MCATs_2

(1)

1 1

Solutions: Terms

Solute, solvent, dissolution, precipitation,

solubility,

solubility, molaritymolarity, , molalitymolality, mole fraction., mole fraction. Solute

Solute: substance in smaller proportion.: substance in smaller proportion. Solvent

Solvent: substance in greater proportion.: substance in greater proportion. Dissolution

Dissolution: process of dissolving.: process of dissolving. Precipitation

Precipitation: reverse of dissolution.: reverse of dissolution. Solubility

(2)

2 2

Solutions: Terms

Molarity

Molarity: moles of solute per : moles of solute per literliter of solution.of solution.

Molality

Molality: moles of solute per : moles of solute per kgkg of solvent.of solvent.

Mole fraction

Mole fraction: fraction of moles of given : fraction of moles of given substance relative to total moles in solution.

substance relative to total moles in solution.

•

• XX_a_a = = nn_a_a//ΣΣnn

What is an

What is an electrolyteelectrolyte?? Free ions in a solution

Free ions in a solution ÖÖ conducts electricity.conducts electricity. What is the

What is the van’tvan’t Hoff factor, Hoff factor, ii??

Number of ions existing after dissolution of

one unit of substance:

(3)

3 3

Colligative

Properties:

_Properties:

Important!

What does a

What does a colligativecolligative property depend property depend upon?

upon?

Number of solute particles in solution (

Number of solute particles in solution (not not identity

identity of solute) of solute) –– van’tvan’t Hoff factor comes Hoff factor comes up big!

up big!

There are three for the MCAT

There are three for the MCAT –– name ‘name ‘emem::

•

• vaporvapor--pressure depression.pressure depression. •

• boilingboiling--point elevation.point elevation. •

(4)

4 4

Vapor Pressure

Definition? Definition?

Sometimes a tough concept

Sometimes a tough concept –– pressure pressure

exerted by the gaseous phase of a liquid that

evaporated from exposed surface of liquid.

My definition: pressure exerted by

molecules leaving liquid phase on the

atmosphere

(5)

5 5

Vapor Pressure

(6)

6 6

Vapor Pressure

Think about my definition…what property of

the liquid will determine vapor pressure?

Intermolecular forces!

High ones will hold liquid in liquid form and

decrease pressure exerted by molecules leaving

liquid.

Low ones will allow liquid to escape into

gaseous form and increase pressure exerted by

molecules as they change phase

(7)

7 7

Vapor Pressure Depression

Now for VP depression…

Given a solution of two liquids, A and B:

•

• Total VP is equal to sum of partial pressures of Total VP is equal to sum of partial pressures of each liquid.

each liquid. •

• Partial pressure of A is equal to proportion of A Partial pressure of A is equal to proportion of A in solution.

in solution.

What is the law called that describes this

situation?

Raoult’s

(8)

8 8

Vapor Pressure Depression

From

From Raoult’sRaoult’s Law, the presence of liquid B Law, the presence of liquid B will lower

will lower PP_a_a by lowering by lowering XX_a_a..

Vapor Pressure Depression:

Vapor Pressure Depression: ∆∆PP_a_a = = ––XX_b_bPP_a_a**

Some deviations from

Some deviations from Raoult’sRaoult’s Law are Law are possible.

possible.

Conceptually, what is going on?

Generally, presence of liquid B will add

intermolecular forces to solution A and make

phase change to gas harder…interference!

(9)

9 9

Boiling Point Elevation

What happens to a liquid when it boils?

Input of energy overcomes intermolecular

forces of liquid to cause phase change.

Relation to VP

Relation to VP ÖÖ boiling point is _{boiling point is}

temperature at which VP =

temperature at which VP = PP_atmosphere_atmosphere •

• equilibrium between liquid pushing on air and equilibrium between liquid pushing on air and vice versa

vice versa…free movement of molecules …free movement of molecules

between phases. between phases.

What causes BP change (elevation)?

(10)

10 10

Boiling Point Elevation

Intermolecular forces!

Addition of solute introduces extra

intermolecular forces between solute and

liquid, making it harder for phase change.

Real life example

Real life example –– cooking pasta.cooking pasta.

What equation quantifies the temp change?

∆

∆TT_b_b = = KK_b_bimim…define the terms!…define the terms!

A constant,

A constant, van’tvan’t Hoff factor, Hoff factor, molalitymolality.. Why does

(11)

11 11

Freezing Point Depression

What happens to liquid when it freezes?

Molecules assemble into an orderly, tightly

Molecules assemble into an orderly, tightly- -packed array.

packed array.

What might the addition of a different

molecule have on this lattice formation?

Interference! More difficult to achieve solid

state

state ÖÖ FP will go down (need to be colder FP will go down (need to be colder to achieve the solid state).

(12)

12 12

Freezing Point Depression

What is the equation that quantifies this

concept?

∆

∆TT_f_f = = ––KK_f_fimim …define the terms!…define the terms!

A constant,

A constant, van’tvan’t Hoff factor, Hoff factor, molalitymolality.. What is the effect of

What is the effect of van’tvan’t Hoff factor here?Hoff factor here? Higher number of ion species will cause

Higher number of ion species will cause

more interference!

(13)

13 13

Kinetics: Some Terms

Define kinetics (Don’t confuse with

thermodynamics!!!).

How fast a reaction occurs…this says

How fast a reaction occurs…this says absolutely nothing

absolutely nothing about spontaneity!!about spontaneity!!

Mechanism, intermediates, rate

Mechanism, intermediates, rate--determining determining step…

step…

Mechanism

Mechanism is a sequence, is a sequence, intermediatesintermediates are are not reactants or products,

not reactants or products, raterate--determining stepdetermining step puts limit on maximum speed.

(14)

14 14

Kinetics: Some Terms

Rate

Rate--determining stepdetermining step: the slowest step in a : the slowest step in a process determines the overall reaction rate.

process determines the overall reaction rate.

In a two step reaction, if step two is slow,

then the speed of step one is irrelevant:

bimolecular

termolecular unlikely!

•

• 2NO 2NO NN₂₂OO₂₂ (fast)(fast) •

• NN₂₂OO₂₂ + O+ O₂₂ 2NO2NO₂₂ (slow)(slow) •

(15)

15 15

Kinetics: Reaction Rate

What three factors determine reaction rate?

They are…

•

• frequency of collisions.frequency of collisions. •

• orientation of colliding molecules.orientation of colliding molecules. •

• energy of molecules.energy of molecules.

The concept of

The concept of activation energyactivation energy……what is what is it and why does it matter?

(16)

16 16

Activation Energy, E

_a_a

Activation energy is the extra “kick” of

energy that a reaction needs to proceed.

MCAT loves

MCAT loves EE_a_adiagrams, but first, what diagrams, but first, what

are

are exothermicexothermic and and endothermicendothermic reactions?reactions? Two diagrams, one for each above type:

Two diagrams, one for each above type:

E_a ∆H _ R P E_a ∆H + R P

(17)

17 17

Activation Energy, E

_a_a

From the activation energy concept, three

statements can be made:

•

• more reactants more reactants ÖÖ fast rate fast rate (more collisions possible). (more collisions possible). •

• higher temperature higher temperature ÖÖ fast fast

rate (more reactant molecules rate (more reactant molecules have sufficient kinetic energy have sufficient kinetic energy

to overcome

to overcome EE_a_a).). •

(18)

18 18

Catalysts

What is a catalyst? What is a catalyst?

Substance that makes reaction go faster by

speeding up rate

speeding up rate--determining step or providing determining step or providing an optimized route to products.

an optimized route to products.

Reactant

Reactant vsvs. catalyst . catalyst ––

catalyst unchanged at

end of reaction.

Catalysts change rate,

not

not thermodynamics thermodynamics

(

(∆∆GG, , ∆∆HH, , ∆∆SS, , etcetc.)..).

E_a

(19)

19 19

Rate Laws

What does a rate law tell us?

The rate at which a reactant disappears.

Consists of

Consists of rate constantrate constant, and , and concentrations of reactants

concentrations of reactants in slow step in slow step only!!

only!!

Rate laws determined

Rate laws determined experimentallyexperimentally..

a

(20)

20 20

Rate Laws

Rate =

Rate = kk[A][A]xx[B][B]yy

x

x ÖÖ order of order of rxnrxn with respect to A.with respect to A.

y

y ÖÖ order of order of rxnrxn with respect to B.with respect to B. x

x++yy ÖÖ overall order of reaction.overall order of reaction.

k

k ÖÖ rate constantrate constant..

Note

Note:: cannot get orders of reactants or rate cannot get orders of reactants or rate

constant from balanced equation.

Need to look at experiment to get rate law.

(21)

21 21

Rate Laws

A + B + C A + B + C D + ED + E Experiment

Experiment [A][A] [B][B] [C][C] Initial Rate, Initial Rate, Ms Ms––11 1 1 0.2 M0.2 M 0.1 M0.1 M 0.05 M0.05 M 1 ₁× 10× 10––33 2 2 0.4 M0.4 M 0.1 M0.1 M 0.05 M0.05 M 2 × 2 × 1010––33 3 3 0.2 M0.2 M 0.2 M0.2 M 0.05 M0.05 M 4 4 × 10× 10––33 4 4 0.2 M0.2 M 0.1 M0.1 M 0.1 M0.1 M 1 1 × 10× 10––33

(22)

22 22

Rate Laws

Look at two experiments where only one

reactant concentration changes.

Determine the factor by which it changes,

and compare to factor by which rate

changes.

[A]

[A] doublesdoubles ÖÖ

order 0. order 0. [B] [B] doublesdoubles ÖÖ [C] [C] doublesdoubles ÖÖ rate

rate doublesdoubles ÖÖ rate

rate quadruplesquadruples ÖÖ rate

rate uneffecteduneffected ÖÖ

order 1.

order 2.

(23)

23 23

Rate Laws

When order is 0, rate does not depend on

concentration of reactant.

Rate law for this example:

Rate law for this example: kk[A][B][A][B]22

Determining

Determining kk…anyone remember the …anyone remember the

formula for finding

formula for finding kk??

Solve the rate law…

Solve the rate law…k k = rate= rate//[A][B][A][B]22

Substitute numbers from any experiment for

[A] and [B].

•

(24)

24 24

The Equilibrium Constant

First, what is the definition of an equilibrium?

The rate of the forward reaction equals reverse.

Given this generic reaction, what is the

Given this generic reaction, what is the KK_eq_eq?? •

• aaAA + b+ bBB '' ccCC + + ddDD

Mass

Mass--action ratio action ratio ÖÖ ratio of products to ratio of products to reactants:

reactants:

Gases and aqueous molecules only!!

[C] [C]cc[D][D]dd [A] [A]aa[B][B]bb K K_eq_eq ==

(25)

25 25

The Equilibrium Constant

The value of

The value of KK_eq_eqfor a reaction is a constant at a for a reaction is a constant at a

given temperature

given temperature ÖÖ temp change causes temp change causes KK_eq_eq

change.

Equilibrium constants, in general, may have

different subscripts (

different subscripts (eqeq, sp, a, b, , sp, a, b, etcetc.) but they all .) but they all

obey the same rules of calculation and temp

dependence.

The value of K

The value of K ÖÖ what values favor react. _{what values favor react.}vsvs. prod.. prod. •

• KK_eq_eq< 1 favors reactants.< 1 favors reactants. •

• KK_eq_eq= 1 balances reactants and products.= 1 balances reactants and products. •

(26)

26 26

The Reaction Quotient

What is the reaction quotient? Symbol?

Ratio of concentrations of products to reactants,

Ratio of concentrations of products to reactants, QQ, ,

but NOT at equilibrium

but NOT at equilibrium ÖÖ differs from _{differs from}KK..

How do the values of

How do the values of KK relate to values of relate to values of QQ??

When

When QQ < < KK ÖÖ proceeding in proceeding in forwardforward direction.direction.

When

When QQ = = KK ÖÖ at equilibriumat equilibrium..

When

(27)

27 27

Le

Châtelier’s

Principle

What does Le

What does Le Châtelier’sChâtelier’s principle state?principle state?

A system at equilibrium will try to neutralize any

imposed change (stress) in order to reestablish

equilibrium.

The effects of different stresses:

•

• Addition of product or reactant.Addition of product or reactant. •

• Removal of product or reactant.Removal of product or reactant. •

• Changing volume and temperature.Changing volume and temperature. •

(28)

28 28

Le

Châtelier’s

_{Châtelier’s}

Principle

_Principle

N N₂₂ (g) + 3 H(g) + 3 H₂₂ (g) (g) '' 2 NH2 NH₃₃ (g) + heat(g) + heat Adding or removing Adding or removing NN₂₂ Adding or removing NH Adding or removing NH₃₃

Reducing volume? Increasing volume?

•

• cut volume, raise pressure Öcut volume, raise pressure Ö favor fewer molesfavor fewer moles •

• raise volume, decrease pressure raise volume, decrease pressure ÖÖ favor greater moles.favor greater moles.

Changing temp? Adding or removing heat.

Inert gas and catalyst

Inert gas and catalyst ÖÖ no change (catalyst no change (catalyst changes rate of forward and reverse equally).

(29)

29 29

Solubility Product

How is the solubility product constant defined?

The extent to which a salt will dissolve in water.

K

K_sp_sp is determined just like is determined just like KK_eq_eq..

Solubility product constant is also temperature

dependent. dependent. M M_n_nXX_m_m(s(s) ) '' nnMMxx++(aq(aq) + ) + mmXXyy––((aqaq)) K K_sp_sp = [= [MMxx++]]nn[X[Xyy––]]mm

(30)

30 30

Solubility Product: Example

The value of the solubility product for copper (I)

chloride is

chloride is KK_sp_sp = 1.2 = 1.2 × × 1010–6–6. Under normal . Under normal

conditions, the maximum concentration of an

aqueous

aqueous CuClCuCl solution will be:solution will be: •

• less than 10less than 10––66 _M_M

•

• greater than 10greater than 10––66 _{M and less than 10}_{M and less than 10}––4 4 _M_M

•

• greater than 10greater than 10––44 _{M and less than 10}_{M and less than 10}––2 2 _M_M

•

(31)

31 31

Ion Product and Common Ion Effect

What is an

What is an ion production product??

Actually just the reaction quotient (

Actually just the reaction quotient (QQ) for ) for

solubility reactions.

Allows us to make predictions just as before…

When

When QQ_sp_sp < < KK_sp_sp ÖÖ more salt can be more salt can be dissolveddissolved..

When

When QQ_sp_sp = = KK_sp_sp ÖÖ solution is solution is saturatedsaturated..

When

When QQ_sp_sp > > KK_sp_sp ÖÖ excess salt will excess salt will precipitateprecipitate..

Think of

(32)

32 32

Ion Product and Common Ion Effect

What is the

What is the common ion effectcommon ion effect??

Disturbing of a solubility equilibrium by adding a

common ion, one that already exists in the

equilibrium.

Follows Le

Follows Le Châtelier’sChâtelier’s principle, causing a principle, causing a decrease in solubility (equilibrium favors the

decrease in solubility (equilibrium favors the

solid and not the aqueous).

Mg(OH)

Mg(OH)₂₂(s) (s) '' MgMg2+ 2+ ₍₍_aq_aq₎₎ _{+ 2OH}_{+ 2OH}––₍₍_aq_aq₎₎_…_…_after_after

OH

(33)

33 33

Acids and Bases: Definitions

Arrhenius

Arrhenius acids/bases, acids/bases, BrønstedBrønsted--Lowry Lowry

acids/bases, Lewis acids/bases…differentiate!

Arrhenius

Arrhenius: acids ionize in water to produce H: acids ionize in water to produce H++

and bases ionize to produce OH

and bases ionize to produce OH––_._.

Brønsted

Brønsted--LowryLowry: acids are proton (H: acids are proton (H++_{) donors}_{) donors}

and bases are proton (H

and bases are proton (H++_{) acceptors.}_{) acceptors.}

Lewis

Lewis: acids are electron pair acceptors and : acids are electron pair acceptors and bases are electron pair donors.

(34)

34 34

Conjugate Acids and Bases

What are conjugate acids and bases, or how

would you recognize them?

When

When BrønstedBrønsted--Lowry acid donates an HLowry acid donates an H++ _the_the

remaining structure is conjugate base.

When

When BrønstedBrønsted--Lowry base accepts an HLowry base accepts an H++ _the_the

new species is the conjugate acid.

Name the acid, base, and conjugates:

•

• NHNH₃₃ + H+ H₂₂O O '' NHNH₄₄++ _{+ OH}_{+ OH}––

(35)

35 35

Strengths of Acids and Bases

Brønsted

Brønsted--Lowry acids/bases placed in two Lowry acids/bases placed in two broad

broad catagoriescatagories: : strongstrong and and weakweak.. What is a

What is a strongstrong acid?acid?

Strong acids dissociate nearly completely. Their

K

K_a_a > 1, favoring products considerably.> 1, favoring products considerably.

Examples (memorize!) For the MCAT, all

others not listed here are considered weak:

•

(36)

36 36

Strengths of Acids and Bases

What is a

What is a weakweak acid?acid?

Beyond the opposite of a strong acid, a weak

acid has a

acid has a KK_a_a < 1 because reactants are favored.< 1 because reactants are favored.

Ranking acid strength: higher

Ranking acid strength: higher KK_a_a means means

stronger acid…same for base strength.

How does this all work for conjugates?

(37)

37 37

Strengths of Conjugate Pairs

Given a

Given a strong acidstrong acid, is conjugate base strong , is conjugate base strong or weak? More importantly, why?

or weak? More importantly, why?

It is weak, because complete forward reaction

means essentially no reverse (conjugate cannot

take up H

take up H++_{) and therefore is weak.}_{) and therefore is weak.}

Same holds true for strong base.

For

For weak acidweak acid??

Conjugate is weak base, but the smaller the

Conjugate is weak base, but the smaller the KK_a_a, ,

the stronger the conjugate base will be, relative

to other weak bases.

(38)

38 38

Strengths of Conjugate Pairs:

Example

Of the following anions, which is the strongest

base? base? • • II–– • • CNCN–– • • NONO₃₃–– • • BrBr––

(39)

39 39

Strengths of Conjugate Pairs:

Example

Of the following, which acid has the weakest

conjugate base? conjugate base? • • HClOHClO₄₄ • • HCOOHHCOOH • • HH₃₃POPO₄₄ • • HH₂₂COCO₃₃

(40)

40 40

Amphoteric

Substances

_Substances

What is an

What is an amphotericamphoteric substance?substance?

A substance that can act as either an

A substance that can act as either an acidacid or a or a base

base..

Where does this happen?

Where does this happen? PolyproticsPolyprotics!!

When a substance has more than one proton to

donate, the conjugate base can either donate

another or accept a free proton.

H

H₃₃POPO₄₄'' HH₂₂POPO₄₄–– _{+ H}_{+ H}+ + _'_' _HPO_HPO 4

(41)

41 41

Ion

-

_-

Product Constant of Water

_{Product Constant of Water}

Water is an

Water is an amphotericamphoteric substance.substance. H

H₂₂OO + + HH₂₂OO '_' HH₃₃OO++ ₊₊_OH_OH–– •

• reacts with itself in reacts with itself in BrønstedBrønsted--Lowry reaction.Lowry reaction.

Anyone know the term for this equilibrium?

Autoionization

Autoionization of water of water ÖÖ K_K_w_w = [H= [H++][OH][OH––]]

Value of

Value of autoionizationautoionization constant at 25 constant at 25 °°C?C?

K

(42)

42 42

pH Calculations: A Shortcut

Normal formula: pH =

Normal formula: pH = ––log[Hlog[H++_]…also,_]…also,

pOH

pOH = = ––log [OHlog [OH––_]_]

Great when [H

Great when [H++_{] = 1}_{] = 1}× × ₁₀₁₀–2–2 _Ö_Ö _{pH =}_{pH =}₂₂

But how about when [H

But how about when [H++_{] = 2.3}_{] = 2.3}×× ₁₀₁₀–4–4 _M?_M?

Use this shortcut…

Use this shortcut… •

• if [Hif [H++_{] =}_{] =}_y_y _×_× ₁₀₁₀––nn M (where M (where _n_n is a whole number).is a whole number).

•

• then pH is between (then pH is between (nn––1) and 1) and nn..

For above example, pH is between 3 and 4!

(43)

43 43

pH Calculations

Remember the shortcut for estimating pH

from [H

from [H++_]._].

When looking at weak acid (base), given its

initial concentration, use the

initial concentration, use the IICCEE method to method to find [H

find [H++_{] and pH.}_{] and pH.}

What is pH of 0.2 M solution of HCN?

•

• KK_a_a = 4.9 = 4.9 × × 1010––1010

The

(44)

44 44

pK

_a_a

and

pK

_b_b

We know pH is

We know pH is ––log [Hlog [H++_{], so what do you}_{], so what do you}

think

think ppKK_a_a or or ppKK_b_b is?is?

Any “p” function represents the

Any “p” function represents the ––log!log! p

pKK_a_a = = ––log log KK_a_a & & ppKK_b_b = = ––log log KK_b_b

High

High KK_a_a or or KK_b_b (closer to 1) means strength, so (closer to 1) means strength, so

how about for

how about for ppKK_a_a or or ppKK_b_b??

Lower is stronger! Think of it like pH

Lower is stronger! Think of it like pH ÖÖ lower is more concentrated.

lower is more concentrated.

Don’t forget

(45)

45 45

Neutralization Reactions

What is a

What is a neutralizationneutralization reaction?reaction?

When an acid and base react to form a

When an acid and base react to form a saltsalt and

and waterwater..

Like when you take antacid (Tums,

Like when you take antacid (Tums, etcetc.)..).

A handy formula for complete neutralization:

•

• aa ×× [A][A] ×× VV_a_a = = bb ×× [B][B] ×× VV_b_b •

• aa is # acidic H, is # acidic H, bb is # H’s base can accept.is # H’s base can accept.

•

(46)

46 46

Neutralization Reactions

How much 0.1 M

How much 0.1 M NaOHNaOH solution needed to solution needed to neutralize 40

neutralize 40 mLmL of 0.3 M of 0.3 M HClHCl?? HCl(aq

HCl(aq) + ) + NaOH(aqNaOH(aq) ) NaCl(aqNaCl(aq) + H) + H₂₂O(l)O(l)

a

a ×× [A] [A] ×× VV_a_a = = bb ×× [B] [B] ×× VV_b_b

1

1 ×× 0.3 0.3 ×× 40 = 1 40 = 1 ×× 0.1 0.1 ×× VV_b_b

Solve for

Solve for VV_b_b to get 120 to get 120 mLmL..

Remember the formula to get you out of

jams…works with neutralization reactions

only to save time over the

(47)

47 47

Indicators

An indicator marks the endpoint of a titration,

but why does it change color?

Indicator is actually weak acid whose

protonated

protonated form is one color and form is one color and deprotonateddeprotonated is another.

is another.

How does this help us?

Don’t forget that the indicator is a weak acid!

(48)

48 48

Indicators

HIn HIn '_' HH++ ₊₊_In_In–– K

K_a_a = [H= [H++][In][In––]/[]/[HInHIn]]……now rearrange:now rearrange:

[H

[H++_]/_]/_K_K a

a = [= [HInHIn]/[In]/[In––]]……and look at ratios:and look at ratios: •

• If [HIf [H++_{] »}_{] »}_K_K a

a, [, [HInHIn] » [In] » [In––]…see ]…see color 1color 1

•

• If [HIf [H++_{] = K}_{] =}_K a

a, [, [HInHIn] = [In] = [In––]…mix of two ]…mix of two ccoolloorrss

•

• If [HIf [H++_{] «}_{] «}_K_K a

a, [, [HInHIn] « [In] « [In––]…see ]…see color 2color 2

So indicator turns colors over short pH range

near its

(49)

49 49

Picking an Indicator

When picking an indicator, use one whose

p

pKK_a_a value lies within the pH range that you value lies within the pH range that you

want to detect.

For indicator,

For indicator, ppKK_a_a +/+/–– 1 represents effective 1 represents effective

range for color change.

So don’t expect to detect a pH change from

3 to 4 if your indicator has a

(50)

50 50

Hydrolysis of Salts

The reaction of a substance (salt/ion) with

water

water is a hydrolysis reaction.is a hydrolysis reaction.

Question…will the hydrolysis result in a

neutral, acidic, or basic salt?

NaCl

NaCl in water in water ÖÖ neutral because neither Naneutral because neither Na++

nor

nor ClCl–– _{will react with water:}_{will react with water:} •

• ClCl–– _{is conjugate base of a strong acid.}_{is conjugate base of a strong acid.}

•

• NaNa++ _{is conjugate acid of a strong base.}_{is conjugate acid of a strong base.}

NH

NH₄₄Cl in water…acidic because NHCl in water…acidic because NH₄₄++ _will_will

react with water (it is a weak acid).

(51)

51 51

Hydrolysis of Salts

NH

NH₄₄CN in water…NHCN in water…NH₄₄++ _{is weak acid, but}_{is weak acid, but}

CN

CN–– _{is good base, so which wins??}_{is good base, so which wins??}

Need to know

Need to know KK_a_a//KK_b_b values!values! K

K_a_a for NHfor NH₄₄++ = 6.3 = 6.3 ×× 1010–10–10

K

K_b_b for CNfor CN–– = 1.6 = 1.6 ×× 1010–5–5

Solution will be basic because CN

Solution will be basic because CN–– _{is better}_{is better}

base than NH

(52)

52 52

What is a buffer?

A solution that resists changing pH when a

small amount of acid or base is added.

The resistance comes from the presence of a

weak acid or base and its conjugate in

roughly equal concentrations.

An expression to remember:

K

K_a_a = = [H[H++]] [conj. base][conj. base]

[acid]

(53)

53 53

Buffer Compensation

A biochemical example: the blood.

The main buffer: carbonic acid:

CO

CO₂₂ + H+ H₂₂O O '_' HH₂₂COCO₃₃'_' HH++ _{+ HCO}_{+ HCO} 3 3––

(54)

54 54

Buffer Compensation

Compensation critical to prevent medical

disorders like acidosis and

disorders like acidosis and alkylosisalkylosis.. Essentially Le

Essentially Le Châtelier’sChâtelier’s principle at work.principle at work. Addition of small amount of acid like

Addition of small amount of acid like HClHCl causes added

causes added HH++ _{to react with present}_{to react with present}_HCO_HCO 3 3––

to form

to form HH₂₂COCO₃₃..

Equilibrium has shifted to the left, but existing

H

H₂₂COCO₃₃prevents large change in pH prevents large change in pH (compensation reverses effects).

(55)

55 55

Buffer Compensation

Provided that acid and its conjugate are in

similar concentration, the pH change is

minimal.

Addition of small amount of base like KOH

causes added OH

causes added OH–– _{to react with present H}_{to react with present H} 2

2COCO33

to form HCO

to form HCO₃₃––_._.

Equilibrium has shifted to the right, but

existing HCO

existing HCO₃₃–– _{prevents large change in pH}_{prevents large change in pH}

(compensation reverses effects).

(56)

56 56

Dealing with Buffers

If [A

If [A––_{] = [HA], then pH =}_{] = [HA], then pH =}_p_p_K_K a a

An ideal buffer works under these

conditions. Small changes to the ratio are

acceptable, but the ratio should stay near 1.

Henderson

Henderson--HasselbalchHasselbalch Equation:Equation: pH

pH = = ppKK_a_a + log+ log [[AA pOHpOH = = ppKK_b_b + log+ log – –_]_] [ [HAHA]] [[AA––_]_] [ [HAHA]]

(57)

57 57

Dealing with Buffers

–

Example

Which of the following compounds could be

added to a solution of HCN to create a buffer?

• • HNOHNO₃₃ • • CaClCaCl₂₂ • • NaCNNaCN • • KOHKOH

(58)

58 58

Acid

-

_-

Base Titrations

_{Base Titrations}

–

_–

Terms

_Terms

Titration

Titration ÖÖ experimental technique to experimental technique to determine concentration or identity of

determine concentration or identity of

unknown weak acid or base by determining

p

pKK_a_a or or ppKK_b_b..

Titrant

Titrant ÖÖ strong acid or base of known strong acid or base of known

concentration that is added to solution of the

unknown base or acid.

Titration curve

Titration curve ÖÖ a plot of pH _{a plot of pH}vsvs. volume of . volume of

titrant

(59)

59 59

Acid

-

_-

Base Titrations

_{Base Titrations}

–

_–

Terms

_Terms

Buffering domain

Buffering domain ÖÖ the section of the the section of the titration curve where the pH changes

titration curve where the pH changes

gradually

gradually before the equivalence point.before the equivalence point.

Equivalence point

Equivalence point ÖÖ the point during a the point during a drastic pH change in which there is

drastic pH change in which there is

complete neutralization of acid and base.

End point

End point ÖÖ one or two drops of one or two drops of titranttitrant past past equivalence point!

equivalence point!

•

• indicator color flip visually detectable:indicator color flip visually detectable:

HIn

_HIn

GF

In

_In

––

(60)

60 60

Titration Curves

–

The Basics

Equivalence point and pH: not necessarily

at pH of 7

at pH of 7 ÖÖ salt hydrolysis?_{salt hydrolysis?}

Looking at the pH of the equivalence point

allows you to determine if unknown is weak

or strong.

Some quick rules for determining the pH at

the equivalence point.

(61)

61 61

Titration Curves

–

_–

The Basics

_{The Basics}

weak acid

weak acid + + strong basestrong base::

•

• pH > 7pH > 7 because product contains because product contains basicbasic salt.salt. •

• HF + HF + NaOHNaOH pp NaNaFF + H+ H₂₂OO

strong acid

strong acid + + weak baseweak base::

•

• pH < 7pH < 7 because product contains because product contains acidicacidic salt.salt. •

• NHNH₃₃ + + HClHCl pp NH_NH₄₄ClCl

strong acid

strong acid + + strong basestrong base::

•

• pH = 7pH = 7 because product contains neutral salt.because product contains neutral salt. •

(62)

62 62

Titration Curves

–

The Basics

Buffer!

Half

Half--equivalence?equivalence? The volume of

The volume of titranttitrant at halfat half--equiv. is half equiv. is half that at equivalence.

that at equivalence.

More importantly, the concentration of

unknown at half equiv is equal to the

concentration of the unknown

concentration of the unknown’’s conjugate at s conjugate at half equivalence.

half equivalence.

•

(63)

63 63

Titration Curves

–

_–

The Basics

_{The Basics}

In that case, the following must be true:

•

• [[HA]HA]_{half equiv}_{half equiv} = [A= [A––]]_{half equiv}_{half equiv} = [HA]= [HA]_i_i

By H

By H--H equation, the pH at half equiv. must H equation, the pH at half equiv. must be equal to

be equal to ppKK_a_a ÖÖ this allows identification of this allows identification of

unknown.

Types of Curves:

•

• Slope to upper right Slope to upper right ÖÖ SASA++SBSB, , WAWA++SBSB •

• Slope to lower right Slope to lower right ÖÖ WBWB++SASA •

(64)

64 64

Titration Curves

(65)

65 65

Titration Curves

(66)

66 66

Titration Curves

(67)

67 67

Titration Curves

–

Examples

Methyl red is an indicator that changes from

red

red to to yellowyellow in pH range in pH range 4.4 4.4 –– 6.26.2. For which . For which of the following titrations would methyl red be

of the following titrations would methyl red be

useful for indicating the equivalence point?

•

• HCN with KOHHCN with KOH •

• NaOHNaOH with HIwith HI •

• CC₆₆HH₅₅COOH with COOH with LiOHLiOH •

(68)

68 68

Titration Curves

–

Examples

Draw the following titration curves, paying

particular attention to shape while labeling

the relevant parts:

•

• CHCH₃₃COOH titrated with KOHCOOH titrated with KOH •

• CC₆₆HH₅₅NHNH₂₂ titrated with HNOtitrated with HNO₃₃ •

(69)

69 69

System and Surroundings

The

The systemsystem is the thing we’re looking at, for is the thing we’re looking at, for

example a melting ice cube or a solid dissolving

into water.

Everything else is known collectively as the

surroundings

surroundings, for example the table the ice cube , for example the table the ice cube sits on and the surrounding air or the beaker that

sits on and the surrounding air or the beaker that

the solution sits in.

System

System + + surroundingssurroundings = = thermodynamic thermodynamic universe

(70)

70 70

System and Surroundings: Energy

Flow

Remember conservation of energy?

When energy flows into a system from the

surroundings (+), the energy of the system

increases and energy of surroundings decreases.

When energy flows out of a system into the

surroundings (

surroundings (––), the energy of the system ), the energy of the system

decreases and energy of surroundings increases.

(71)

71 71

First Law of Thermodynamics

The First Law

The First Law ÖÖ anybody know it?anybody know it?

Total energy of the universe is constant, so

energy can be transferred but not created or

destroyed.

Transfer of energy can occur as heat or work:

∆

∆EE = = qq + + ww ÖÖ ww = = PP∆∆VV

Energy input

Energy input ÖÖ positive _positiveqq (heat (heat absorbedabsorbed) and ) and ww

(work done

(work done onon system).system). Energy out

Energy out ÖÖ negative _negativeqq (heat (heat releasedreleased) and ) and ww

(work done

(72)

72 72

First Law of Thermodynamics

Energy transfer in car air conditioner.

Air taken in through hose and moves over fins of

evaporator.

This hot air evaporates refrigerant inside (heat

transfer) which travels to condenser and is

changed back into liquid (heat transfer).

Air which was cooled when heating the fins of

the evaporator flows into the car’s interior.

No net change in energy for system…refrigerant

heated but then cooled to its original state.

(73)

73 73

First Law of Thermodynamics

Adiabatic and

Adiabatic and isothermicisothermic?? Isothermic

Isothermic: expansion or compression of gas at : expansion or compression of gas at constant temperature (heat input or output).

constant temperature (heat input or output).

Adiabatic

Adiabatic: expansion or compression of gas : expansion or compression of gas

without a heat exchange (temperature changes;

q

q = 0 = 0

Ö

∆∆EE = = ww))

Adiabatically expanding gases cool while

adiabatically compressing gases warm.

Adiabatic principle at work: snow

Adiabatic principle at work: snow--making making machine.

(74)

74 74

First Law of Thermodynamics

Snow

Snow--making machines contain mixture of making machines contain mixture of compressed air and water vapor (20

compressed air and water vapor (20 atmatm).). When sprayed into the air, the mixture

When sprayed into the air, the mixture

undergoes a huge pressure change and expands

so rapidly that essentially no heat is exchanged

between system and surroundings (adiabatic

expansion).

Adiabatic expanding gases cool, so the water

vapor changes almost instantaneously to snow.

(75)

75 75

Second Law of

Thermodynamics

The Second Law

The Second Law ÖÖ what does it state?what does it state?

All processes tend to run in a direction that leads

to maximum disorder. If a process is

spontaneous in one direction, then the reverse

cannot be spontaneous

cannot be spontaneous ÖÖ makes sense!_{makes sense!}

From here, we can discuss

From here, we can discuss entropyentropy, , enthalpyenthalpy, , and

(76)

76 76

Entropy: Disorder or Randomness

∆

∆SS = = SS_products_products –– SS_reactants_reactants

•

• Increasing randomness (decreasing order) results in a Increasing randomness (decreasing order) results in a positive

positive ∆∆SS.. •

• Decreasing randomness (increasing order) results in Decreasing randomness (increasing order) results in negative

negative ∆∆SS..

What are some situations in which entropy is

predictable?

Liquids have more than solids, gases more than

liquids, particles in solution more than solids,

two moles more than one mole.

(77)

77 77

Enthalpy: Heat Energy

Two important principles

Two important principles ÖÖ bond formation bond formation

releases energy (not needed to ensure stability),

energy must be added to break a bond.

∆

∆HH = = HH_products_products –– HH_reactants_reactants

Exothermic

Exothermic: heat released because products of : heat released because products of reaction have stronger, more stable bonds than

reaction have stronger, more stable bonds than

reactants.

Endothermic

Endothermic: heat absorbed because products of : heat absorbed because products of reaction have weaker bonds and energy needed

reaction have weaker bonds and energy needed

to transition to this more unstable state.

(78)

78 78

Enthalpy and Heat of

Formation

Amount of energy required to make

Amount of energy required to make one moleone mole of of a compound from pure elements in their natural

a compound from pure elements in their natural

or standard state.

If

If ––

∆

HH, heat given off…if , heat given off…if ++

∆

HH, heat required., heat required.

What is standard state?

T

T = 298 K, = 298 K, PP = 1 = 1 atmatm, [X] = 1 M., [X] = 1 M.

Designated by

Designated by °° superscript, standard state is superscript, standard state is condition for determining heat of formation,

condition for determining heat of formation,

enthalpies, constants,

(79)

79 79

More Enthalpy

Each reaction has own

Each reaction has own ∆∆HH ÖÖ doubling reactants doubling reactants

will double the heat required or released.

Hess’s Law

Hess’s Law of Heat Summation, Remember?of Heat Summation, Remember? If a reaction occurs in several steps, the sum of

If a reaction occurs in several steps, the sum of

the energies absorbed or released in all steps will

be the same as the overall reaction.

If reaction is reversed, the sign of

If reaction is reversed, the sign of ∆∆HH reverses reverses

too.

If equation multiplied by constant, so too is

(80)

80 80

Hess’s Law Example

C(s) + C(s) + 22HH₂₂(g)(g) CH CH₄₄(g)(g) CO CO₂₂(g) + (g) + 22HH₂₂O(g)O(g) Direct path

Direct path ∆∆HHnetnet

= = ∆∆HH₁₁ + + 22∆∆HH₂₂ + (+ (––∆∆HH₃₃)) Indirect path Indirect path + 2O + 2O₂₂(g)(g) ∆ ∆HH_rx_rx = = ∆∆HH₁₁ + + 22∆∆HH₂₂ – – 2O2O₂₂(g)(g) ∆ ∆HH_rx_rx = = ––∆∆HH₃₃

(81)

81 81

Gibbs Free Energy

The energy available to do useful work from a

chemical reaction.

Spontaneity determined by enthalpy and entropy,

so spontaneity can be accounted for by

so spontaneity can be accounted for by ∆∆GG..

∆ ∆GG = = ∆∆HH –– TT∆∆SS When… When… • • ∆∆G < 0G < 0 ÖÖ spontaneousspontaneous • • ∆∆G = 0G = 0 ÖÖ at equilibriumat equilibrium •

(82)

82 82

∆

G and Temperature

∆

∆GG = = ∆∆HH –– TT∆∆SS ∆

∆HH ∆∆SS ∆∆GG Reaction is…Reaction is…

– – ++ –– SpontaneousSpontaneous + + ++ –– at high at high TT + + at low at low TT Spontaneous Spontaneous Non

Non--spontaneousspontaneous

–

– –– + + at high at high TT

–

– at low at low TT

Non

Non--spontaneousspontaneous Spontaneous

Spontaneous

+

(83)

83 83

Reaction Energy Diagrams Revisited

Two different diagrams

Two different diagrams ÖÖ endothermic and endothermic and exothermic.

exothermic.

∆

∆GG ≈ ≈ ∆∆HH when when TT∆∆SS is small.is small. Do not

Do not confuse confuse ∆∆GG, , ∆∆HH, or , or ∆∆SS with rate or with rate or EE_a_a!! •

• catalysts lower catalysts lower EE_a_a, but don’t touch , but don’t touch ∆∆G!G!

Reversibility

Reversibility: the reverse of any reaction has the : the reverse of any reaction has the same magnitude for all thermodynamic values

same magnitude for all thermodynamic values

(opposite signs) and the same reaction pathway:

•

• Coming from products side, Coming from products side, EE_a_a is now forward Eis now forward E_a_a

plus