton from the HCl, so the water is the basebase..

In the reverse direction:In the reverse direction:

• The HThe H33OO++ donatesdonates a proton to Cl a proton to Cl-- ion, so the H ion, so the H33OO++ is the is the acidacid

• The ClThe Cl-- ion ion acceptsaccepts a proton from the H a proton from the H33OO++, so the Cl, so the Cl-- ion is the ion is the basebase..

AcidAcidAcidAcid BaseBaseBaseBase BaseBaseBaseBase AcidAcidAcidAcidCl- + H3O+HCl + H2O

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The Brønsted-Lowry definition also more accurately The Brønsted-Lowry definition also more accurately describes the hydrogen ion (Hdescribes the hydrogen ion (H++) as being attached to a water ) as being attached to a water molecule to form a hydronium ion (Hmolecule to form a hydronium ion (H33OO++))

• In the Brønsted Lowry theoryIn the Brønsted Lowry theory

H+ O H

H

+ O H

H

H

hydronium ionhydronium ionhydronium ionhydronium ion

pH =−log H3O+⎡⎣ ⎤⎦( )

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The species that differ from one another by only a protonThe species that differ from one another by only a proton(H(H++), are called ), are called conjugates conjugates

Examples include:Examples include:• NHNH44

+ + is the is the conjugate acidconjugate acid of NH of NH33..

• HH33OO++ is the is the conjugate acidconjugate acid of H of H22OO

• NHNH33 is the is the conjugate baseconjugate base of NH of NH44++

• HH22O is the O is the conjugate baseconjugate base of H of H33OO++

• HH22O is the O is the conjugate acidconjugate acid of OH of OH--

A substance such as HA substance such as H22O, that can serve as both an acid and O, that can serve as both an acid and a base, is called a base, is called amphotericamphoteric..

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For a a more in depthFor a a more in depthdiscussion on the Arrhenius definitiondiscussion on the Arrhenius definitionof acids and basesof acids and basessee the Unit 6see the Unit 6Elaboration - Definitions of Acids and Bases-Elaboration - Definitions of Acids and Bases-Part III: Brønsted-Lowry DefinitioPart III: Brønsted-Lowry Definitionn

http://www.chem.uwec.edu/Chem150_S07/elaborations/unit6/unit6-c-Br%C3%B8nsted-Lowry.html



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Definitions of acids and bases include:Definitions of acids and bases include:

AcidAcid BaseBase




ArrhreniusArrhreniusReleases HReleases H++


Releases OHReleases OH--


Brønsted-LowryBrønsted-LowryDonate aDonate a

proton (Hproton (H++))to a B-L baseto a B-L base

Accept aAccept aproton (Hproton (H++))

from a B-L acidfrom a B-L acid

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EquilibriumEquilibrium

In Unit 4 we discussed that,In Unit 4 we discussed that,

• The spontaneity, or direction, of a reaction is determined by The spontaneity, or direction, of a reaction is determined by the change in the free energy for a a reaction, Δthe change in the free energy for a a reaction, ΔGG..

• The rate that a reaction moves in the spontaneous The rate that a reaction moves in the spontaneous direction is determined by the activation energy, direction is determined by the activation energy, EEactact..

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Free Energy and Reaction Rates (Unit 4)Free Energy and Reaction Rates (Unit 4)

The height of this hill is called the The height of this hill is called the activation energyactivation energy, , EEactact..• The activation energy has no effect on the overall change The activation energy has no effect on the overall change

in the free energy for the reaction.in the free energy for the reaction.

FreeFreeEnergyEnergy

(G)(G)


(G)(G)

Progress ofProgress ofreactionreaction


Α Α → → BBΑ Α → → BB

AAAA

ΒΒΒΒ

ΔΔGG < 0 < 0spontaneousspontaneous

ΔΔGG < 0 < 0spontaneousspontaneous

EEactact > 0 > 0EEactact > 0 > 0

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For acid-base reactions, the activation energy is low so they For acid-base reactions, the activation energy is low so they proceed at a fairly fast rate in the spontaneous, or favorable, proceed at a fairly fast rate in the spontaneous, or favorable, direction.direction.

• For this reaction, the forward direction is the favorable For this reaction, the forward direction is the favorable direction.direction.


(G)(G)


(G)(G)




ΒΒΒΒ

AAAA

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As the reaction proceeds the the forward directionAs the reaction proceeds the the forward direction• The concentration of the reactant, A, decreases, whileThe concentration of the reactant, A, decreases, while• The concentration of the product, B, increases.The concentration of the product, B, increases.


(G)(G)


(G)(G)




ΒΒΒΒ

AAAA

2929


As the the concentration of the reactant, A, decreases, and As the the concentration of the reactant, A, decreases, and the concentration of the product, B, increases.the concentration of the product, B, increases.• The free energy of the reactant decreases, whileThe free energy of the reactant decreases, while• The free energy of the product increasesThe free energy of the product increases


(G)(G)


(G)(G)




ΒΒΒΒ

AAAA

3030


When the free energy of the reactants and the products are equal, When the free energy of the reactants and the products are equal, the concentrations of the products and reactants stop changing.the concentrations of the products and reactants stop changing.• The reaction is said to have reached The reaction is said to have reached equilibriumequilibrium..• The double-headed arrow is used to indicate this in the reaction The double-headed arrow is used to indicate this in the reaction

equationequation


(G)(G)


(G)(G)



ΒΒΒΒ

AAAA

Α Α ⇄ BBΑ Α ⇄ BB

3131


At equilibrium, the rate of the forward and reverse reactions At equilibrium, the rate of the forward and reverse reactions are the same.are the same.• The ratio of the equilibrium concentrations of the products The ratio of the equilibrium concentrations of the products

to the reactants is a constant that is a characteristic of the to the reactants is a constant that is a characteristic of the reaction.reaction.

• This constant is called the This constant is called the equilibrium constantequilibrium constant, , KKeqeq::

Keq =C[ ]

c D[ ]d

A[ ]a B[ ]

b

aA + bB ⏐ →⏐← ⏐⏐ cC + dD

3232


Example: The Haber reaction, which is used covert gaseous Example: The Haber reaction, which is used covert gaseous nitrogen to gaseous ammonia.nitrogen to gaseous ammonia.

• This reaction is used in the production of ammonia This reaction is used in the production of ammonia fertilizer.fertilizer.

N2 (g)+ 3 H2 (g) ⏐ →⏐← ⏐⏐ 2 NH3 (g)

Keq =NH3[ ]

2

N2[ ] H2[ ]3 =6.9 x 105

3333


If the quantity of a reactant or product is unaffected by the If the quantity of a reactant or product is unaffected by the reaction, it is not included in the equilibrium constant reaction, it is not included in the equilibrium constant expression.expression.• Example: An aqueous phase reaction in which one of the Example: An aqueous phase reaction in which one of the

reactants or products is a solid:reactants or products is a solid:

• The PbIThe PbI22 is not included because it is a solid and its is not included because it is a solid and its quantity has no effect on the aqueous concentrationsquantity has no effect on the aqueous concentrationsof Pbof Pb2+2+ or I or I--..

PbI2 (s) ⏐ →⏐← ⏐⏐ Pb2+(aq) + 2 I– (aq)

Keq = Pb2+⎡⎣ ⎤⎦ I–⎡⎣ ⎤⎦2=7.1 x 10-9

3434


If the quantity of a reactant or product is unaffected by the If the quantity of a reactant or product is unaffected by the reaction, it is not included in the equilibrium constant reaction, it is not included in the equilibrium constant expression.expression.• Example: Brønsted-Lowry reaction acid-base reaction for Example: Brønsted-Lowry reaction acid-base reaction for

aqueous ammonia:aqueous ammonia:

• The HThe H22O is not included in the equilibrium constant O is not included in the equilibrium constant expression because, as the solvent, its relative expression because, as the solvent, its relative concentration is so large that it concentration is unaffected concentration is so large that it concentration is unaffected by the reaction.by the reaction.

NH3 (aq) + H2O (l) ⏐ →⏐← ⏐⏐ NH4+ (aq) + OH– (aq)

Keq =NH4

+⎡⎣ ⎤⎦ OH–⎡⎣ ⎤⎦NH3[ ]

=1.78 x 10–5

3535


The equilibrium constant can be used to predict the relative The equilibrium constant can be used to predict the relative amounts of product and reactant at equilibriumamounts of product and reactant at equilibrium

3636

Thanksgiving Demo-Red CabbageThanksgiving Demo-Red Cabbage

http://web.chemistry.gatech.edu/~williams/bCourse_Information/red_cabbage_pH_indicator/cabbage.html




3737



Anthocyanins-red to blueAnthocyanins-red to blue




3838



Anthoxanthins-yellow at high pH




3939

Le Châtelier’s PrincipleLe Châtelier’s Principle

Le Châtelier’s Principle states that Le Châtelier’s Principle states that when reversible reaction is when reversible reaction is pushed out of equilibrium, the reaction responds to pushed out of equilibrium, the reaction responds to reestablish equilibriumreestablish equilibrium..

4040



4141



4242

Ionization of WaterIonization of Water

We have seen that water can serve as both an acid and a We have seen that water can serve as both an acid and a base.base.

• This means that water is This means that water is amphotericamphoteric..

AcidAcidAcidAcidBaseBaseBaseBase BaseBaseBaseBaseAcidAcidAcidAcid

NH3 + OHH2O+ NH4

AcidAcidAcidAcid BaseBaseBaseBase BaseBaseBaseBase AcidAcidAcidAcid

Cl- + H3O+HCl + H2O

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Pure water has a Pure water has a pHpH of 7.0, because water has a small, but of 7.0, because water has a small, but significant, tendency to dissociate to form Hsignificant, tendency to dissociate to form H++ ions and OH ions and OH-- ions.ions.

• This fulfills the Arrhenius definition of both an acid and a This fulfills the Arrhenius definition of both an acid and a base.base.

H2O (l) H+ (aq) + OH- (aq)55.5 M55.5 M55.5 M55.5 M 0.0000001 M0.0000001 M0.0000001 M0.0000001 M 0.0000001 M0.0000001 M0.0000001 M0.0000001 M

4444


The fraction of the water molecules that dissociate is very The fraction of the water molecules that dissociate is very small:small:

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

55.5 M55.5 M55.5 M55.5 M 0.0000001 M0.0000001 M0.0000001 M0.0000001 M 0.0000001 M0.0000001 M0.0000001 M0.0000001 M

number of H+ ions

number of H2O molecules

=1 H+ ion

555,000,000 H2O molecules

4545


It is the concentration of HIt is the concentration of H++ ions in pure water that is ions in pure water that is responsible for pure water having a responsible for pure water having a pH pH of 7.0of 7.0

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

55.5 M55.5 M55.5 M55.5 M 0.0000001 M0.0000001 M0.0000001 M0.0000001 M 0.0000001 M0.0000001 M0.0000001 M0.0000001 M

pH =−log H+⎡⎣ ⎤⎦( )

=−log 0.0000001( )

=−log 1 x 10-7( )

pH =7.0

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We can also use the Brønsted-Lowry definitionWe can also use the Brønsted-Lowry definition

H O

H

H O

H

+ H O

H

O

H

+H

H2O H3O+H2O OH-

pH =−log H3O+⎡⎣ ⎤⎦( )

=−log 0.0000001( )

=−log 1 x 10-7( )

pH =7.0

acidacidacidacid basebasebasebase acidacidacidacidbasebasebasebase

4747


The equilibrium constant for the ionization of waterThe equilibrium constant for the ionization of water

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

55.5 M55.5 M55.5 M55.5 M 0.0000001 M0.0000001 M0.0000001 M0.0000001 M 0.0000001 M0.0000001 M0.0000001 M0.0000001 M

Keq= H+⎡⎣ ⎤⎦ OH-⎡⎣ ⎤⎦

= H3O+⎡⎣ ⎤⎦ OH–⎡⎣ ⎤⎦

= 1 x 10-7( ) 1 x 10-7( )

Keq =1 x 10-14

Kw =1 x 10-14

4848


Le Châtelier’s Principle tells us if an acid causes the HLe Châtelier’s Principle tells us if an acid causes the H33OO++ ion concentration to increase, the OHion concentration to increase, the OH-- ion concentration ion concentration will decrease in order to re-establish the equilibrium, as will decrease in order to re-establish the equilibrium, as defined by defined by KKww::

• Adding acid will cause the OHAdding acid will cause the OH-- concentration to concentration to decrease.decrease.

OH -⎡⎣ ⎤⎦=Kw

H3O+⎡⎣ ⎤⎦

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Likewise, if a base causes the Likewise, if a base causes the OHOH-- ion concentration to ion concentration to increase, the increase, the HH33OO+ + ion concentration will decrease in ion concentration will decrease in order to re-establish the equlibrium, as defined by order to re-establish the equlibrium, as defined by KKww::

• Adding base will cause the HAdding base will cause the H33OO++ concentration to concentration to decrease.decrease.

• This is why bases causes the This is why bases causes the pHpH of a solution to of a solution to increase.increase.

H3O+⎡⎣ ⎤⎦=

Kw

OH-⎡⎣ ⎤⎦

5050

The pH ScaleThe pH Scale

Pure water with a Pure water with a pH pH of 7.0, is defined as having a of 7.0, is defined as having a neutralneutral pHpH..

• Acids cause the HAcids cause the H33OO++ ion concentration to increase, and, ion concentration to increase, and, consequently, the consequently, the pH pH to decrease from 7.0. to decrease from 7.0. acidicacidic conditionsconditions

• Bases cause the OHBases cause the OH-- ion concentration to increase, and , ion concentration to increase, and , consequently the [Hconsequently the [H33OO++] to decrease the ] to decrease the pH pH toto increase increase from; from; basicbasic conditions. conditions.

H3O+⎡⎣ ⎤⎦=

Kw

OH-⎡⎣ ⎤⎦

5151

The pH ScaleThe pH Scale

5252

Strengths of Acids and BasesStrengths of Acids and Bases

Different acids have different strengths, meaning, that for a Different acids have different strengths, meaning, that for a given concentration they produce different [Hgiven concentration they produce different [H33OO++] ] concentrations.concentrations.• Strong acidsStrong acids produce the maximum [H produce the maximum [H33OO++] concentration: ] concentration:

they completely dissociate to produce a [Hthey completely dissociate to produce a [H33OO++] ] concentration that is equal to the concentration of the acid, concentration that is equal to the concentration of the acid, times the number of protons that the acid releases times the number of protons that the acid releases (donates).(donates).

HCl (aq) + H2O (l) Cl- (aq) + H3O+ (aq)

~ 0% ~ 100%

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Different acids have different strengths, meaning, that for a Different acids have different strengths, meaning, that for a given concentration they produce different [Hgiven concentration they produce different [H33OO++] ] concentrations.concentrations.• Weak acidsWeak acids produce less than the maximum [H produce less than the maximum [H33OO++] ]

concentration: they dissociate only partially to produce a concentration: they dissociate only partially to produce a [H[H33OO++] concentration that is less to the concentration of the ] concentration that is less to the concentration of the acid.acid.

CH3COOH(aq) + H2O (l) CH3COO- (aq) + H3O+ (aq)

~ 96% ~ 4%

5454


Determining the Determining the pHpH of solutions is weak acids is less straight- of solutions is weak acids is less straight-forward than for strong acids.forward than for strong acids.• It requires using the equilibrium constant for the acid-base It requires using the equilibrium constant for the acid-base

reaction of the acid with water.reaction of the acid with water.

• The equilibrium constant for this type of reaction is called The equilibrium constant for this type of reaction is called the the acidity constantacidity constant, , KKaa..

CH3COOH(aq) + H2O (l) CH3COO- (aq) + H3O+ (aq)

Ka=Keq =

CH3COO–⎡⎣ ⎤⎦ H3O+⎡⎣ ⎤⎦

CH3COOH⎡⎣ ⎤⎦

5555


We will not be using We will not be using KKaa to determine the to determine the pHpH values of weak values of weak acid solutions in this course.acid solutions in this course.• Instead, we will use Instead, we will use KKaa values to compare the relative values to compare the relative

strengths of different acids.strengths of different acids.• The smaller the The smaller the KKaa value, the weaker the acid.value, the weaker the acid.

Like with hydrogen ion concentrations, Like with hydrogen ion concentrations, KKaa values are also values are also expressed as expressed as pKpKaa (= -log((= -log(KKaa)) values.)) values.• The higher the The higher the pKpKaa value, the weaker the acid.value, the weaker the acid.

5656


5757


For a more in depthFor a more in depthdiscussion of the strengths ofdiscussion of the strengths ofacids and basesacids and basessee the Unit 6see the Unit 6Elaboration - Acid and Base StrengthElaboration - Acid and Base Strength

http://www.chem.uwec.edu/Chem150_S07/elaborations/unit6/unit6-d-acid-strength.html

5858


The strength of bases can be compared by looking at the The strength of bases can be compared by looking at the relative strength of their conjugate acid.relative strength of their conjugate acid.

• The stronger the conjugate acid, the weaker the base.The stronger the conjugate acid, the weaker the base.

5959


6060

Neutralizing Acids and BasesNeutralizing Acids and Bases

As we saw from the definitions of acids an bases, they As we saw from the definitions of acids an bases, they behave as opposites.behave as opposites.• The one can neutralize the effects of the other.The one can neutralize the effects of the other.• When equal quantities of either a strong acid and a strong When equal quantities of either a strong acid and a strong

base, or of a weak acid and a weak base of comparable base, or of a weak acid and a weak base of comparable strength, are mixed, the result is a neutral salt solution:strength, are mixed, the result is a neutral salt solution:

HCl (aq) + NaOH (aq) +NaCl (aq) H2O (aq)

acid base salt water

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The neutralization of the an acid by a base, or a base by an The neutralization of the an acid by a base, or a base by an acid, can be be used to determine the unknown concentration acid, can be be used to determine the unknown concentration of an acid or base.of an acid or base.• This is because while one is neutralizing the other, the This is because while one is neutralizing the other, the pH pH

will change slowly with each addition of acid or base, until will change slowly with each addition of acid or base, until an equivalent amount has been added.an equivalent amount has been added.

• At that point, the At that point, the pH pH will change abruptly, making it easy to will change abruptly, making it easy to determine when an equivalent amount has been added.determine when an equivalent amount has been added.

• Determining an unknown concentration in this way is called Determining an unknown concentration in this way is called a a titrationtitration..

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If a known volume of a HCl solution is If a known volume of a HCl solution is titrated titrated with a NaOH with a NaOH solution of known concentration, the concentration of the HCL solution of known concentration, the concentration of the HCL can be determined:can be determined:

HCl (aq) + NaOH (aq) +NaCl (aq) H2O (aq)

acid base salt water

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This can be demonstrated with the Virtual Laboratory This can be demonstrated with the Virtual Laboratory Simulator.Simulator.

The titrationThe titrationofof

25 mL25 mLofof

0.1 M HCl0.1 M HClwithwith

0.1 M NaOH0.1 M NaOH

The titrationThe titrationofof

25 mL25 mLofof

0.1 M HCl0.1 M HClwithwith

0.1 M NaOH0.1 M NaOH

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If the If the pHpH is recorded with each addition of NaOH, the is recorded with each addition of NaOH, the following plot is obtained:following plot is obtained:

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For a more in depthFor a more in depthdiscussion of the neutralization ofdiscussion of the neutralization ofacids and basesacids and basessee the Unit 6see the Unit 6Elaboration - Neutralization of Acids and BasesElaboration - Neutralization of Acids and Bases

http://www.chem.uwec.edu/Chem150_S07/elaborations/unit6/unit6-e-neutralization.html

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Effect of pH on the Acid and Conjugate Base Concentrations


The expression for the acidity constant can be rearrange to The expression for the acidity constant can be rearrange to obtain the following equation.obtain the following equation.

HA (aq) + H2O (l) A– (aq) + H3O+ (aq)

acid conjugate base

Ka=

A –⎡⎣ ⎤⎦ H3O+⎡⎣ ⎤⎦

HA⎡⎣ ⎤⎦

pH =pKa + logA –⎡⎣ ⎤⎦HA⎡⎣ ⎤⎦

⎛

⎝⎜⎜

⎞

⎠⎟⎟

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• When [AWhen [A--] = [HA]] = [HA][A[A--]/[HA] = 1, and log([A]/[HA] = 1, and log([A--]/[HA]) = 0]/[HA]) = 0

pH =pKa + logA –⎡⎣ ⎤⎦HA⎡⎣ ⎤⎦

⎛

⎝⎜⎜

⎞

⎠⎟⎟

pH =pKa

when A -⎡⎣ ⎤⎦= HA⎡⎣ ⎤⎦

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• When [AWhen [A--] > [HA] (there is more conjugate base than acid)] > [HA] (there is more conjugate base than acid)[A[A--]/[HA] > 1, and log([A]/[HA] > 1, and log([A--]/[HA]) > 0]/[HA]) > 0

pH =pKa + logA –⎡⎣ ⎤⎦HA⎡⎣ ⎤⎦

⎛

⎝⎜⎜

⎞

⎠⎟⎟

pH > pKa

when A -⎡⎣ ⎤⎦> HA⎡⎣ ⎤⎦

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• When [AWhen [A--] < [HA] (there is more acid than conjugate base)] < [HA] (there is more acid than conjugate base)[A[A--]/[HA] < 1, and log([A]/[HA] < 1, and log([A--]/[HA]) < 0]/[HA]) < 0

pH =pKa + logA –⎡⎣ ⎤⎦HA⎡⎣ ⎤⎦

⎛

⎝⎜⎜

⎞

⎠⎟⎟

pH < pKa

when A -⎡⎣ ⎤⎦< HA⎡⎣ ⎤⎦

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BuffersBuffers

Buffers are used to resist changes in Buffers are used to resist changes in pHpH..• A buffer is a mixture of a weak acid, HA, and its conjugate A buffer is a mixture of a weak acid, HA, and its conjugate

base, Abase, A--..

• When a strong acid (HWhen a strong acid (H33OO--) or strong base (OH) or strong base (OH--) is added to ) is added to a buffer solution, it reacts with the weak base (Aa buffer solution, it reacts with the weak base (A --) or weak ) or weak acid (HA), to replace that strong acid or base with a weak acid (HA), to replace that strong acid or base with a weak acid or base.acid or base.

HA (aq) + H2O (l) A– (aq) + H3O+ (aq)

acid conjugate base

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BuffersBuffers

Buffers are used to resist changes in Buffers are used to resist changes in pHpH..• When a When a strong acidstrong acid is added to a buffer solution, it causes is added to a buffer solution, it causes

the Hthe H33OO-- to increase. According to Le Châtelier’s Principle, to increase. According to Le Châtelier’s Principle, the equlibrium shifts to the left to produce more of the weak the equlibrium shifts to the left to produce more of the weak acid, HA:acid, HA:

HA (aq) + H2O (l) A– (aq) + H3O+ (aq)

weak acid weak base

strongacid

added

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BuffersBuffers

Buffers are used to resist changes in Buffers are used to resist changes in pHpH..• When a When a strong basestrong base is added to a buffer solution, it is added to a buffer solution, it

causes the OHcauses the OH-- to increase. The OH to increase. The OH-- reacts with the H reacts with the H33OO++ to produce Hto produce H22O and, according to Le Châtelier’s Principle, O and, according to Le Châtelier’s Principle, shifts the equlibrium to the right to replace lost Hshifts the equlibrium to the right to replace lost H33OO--

and and increase the concentration of the weak base, Aincrease the concentration of the weak base, A --..

HA (aq) + H2O (l) A– (aq) + H3O+ (aq)

weak acid weak base

strongbase

added

+

OH-

2 H2O

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BuffersBuffers

Buffers buffer best when the concentrations of the weak acid, Buffers buffer best when the concentrations of the weak acid, HA, and its conjugate base, AHA, and its conjugate base, A--, are equal., are equal.

• We saw earlier that when [HA] = [AWe saw earlier that when [HA] = [A--], the ], the pHpH = = pKpKaa

• Therefore, buffers buffer best when Therefore, buffers buffer best when pH = pKpH = pKaa

pH =pKa + logA –⎡⎣ ⎤⎦HA⎡⎣ ⎤⎦

⎛

⎝⎜⎜

⎞

⎠⎟⎟

pH =pKa when A –⎡⎣ ⎤⎦= HA⎡⎣ ⎤⎦

0000

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BuffersBuffers

The blood’s The blood’s pHpH must be carefully regulated at around 7.4 must be carefully regulated at around 7.4• This is done with the bicarbonate buffer system.This is done with the bicarbonate buffer system.• The source of the weak acid (HThe source of the weak acid (H22COCO33), and conjugate base ), and conjugate base

(HCO(HCO33--), is the CO), is the CO22 gas in our lungs: gas in our lungs:

CO2 (g) CO2 (aq)

CO2 (aq) H2O (l)+ H2CO3 (aq)

H2CO3 (aq) H2O (l)+ HCO3- (aq) + H3O+ (aq)

weak acid weak base

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BuffersBuffers

Acidosis is the state where the blood’s Acidosis is the state where the blood’s pHpH drops below 7.4 drops below 7.4• Mild acidosis results in light-headedness.Mild acidosis results in light-headedness.• Sever acidosis can result coma and death.Sever acidosis can result coma and death.Acidosis can results from respiratory causes that increase the COAcidosis can results from respiratory causes that increase the CO22 concentrations in the lungconcentrations in the lung• PneumoniaPneumonia• EmphysemaEmphysema• Cystic fibrosisCystic fibrosis• Holding your breathHolding your breathAcidosis can also result from metabolic causes that increase the Acidosis can also result from metabolic causes that increase the concentrations of acids in the bloodconcentrations of acids in the blood• DiabetesDiabetes• Excessive alcohol consumptionExcessive alcohol consumption• ShockShock

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BuffersBuffers

Alkadosis is the state where the blood’s Alkadosis is the state where the blood’s pHpH rises above 7.4 rises above 7.4• Mild acidosis results in headaches, nervousness and cramps.Mild acidosis results in headaches, nervousness and cramps.• Severe acidosis can result convulsions and death.Severe acidosis can result convulsions and death.Alkadosis can results from respiratory causes that decrease the Alkadosis can results from respiratory causes that decrease the COCO22 concentrations in the lung concentrations in the lung• HyperventilationHyperventilationAlkadosis can also result from metabolic causes that increase the Alkadosis can also result from metabolic causes that increase the concentrations of acids in the bloodconcentrations of acids in the blood• Excessive antiacid consumptionExcessive antiacid consumption• ConstipationConstipation

The EndThe End

Documents

Chem 150 Unit 6 - Chemical Properties II Acids and Bases Water has played a major role in our discussions so far this semester; and for good reason, water