11

Chemistry ReviewChemistry ReviewRound 4 (final countdown)Round 4 (final countdown)

Buffers, Ksp, Thermo, and ElectroBuffers, Ksp, Thermo, and Electro

22

Chapter 15Chapter 15

Applying equilibriumApplying equilibrium

33

The Common Ion EffectThe Common Ion Effect When the salt with the anion of a When the salt with the anion of a

weak acid is added to that acid,weak acid is added to that acid, It reverses the dissociation of the It reverses the dissociation of the

acid.acid. Lowers the percent dissociation of Lowers the percent dissociation of

the acid.the acid. The same principle applies to salts The same principle applies to salts

with the cation of a weak base..with the cation of a weak base.. The calculations are the same as The calculations are the same as

last chapter. last chapter.

44

Buffered solutionsBuffered solutions A solution that resists a change in pH.A solution that resists a change in pH. Either a weak acid and its salt or a Either a weak acid and its salt or a

weak base and its salt.weak base and its salt. We can make a buffer of any pH by We can make a buffer of any pH by

varying the concentrations of these varying the concentrations of these solutions.solutions.

Same calculations as before.Same calculations as before. Calculate the pH of a solution that Calculate the pH of a solution that

is .50 M HAc and .25 M NaAc (Ka = 1.8 is .50 M HAc and .25 M NaAc (Ka = 1.8 x 10x 10-5-5))

55

Adding a strong acid or baseAdding a strong acid or base Do the stoichiometry first. (BAAM)Do the stoichiometry first. (BAAM) A strong base will grab protons A strong base will grab protons

from the weak acid reducing [HA]from the weak acid reducing [HA]00

A strong acid will add its proton to A strong acid will add its proton to the anion of the salt reducing [Athe anion of the salt reducing [A--]]00

Then do the equilibrium problem.Then do the equilibrium problem. What is the pH of 1.0 L of the What is the pH of 1.0 L of the

previous solution when 0.010 mol previous solution when 0.010 mol of solid NaOH is added?of solid NaOH is added?

66

General equationGeneral equation Ka = [HKa = [H++] [A] [A--]]

[HA][HA] so [Hso [H++] = Ka [HA]] = Ka [HA]

[A [A--]] The [HThe [H++] depends on the ratio [HA]/[A] depends on the ratio [HA]/[A--]] taking the negative log of both sidestaking the negative log of both sides pH = -log(Ka [HA]/[ApH = -log(Ka [HA]/[A--])]) pH = -log(Ka)-log([HA]/[ApH = -log(Ka)-log([HA]/[A--])]) pH = pKa + log([ApH = pKa + log([A--]/[HA])]/[HA])

77

This is called the Henderson-This is called the Henderson-Hasselbach equationHasselbach equation

pH = pKa + log([ApH = pKa + log([A--]/[HA])]/[HA]) pH = pKa + log(base/acid)pH = pKa + log(base/acid) Calculate the pH of the following Calculate the pH of the following

mixturesmixtures 0.75 M lactic acid (HC0.75 M lactic acid (HC33HH55OO33) and 0.25 ) and 0.25

M sodium lactate (Ka = 1.4 x 10M sodium lactate (Ka = 1.4 x 10-4-4)) 0.25 M NH0.25 M NH33 and 0.40 M NH and 0.40 M NH44ClCl

(Kb = 1.8 x 10(Kb = 1.8 x 10-5-5))

88

Buffer capacityBuffer capacity The pH of a buffered solution is The pH of a buffered solution is

determined by the ratio [Adetermined by the ratio [A--]/[HA].]/[HA]. As long as this doesn’t change As long as this doesn’t change

much the pH won’t change much.much the pH won’t change much. The more concentrated these two The more concentrated these two

are the more Hare the more H++ and OH and OH-- the the solution will be able to absorb.solution will be able to absorb.

Larger concentrations bigger Larger concentrations bigger buffer capacity.buffer capacity.

99

Buffer CapacityBuffer Capacity Calculate the change in pH that Calculate the change in pH that

occurs when 0.010 mol of HCl(g) is occurs when 0.010 mol of HCl(g) is added to 1.0L of each of the added to 1.0L of each of the following:following:

5.00 M HAc and 5.00 M NaAc5.00 M HAc and 5.00 M NaAc 0.050 M HAc and 0.050 M NaAc0.050 M HAc and 0.050 M NaAc Ka= 1.8x10Ka= 1.8x10-5-5

1010

Buffer capacityBuffer capacity The best buffers have a ratioThe best buffers have a ratio

[A [A--]/[HA] = 1]/[HA] = 1 This is most resistant to changeThis is most resistant to change True when [ATrue when [A--] = [HA]] = [HA] Make pH = pKa (since log1=0)Make pH = pKa (since log1=0)

1111

TitrationsTitrations Millimole (mmol) = 1/1000 molMillimole (mmol) = 1/1000 mol Molarity = mmol/mL = mol/L Molarity = mmol/mL = mol/L Makes calculations easier because Makes calculations easier because

we will rarely add Liters of solution.we will rarely add Liters of solution. Adding a solution of known Adding a solution of known

concentration until the substance concentration until the substance being tested is consumed.being tested is consumed.

This is called the equivalence point.This is called the equivalence point. Graph of pH vs. mL is a titration Graph of pH vs. mL is a titration

curve.curve.

1212

Strong acid with Strong BaseStrong acid with Strong Base Do the stoichiometry.Do the stoichiometry. There is no equilibrium .There is no equilibrium . They both dissociate completely.They both dissociate completely. The titration of 50.0 mL of 0.200 M The titration of 50.0 mL of 0.200 M

HNOHNO33 with 0.100 M NaOH with 0.100 M NaOH

Analyze the pHAnalyze the pH

1313

Weak acid with Strong baseWeak acid with Strong base There is an equilibrium.There is an equilibrium. Do stoichiometry.Do stoichiometry. Then do equilibrium.Then do equilibrium. Titrate 50.0 mL of 0.10 M HF Titrate 50.0 mL of 0.10 M HF

(Ka = 7.2 x 10(Ka = 7.2 x 10-4-4) with 0.10 M ) with 0.10 M NaOH NaOH

1414

Titration CurvesTitration Curves

1515

pH

mL of Base added

7

Strong acid with strong BaseStrong acid with strong Base Equivalence at pH 7Equivalence at pH 7

1616

pH

mL of Base added

>7

Weak acid with strong Base Equivalence at pH >7

1717

pH

mL of Base added

7

Strong base with strong acid

Equivalence at pH 7

1818

pH

mL of Base added

<7

Weak base with strong acid

Equivalence at pH <7

1919

SummarySummary Strong acid and base just Strong acid and base just

stoichiometry.stoichiometry. Determine Ka, use for 0 mL baseDetermine Ka, use for 0 mL base Weak acid before equivalence pointWeak acid before equivalence point

–Stoichiometry first–Then Henderson-Hasselbach

Weak acid at equivalence point KbWeak acid at equivalence point Kb Weak base after equivalence - Weak base after equivalence -

leftover strong base.leftover strong base.

2020

SummarySummary Determine Ka, use for 0 mL acid.Determine Ka, use for 0 mL acid. Weak base before equivalence Weak base before equivalence

point.point.–Stoichiometry first

–Then Henderson-Hasselbach Weak base at equivalence point Weak base at equivalence point

Ka.Ka. Weak base after equivalence - Weak base after equivalence -

leftover strong acid.leftover strong acid.

2121

Solubility EquilibriaSolubility Equilibria

Will it all dissolve, and if not, Will it all dissolve, and if not, how much?how much?

2222

All dissolving is an equilibrium.All dissolving is an equilibrium. If there is not much solid it will all If there is not much solid it will all

dissolve.dissolve. As more solid is added the solution As more solid is added the solution

will become saturated.will become saturated. SolidSolid dissolveddissolved The solid will precipitate as fast as The solid will precipitate as fast as

it dissolves .it dissolves . EquilibriumEquilibrium

2323

General equationGeneral equation MM++ stands for the cation (usually stands for the cation (usually

metal).metal). NmNm-- stands for the anion (a nonmetal).stands for the anion (a nonmetal).

But the concentration of a solid doesn’t But the concentration of a solid doesn’t change.change.

KKspsp = [M = [M++]]aa[Nm[Nm--]]bb

Called the Called the solubility productsolubility product for each for each compound.compound.

2424

Watch outWatch out Solubility is not the same as solubility Solubility is not the same as solubility

product.product. Solubility product is an equilibrium Solubility product is an equilibrium

constant.constant. it doesn’t change except with it doesn’t change except with

temperature.temperature. Solubility is an equilibrium position for Solubility is an equilibrium position for

how much can dissolve.how much can dissolve. A common ion can change this.A common ion can change this.

2525

Calculating KCalculating Kspsp The solubility of iron(II) oxalate The solubility of iron(II) oxalate

FeCFeC22OO4 4 is 65.9 mg/L is 65.9 mg/L

The solubility of LiThe solubility of Li22COCO33 is 5.48 g/L is 5.48 g/L

2626

Calculating SolubilityCalculating Solubility The solubility is determined by The solubility is determined by

equilibrium.equilibrium. Its an equilibrium problem.Its an equilibrium problem. Calculate the solubility of SrSOCalculate the solubility of SrSO44, ,

with a Ksp of 3.2 x 10with a Ksp of 3.2 x 10-7 -7 in M and in M and g/L.g/L.

Calculate the solubility of AgCalculate the solubility of Ag22CrOCrO44, ,

with a Ksp of 9.0 x 10with a Ksp of 9.0 x 10-12 -12 in M and in M and g/L.g/L.

2727

Relative solubilitiesRelative solubilities Ksp will only allow us to compare Ksp will only allow us to compare

the solubility of solids the at fall the solubility of solids the at fall apart into the same number of apart into the same number of ions.ions.

The bigger the Ksp of those the The bigger the Ksp of those the more soluble.more soluble.

If they fall apart into different If they fall apart into different number of pieces you have to do number of pieces you have to do the math.the math.

2828

Common Ion EffectCommon Ion Effect If we try to dissolve the solid in a If we try to dissolve the solid in a

solution with either the cation or anion solution with either the cation or anion already present less will dissolve.already present less will dissolve.

Calculate the solubility of SrSOCalculate the solubility of SrSO44, with , with

a Ksp of 3.2 x 10a Ksp of 3.2 x 10-7 -7 in M and g/L in a in M and g/L in a solution of 0.010 M Nasolution of 0.010 M Na22SOSO4.4.

Calculate the solubility of SrSOCalculate the solubility of SrSO44, with , with

a Ksp of 3.2 x 10a Ksp of 3.2 x 10-7 -7 in M and g/L in a in M and g/L in a solution of 0.010 M SrNOsolution of 0.010 M SrNO3.3.

2929

PrecipitationPrecipitation Ion Product, Q =[MIon Product, Q =[M++]]aa[Nm[Nm--]]bb If Q>Ksp a precipitate forms.If Q>Ksp a precipitate forms. If Q<Ksp No precipitate.If Q<Ksp No precipitate. If Q = Ksp equilibrium.If Q = Ksp equilibrium. A solution of 750.0 mL of 4.00 x 10A solution of 750.0 mL of 4.00 x 10-3-3M M

Ce(NOCe(NO33))33 is added to 300.0 mL of is added to 300.0 mL of

2.00 x 102.00 x 10-2-2M KIOM KIO33. Will Ce(IO. Will Ce(IO33))33 (Ksp= (Ksp=

1.9 x 101.9 x 10-10-10M)precipitate and if so, M)precipitate and if so, what is the concentration of the ions?what is the concentration of the ions?

3030

Selective PrecipitationsSelective Precipitations Used to separate mixtures of metal Used to separate mixtures of metal

ions in solutions.ions in solutions. Add anions that will only precipitate Add anions that will only precipitate

certain metals at a time.certain metals at a time. Used to purify mixtures.Used to purify mixtures.

Often use HOften use H22S because in acidic S because in acidic

solution Hgsolution Hg+2+2, Cd, Cd+2+2, Bi, Bi+3+3, Cu, Cu+2+2, ,

SnSn+4+4 will precipitate. will precipitate.

3131

Selective PrecipitationSelective Precipitation In Basic adding OHIn Basic adding OH--solution Ssolution S-2-2 will will

increase so more soluble sulfides increase so more soluble sulfides will precipitate.will precipitate.

CoCo+2+2, Zn, Zn+2+2, Mn, Mn+2+2, Ni, Ni+2+2, Fe, Fe+2+2, , Cr(OH)Cr(OH)33, Al(OH), Al(OH)33

3232

B

3333

D

3434

A

3535

D

3636

E

3737

3838

3939

4040

Chapter 6Chapter 6

EnergyEnergy

ThermodynamicsThermodynamics

4141

Energy is...Energy is... The ability to do work.The ability to do work. Conserved.Conserved. made of heat and work.made of heat and work. a state function.a state function. independent of the path, or how you independent of the path, or how you

get from point A to B.get from point A to B. Work is a force acting over a distance.Work is a force acting over a distance. Heat is energy transferred between Heat is energy transferred between

objects because of temperature objects because of temperature difference.difference.

4242

The universeThe universe is divided into two halves.is divided into two halves. the system and the surroundings.the system and the surroundings. The system is the part you are The system is the part you are

concerned with.concerned with. The surroundings are the rest.The surroundings are the rest. Exothermic reactions release Exothermic reactions release

energy to the surroundings.energy to the surroundings. Endo thermic reactions absorb Endo thermic reactions absorb

energy from the surroundings.energy from the surroundings.

4343

CH + 2O CO + 2H O + Heat4 2 2 2

CH + 2O 4 2

CO + 2 H O 2 2

Pote

nti

al en

erg

y

Heat

4444

N + O2 2

Pote

nti

al en

erg

y

Heat

2NO

N + O 2NO2 2 + heat

4545

DirectionDirection Every energy measurement has Every energy measurement has

three parts.three parts.

1.1. A unit ( Joules of calories).A unit ( Joules of calories).

2.2. A number how many.A number how many.

3.3. and a sign to tell direction.and a sign to tell direction. negative - exothermicnegative - exothermic positive- endothermicpositive- endothermic

4646

System

Surroundings

Energy

E <0

4747

System

Surroundings

Energy

E >0

4848

Same rules for heat and workSame rules for heat and work Heat given off is negative.Heat given off is negative. Heat absorbed is positive.Heat absorbed is positive. Work done by system on Work done by system on

surroundings is positive.surroundings is positive. Work done on system by Work done on system by

surroundings is negative.surroundings is negative. Thermodynamics- The study of Thermodynamics- The study of

energy and the changes it energy and the changes it undergoes.undergoes.

4949

First Law of ThermodynamicsFirst Law of Thermodynamics The energy of the universe is The energy of the universe is

constant.constant. Law of conservation of energy.Law of conservation of energy. q = heatq = heat w = workw = work E = q + wE = q + w Take the systems point of view to Take the systems point of view to

decide signs.decide signs.

5050

What is work?What is work? Work is a force acting over a Work is a force acting over a

distance.distance. w= F x w= F x dd P = F/ areaP = F/ area d = V/aread = V/area w= (P x area) x w= (P x area) x (V/area)= P (V/area)= PVV Work can be calculated by Work can be calculated by

multiplying pressure by the change multiplying pressure by the change in volume at constant pressure.in volume at constant pressure.

units of liter - atm L-atmunits of liter - atm L-atm

5151

Work needs a signWork needs a sign If the volume of a gas increases, If the volume of a gas increases,

the system has done work on the the system has done work on the surroundings.surroundings.

work is negativework is negative w = - Pw = - PVV Expanding work is negative.Expanding work is negative. Contracting, surroundings do work Contracting, surroundings do work

on the system w is positive.on the system w is positive. 1 L atm = 101.3 J1 L atm = 101.3 J

5252

ExamplesExamples What amount of work is done What amount of work is done

when 15 L of gas is expanded to when 15 L of gas is expanded to 25 L at 2.4 atm pressure?25 L at 2.4 atm pressure?

If 2.36 J of heat are absorbed by If 2.36 J of heat are absorbed by the gas above. what is the change the gas above. what is the change in energy?in energy?

How much heat would it take to How much heat would it take to change the gas without changing change the gas without changing the internal energy of the gas? the internal energy of the gas?

5353

EnthalpyEnthalpy abbreviated Habbreviated H H = E + PV (that’s the definition)H = E + PV (that’s the definition) at constant pressure.at constant pressure. H = H = E + PE + PVV

the heat at constant pressure qthe heat at constant pressure qpp can can

be calculated frombe calculated from

E = qE = qpp + w = q + w = qpp - P - PVV

qqpp = = E + P E + P V = V = HH

5454

CalorimetryCalorimetry Measuring heat.Measuring heat. Use a calorimeter.Use a calorimeter. Two kindsTwo kinds Constant pressure calorimeter Constant pressure calorimeter

(called a coffee cup calorimeter)(called a coffee cup calorimeter) heat capacity for a material, C is heat capacity for a material, C is

calculated calculated C= heat absorbed/ C= heat absorbed/ T = T = H/ H/ TT specific heat capacity = C/mass specific heat capacity = C/mass

5555

CalorimetryCalorimetry molar heat capacity = C/molesmolar heat capacity = C/moles heat = specific heat x m x heat = specific heat x m x TT heat = molar heat x moles x heat = molar heat x moles x TT Make the units work and you’ve done Make the units work and you’ve done

the problem right.the problem right. A coffee cup calorimeter measures A coffee cup calorimeter measures H.H. An insulated cup, full of water. An insulated cup, full of water. The specific heat of water is 1 cal/gºCThe specific heat of water is 1 cal/gºC Heat of reaction= Heat of reaction= H = sh x mass x H = sh x mass x TT

5656

ExamplesExamples The specific heat of graphite is 0.71 The specific heat of graphite is 0.71

J/gºC. Calculate the energy needed to J/gºC. Calculate the energy needed to raise the temperature of 75 kg of raise the temperature of 75 kg of graphite from 294 K to 348 K.graphite from 294 K to 348 K.

A 46.2 g sample of copper is heated to A 46.2 g sample of copper is heated to 95.4ºC and then placed in a 95.4ºC and then placed in a calorimeter containing 75.0 g of water calorimeter containing 75.0 g of water at 19.6ºC. The final temperature of at 19.6ºC. The final temperature of both the water and the copper is both the water and the copper is 21.8ºC. What is the specific heat of 21.8ºC. What is the specific heat of copper?copper?

5757

PropertiesProperties intensive properties not related to intensive properties not related to

the amount of substance.the amount of substance. density, specific heat, density, specific heat,

temperature.temperature. Extensive property - does depend Extensive property - does depend

on the amount of stuff.on the amount of stuff. Heat capacity, mass, heat from a Heat capacity, mass, heat from a

reaction.reaction.

5858

Hess’s LawHess’s Law Enthalpy is a state function.Enthalpy is a state function. It is independent of the path.It is independent of the path. We can add equations to to come We can add equations to to come

up with the desired final product, up with the desired final product, and add the and add the HH

Two rulesTwo rules If the reaction is reversed the sign If the reaction is reversed the sign

of of H is changedH is changed If the reaction is multiplied, so is If the reaction is multiplied, so is HH

5959

N2 2O2

O2 NO2

68 kJ

NO2180 kJ

-112 kJ

H (

kJ)

6060

Standard EnthalpyStandard Enthalpy The enthalpy change for a reaction The enthalpy change for a reaction

at standard conditions (25ºC, 1 at standard conditions (25ºC, 1 atm , 1 M solutions)atm , 1 M solutions)

Symbol Symbol HºHº When using Hess’s Law, work by When using Hess’s Law, work by

adding the equations up to make it adding the equations up to make it look like the answer. look like the answer.

The other parts will cancel out.The other parts will cancel out.

6161

H (g) + 1

2O (g) H (l) 2 2 2O

C(s) + O (g) CO (g) 2 2Hº= -394 kJ

Hº= -286 kJ

C H (g) + 5

2O (g) 2CO (g) + H O( ) 2 2 2 2 2 l

ExampleExample GivenGiven

calculate calculate Hº for this reactionHº for this reaction

Hº= -1300. kJ

2C(s) + H (g) C H (g) 2 2 2

6262

ExampleExample

O (g) + H (g) 2OH(g) 2 2 O (g) 2O(g)2 H (g) 2H(g)2

O(g) + H(g) OH(g)

Given

Calculate Hº for this reaction

Hº= +77.9kJHº= +495 kJ

Hº= +435.9kJ

6363

Standard Enthalpies of FormationStandard Enthalpies of Formation Hess’s Law is much more useful if you Hess’s Law is much more useful if you

know lots of reactions.know lots of reactions. Made a table of standard heats of Made a table of standard heats of

formation. The amount of heat needed formation. The amount of heat needed to for 1 mole of a compound from its to for 1 mole of a compound from its elements in their standard states.elements in their standard states.

Standard states are 1 atm, 1M and Standard states are 1 atm, 1M and 25ºC25ºC

For an element it is 0For an element it is 0 There is a table in Appendix 4 (pg A22)There is a table in Appendix 4 (pg A22)

6464

Standard Enthalpies of FormationStandard Enthalpies of Formation Need to be able to write the Need to be able to write the

equations.equations. What is the equation for the What is the equation for the

formation of NOformation of NO22 ? ?

½N½N2 2 (g) + O(g) + O22 (g) (g) NO NO22 (g) (g)

Have to make Have to make one mole one mole to meet to meet the definition.the definition.

Write the equation for the Write the equation for the formation of methanol CHformation of methanol CH33OH.OH.

6565

Since we can manipulate the Since we can manipulate the equationsequations

We can use heats of formation to We can use heats of formation to figure out the heat of reaction.figure out the heat of reaction.

Lets do it with this equation.Lets do it with this equation. CC22HH55OH +3OOH +3O22(g) (g) 2CO 2CO22 + 3H + 3H22OO

which leads us to this rule.which leads us to this rule.

6666

Since we can manipulate the Since we can manipulate the equationsequations

We can use heats of formation to We can use heats of formation to figure out the heat of reaction.figure out the heat of reaction.

Lets do it with this equation.Lets do it with this equation. CC22HH55OH +3OOH +3O22(g) (g) 2CO 2CO22 + 3H + 3H22OO

which leads us to this rule.which leads us to this rule.

( H products) - ( H reactants) = Hfo

fo o

6767

Chapter 16Chapter 16

Spontaneity, entropy and free Spontaneity, entropy and free energyenergy

6868

SpontaneousSpontaneous A reaction that will occur without A reaction that will occur without

outside intervention.outside intervention. We can’t determine how fast.We can’t determine how fast. We need both thermodynamics and We need both thermodynamics and

kinetics to describe a reaction kinetics to describe a reaction completely.completely.

Thermodynamics compares initial Thermodynamics compares initial and final states.and final states.

Kinetics describes pathway Kinetics describes pathway between.between.

6969

ThermodynamicsThermodynamics 1st Law- the energy of the 1st Law- the energy of the

universe is constant.universe is constant. Keeps track of thermodynamics Keeps track of thermodynamics

doesn’t correctly predict doesn’t correctly predict spontaneity.spontaneity.

EntropyEntropy (S) is disorder or (S) is disorder or randomnessrandomness

2nd Law the entropy of the 2nd Law the entropy of the universe increases.universe increases.

7070

EntropyEntropy Defined in terms of probability.Defined in terms of probability. Substances take the arrangement Substances take the arrangement

that is most likely.that is most likely. The most likely is the most The most likely is the most

random.random. Calculate the number of Calculate the number of

arrangements for a system.arrangements for a system.

7171

2 possible 2 possible arrangementsarrangements

50 % chance 50 % chance of finding the of finding the left emptyleft empty

7272

4 possible arrangements

25% chance of finding the left empty

50 % chance of them being evenly dispersed

7373

4 possible arrangements

8% chance of finding the left empty

50 % chance of them being evenly dispersed

7474

GasesGases Gases completely fill their chamber Gases completely fill their chamber

because there are many more because there are many more ways to do that than to leave half ways to do that than to leave half empty.empty.

SSsolid solid <S<Sliquid liquid <<S<<Sgasgas there are many more ways for the there are many more ways for the

molecules to be arranged as a molecules to be arranged as a liquid than a solid.liquid than a solid.

Gases have a huge number of Gases have a huge number of positions possible.positions possible.

7575

Gibb's Free EnergyGibb's Free Energy G=H-TSG=H-TS Never used this way.Never used this way. G=G=H-TH-TS at constant S at constant

temperaturetemperature Divide by -TDivide by -T --G/T = -G/T = -H/T-H/T-SS --G/T = G/T = SSsurrsurr + + S S --G/T = G/T = SSunivuniv If If G is negative at constant T and G is negative at constant T and

P, the Process is spontaneous.P, the Process is spontaneous.

7676

Let’s CheckLet’s Check For the reaction HFor the reaction H22O(s) O(s) H H22O(l)O(l)

Sº = 22.1 J/K mol Sº = 22.1 J/K mol Hº =6030 Hº =6030 J/molJ/mol

Calculate Calculate G at 10ºC and -10ºCG at 10ºC and -10ºC Look at the equation Look at the equation G=G=H-TH-TSS Spontaneity can be predicted from Spontaneity can be predicted from

the sign of the sign of H and H and S.S.

7777

G=G=H-TH-TSSHS Spontaneous?

+ - At all Temperatures

+ + At high temperatures, “entropy driven”

- - At low temperatures, “enthalpy driven”

+- Not at any temperature,Reverse is spontaneous

7878

Third Law of ThermoThird Law of Thermo The entropy of a pure crystal at 0 K The entropy of a pure crystal at 0 K

is 0.is 0. Gives us a starting point.Gives us a starting point. All others must be>0.All others must be>0. Standard Entropies Sº ( at 298 K and Standard Entropies Sº ( at 298 K and

1 atm) of substances are listed.1 atm) of substances are listed. Products - reactants to find Products - reactants to find Sº (a Sº (a

state function).state function). More complex molecules higher Sº.More complex molecules higher Sº.

7979

Free Energy in ReactionsFree Energy in Reactions Gº = standard free energy change.Gº = standard free energy change. Free energy change that will occur if Free energy change that will occur if

reactants in their standard state turn reactants in their standard state turn to products in their standard state.to products in their standard state.

Can’t be measured directly, can be Can’t be measured directly, can be calculated from other measurements.calculated from other measurements.

Gº=Gº=Hº-THº-TSºSº Use Hess’s Law with known reactions.Use Hess’s Law with known reactions.

8080

Free Energy in ReactionsFree Energy in Reactions There are tables of There are tables of GºGºff . .

Products-reactants because it is a Products-reactants because it is a state function.state function.

The standard free energy of The standard free energy of formation for any element in its formation for any element in its standard state is 0.standard state is 0.

Remember- Spontaneity tells us Remember- Spontaneity tells us nothing about rate.nothing about rate.

8181

Free energy and PressureFree energy and Pressure G = G = Gº +RTln(Q) where Q is the Gº +RTln(Q) where Q is the

reaction quotients (P of the products /P of reaction quotients (P of the products /P of the reactants).the reactants).

CO(g) + 2HCO(g) + 2H22(g) (g) CH CH33OH(l)OH(l)

Would the reaction be spontaneous at Would the reaction be spontaneous at 25ºC with the H25ºC with the H22 pressure of 5.0 atm and pressure of 5.0 atm and

the CO pressure of 3.0 atm?the CO pressure of 3.0 atm? GºGºff CH CH33OH(l) = -166 kJ OH(l) = -166 kJ

GºGºff CO(g) = -137 kJ CO(g) = -137 kJ GºGºff H H22(g) = 0 kJ (g) = 0 kJ

8282

How far?How far? G tells us spontaneity at current G tells us spontaneity at current

conditions. When will it stop?conditions. When will it stop? It will go to the lowest possible free It will go to the lowest possible free

energy which may be an energy which may be an equilibrium.equilibrium.

At equilibrium At equilibrium G = 0, Q = KG = 0, Q = K Gº = -RTlnKGº = -RTlnK

8383

Gº KGº K=0=0 =1=1

<0<0 >0>0

>0>0 <0<0

8484

Temperature dependence of KTemperature dependence of K

Gº= -RTlnK = Gº= -RTlnK = Hº - THº - TSºSº ln(K) = ln(K) = Hº/R(1/T)+ Hº/R(1/T)+ Sº/RSº/R A straight line of lnK vs 1/TA straight line of lnK vs 1/T

8585

Free energy And WorkFree energy And Work Free energy is that energy free to Free energy is that energy free to

do work.do work. The maximum amount of work The maximum amount of work

possible at a given temperature possible at a given temperature and pressure.and pressure.

Never really achieved because Never really achieved because some of the free energy is some of the free energy is changed to heat during a change, changed to heat during a change, so it can’t be used to do work.so it can’t be used to do work.

8686

C

8787E

8888

D

8989

E

9090

D

9191

9292

9393

ElectrochemistryElectrochemistry

Applications of RedoxApplications of Redox

9494

ReviewReview Oxidation reduction reactions Oxidation reduction reactions

involve a transfer of electrons.involve a transfer of electrons. OIL- RIGOIL- RIG Oxidation Involves GainOxidation Involves Gain Reduction Involves LossReduction Involves Loss LEO-GER LEO-GER Lose Electrons OxidationLose Electrons Oxidation Gain Electrons ReductionGain Electrons Reduction

9595

ApplicationsApplications Moving electrons is electric current.Moving electrons is electric current. 8H8H+++MnO+MnO44

--+ 5Fe+ 5Fe+2 +2 +5e+5e-- Mn Mn+2 +2 + 5Fe+ 5Fe+3 +3 +4H+4H22OO

Helps to break the reactions into half Helps to break the reactions into half reactions.reactions.

8H8H+++MnO+MnO44--+5e+5e-- Mn Mn+2 +2 +4H+4H22OO

5(Fe5(Fe+2+2 Fe Fe+3 +3 + e+ e-- ) ) In the same mixture it happens In the same mixture it happens

without doing useful work, but if without doing useful work, but if separateseparate

9696

H+

MnO4-

Fe+2

Connected this way the reaction Connected this way the reaction startsstarts

Stops immediately because charge Stops immediately because charge builds up.builds up.

9797

H+

MnO4-

Fe+2

Galvanic CellGalvanic Cell

Salt Bridge allows current to flow

9898

H+

MnO4-

Fe+2

e -

Electricity travels in a complete Electricity travels in a complete circuitcircuit

Instead of a salt bridgeInstead of a salt bridge

9999

H+

MnO4-

Fe+2

Porous Disk

100100

oxidation

reduction

e-

e-

e- e-

e-

e-

Anode Cathode

101101

Cell PotentialCell Potential Species being reduced pushes the electron.Species being reduced pushes the electron. Species being oxidized pulls the electron. Species being oxidized pulls the electron. The push or pull (“driving force”) is The push or pull (“driving force”) is

called the cell potential called the cell potential EEcellcell

Also called the electromotive force Also called the electromotive force (emf) (emf)

Unit is the volt(V) Unit is the volt(V) = 1 joule of work/coulomb of charge= 1 joule of work/coulomb of charge Measured with a voltmeterMeasured with a voltmeter

102102

Zn+2

SO4-2

1 M HCl

Anode

0.76

1 M ZnSO4

H+

Cl-

H2 in

Cathode

103103

1 M HCl

H+

Cl-

H2 in

Standard Hydrogen ElectrodeStandard Hydrogen Electrode This is the This is the

reference all reference all other oxidations other oxidations are compared toare compared to

EEº = 0º = 0 º indicates º indicates

standard states of standard states of 25ºC, 25ºC, 1 atm, 1 atm, 1 M solutions.1 M solutions.

104104

Cell PotentialCell Potential Zn(s) + CuZn(s) + Cu+2 +2 (aq)(aq) Zn Zn+2+2(aq)(aq) + Cu(s) + Cu(s) The total cell potential is the sum of The total cell potential is the sum of

the potential at each electrode.the potential at each electrode.

EEºº cellcell = = EEººZnZn Zn Zn+2+2 + + EEºº CuCu+2+2 CuCu

We can look up reduction potentials in We can look up reduction potentials in a table.a table.

One of the reactions must be One of the reactions must be reversed, so change it sign.reversed, so change it sign.

105105

Cell PotentialCell Potential Determine the cell potential for a galvanic Determine the cell potential for a galvanic

cell based on the redox reaction.cell based on the redox reaction. Cu(s) + FeCu(s) + Fe+3+3(aq)(aq) Cu Cu+2+2(aq)(aq) + Fe + Fe+2+2(aq)(aq)

FeFe+3+3(aq)(aq) + e+ e-- Fe Fe+2+2(aq) (aq) EEº = 0.77 Vº = 0.77 V CuCu+2+2(aq)+2e(aq)+2e-- Cu(s) Cu(s) EEº = 0.34 Vº = 0.34 V Cu(s) Cu(s) CuCu+2+2(aq)+2e(aq)+2e-- EEº = -0.34 º = -0.34

VV 2Fe2Fe+3+3(aq)(aq) + 2e+ 2e-- 2Fe 2Fe+2+2(aq) (aq) EEº = 0.77 Vº = 0.77 V

106106

Line NotationLine Notation solidsolidAqueousAqueousAqueousAqueoussolidsolid Anode on the leftAnode on the leftCathode on the rightCathode on the right Single line different phases.Single line different phases. Double line porous disk or salt bridge.Double line porous disk or salt bridge. If all the substances on one side are If all the substances on one side are

aqueous, a platinum electrode is aqueous, a platinum electrode is indicated.indicated.

For the last reactionFor the last reaction Cu(s)Cu(s)CuCu+2+2(aq)(aq)FeFe+2+2(aq),Fe(aq),Fe+3+3(aq)(aq)Pt(s)Pt(s)

107107

Galvanic CellGalvanic Cell The reaction always runs The reaction always runs

spontaneously in the direction that spontaneously in the direction that produced a positive cell potential.produced a positive cell potential.

Four things for a complete Four things for a complete description.description.

1)1) Cell PotentialCell Potential

2)2) Direction of flowDirection of flow

3)3) Designation of anode and cathodeDesignation of anode and cathode

4)4) Nature of all the components- Nature of all the components- electrodes and ionselectrodes and ions

108108

Potential, Work and Potential, Work and GG Gº = -nFGº = -nFE E ºº if if E E º < 0, then º < 0, then Gº > 0 spontaneousGº > 0 spontaneous if if E E º > 0, then º > 0, then Gº < 0 nonspontaneousGº < 0 nonspontaneous In fact, reverse is spontaneous.In fact, reverse is spontaneous. Calculate Calculate Gº for the following reaction:Gº for the following reaction: CuCu+2+2(aq)+ Fe(s) (aq)+ Fe(s) Cu(s)+ FeCu(s)+ Fe+2+2(aq)(aq)

FeFe+2+2(aq)(aq) + e+ e--Fe(s)Fe(s) EEº = 0.44 Vº = 0.44 V CuCu+2+2(aq)+2e(aq)+2e-- Cu(s) Cu(s) EEº = 0.34 Vº = 0.34 V

109109

Cell Potential and Cell Potential and ConcentrationConcentration

Qualitatively - Can predict direction of Qualitatively - Can predict direction of

change in change in EE from LeChâtelier. from LeChâtelier. 2Al(s) + 3Mn2Al(s) + 3Mn+2+2(aq) (aq) 2Al 2Al+3+3(aq) + 3Mn(s)(aq) + 3Mn(s)

Predict if Predict if EEcellcell will be greater or less than will be greater or less than

EEººcellcell if [Al if [Al+3+3] = 1.5 M and [Mn] = 1.5 M and [Mn+2+2] = 1.0 M] = 1.0 M

if [Alif [Al+3+3] = 1.0 M and [Mn] = 1.0 M and [Mn+2+2] = 1.5M] = 1.5M if [Alif [Al+3+3] = 1.5 M and [Mn] = 1.5 M and [Mn+2+2] = 1.5 M ] = 1.5 M

110110

The Nernst EquationThe Nernst Equation G = G = Gº +RTln(Q)Gº +RTln(Q) -nF-nFEE = -nF = -nFEEº + RTln(Q)º + RTln(Q)

EE = = EEº - º - RTRTln(Q)ln(Q)

nF nF 2Al(s) + 3Mn2Al(s) + 3Mn+2+2(aq) (aq) 2Al 2Al+3+3(aq) + (aq) +

3Mn(s) 3Mn(s) EEº = 0.48 Vº = 0.48 V Always have to figure out n by balancing.Always have to figure out n by balancing. If concentration can gives voltage, If concentration can gives voltage,

then from voltage we can tell then from voltage we can tell concentration.concentration.

111111

The Nernst EquationThe Nernst Equation As reactions proceed concentrations of As reactions proceed concentrations of

products increase and reactants decrease.products increase and reactants decrease. Reach equilibrium where Q = K and Reach equilibrium where Q = K and

EEcellcell = 0 = 0

0 = 0 = EEº - º - RTRTln(K)ln(K) nF nF

EEº = º = RTRTln(K)ln(K) nF nF

nFnFEEº º = ln(K)= ln(K) RTRT

112112

CorrosionCorrosion Rusting - spontaneous oxidation.Rusting - spontaneous oxidation. Most structural metals have reduction Most structural metals have reduction

potentials that are less positive than potentials that are less positive than OO22 . .

Fe Fe Fe Fe+2+2 +2e+2e-- EEº= 0.44 Vº= 0.44 V OO22 + 2H + 2H22O + 4eO + 4e- - 4OH4OH-- EEº= 0.40 Vº= 0.40 V

FeFe+2+2 + O + O2 2 + H+ H22O O FeFe2 2 OO3 3 + H+ H++

Reaction happens in two places.Reaction happens in two places.

113113

Water Rust

Iron Dissolves- Fe Fe+2

e-

Salt speeds up process by increasing conductivity

114114

Preventing CorrosionPreventing Corrosion Coating to keep out air and water.Coating to keep out air and water. Galvanizing - Putting on a zinc coatGalvanizing - Putting on a zinc coat Has a lower reduction potential, so Has a lower reduction potential, so

it is more. easily oxidized.it is more. easily oxidized. Alloying with metals that form oxide Alloying with metals that form oxide

coats.coats. Cathodic Protection - Attaching Cathodic Protection - Attaching

large pieces of an active metal like large pieces of an active metal like magnesium that get oxidized magnesium that get oxidized instead.instead.

115115

Running a galvanic cell backwards.Running a galvanic cell backwards. Put a voltage bigger than the Put a voltage bigger than the

potential and reverse the direction potential and reverse the direction of the redox reaction.of the redox reaction.

Used for electroplating.Used for electroplating.

ElectrolysisElectrolysis

116116

1.0 M Zn+2

e- e-

Anode Cathode

1.10

Zn Cu1.0 M Cu+2

117117

1.0 M Zn+2

e- e-

AnodeCathode

A battery >1.10V

Zn Cu1.0 M Cu+2

118118

Calculating platingCalculating plating Have to count charge.Have to count charge. Measure current Measure current II (in amperes) (in amperes) 1 amp = 1 coulomb of charge per 1 amp = 1 coulomb of charge per

secondsecond q = q = II x t x t q/nF = moles of metalq/nF = moles of metal Mass of plated metalMass of plated metal How long must 5.00 amp current be How long must 5.00 amp current be

applied to produce 15.5 g of Ag from applied to produce 15.5 g of Ag from AgAg++

119119

Other usesOther uses Electroysis of water.Electroysis of water. Seperating mixtures of ions.Seperating mixtures of ions. More positive reduction potential More positive reduction potential

means the reaction proceeds means the reaction proceeds forward. forward.

We want the reverse.We want the reverse. Most negative reduction potential Most negative reduction potential

is easiest to plate out of solution.is easiest to plate out of solution.

120120

B

121121

A

122122

123123

124124