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Flashcards in Chemistry 2 Deck (160):
1

Common forms of ΔH

  • Define ΔHformation
    • If number is NEGATIVE, formation of the cpd is an ______ process
    • If number is POSITIVE, formation of the cpd is an _____ process 

 

ΔHformation

  • The enthalpy value for the formation of a compound 
    • ...from its elements in their standard states

If ΔHformation value is NEGATIVE:

  • Formation of the cpd is an EXOthermic process

If ΔHformation value is POSITIVE: 

  • Formation of the cpd is an ENDOthermic process

2

Specific heat capacity definition

describes energy absorption for ONE individual substance

2

∆Hvaporization definition

enthalpy value associated with phase change from liquid to gas(therefore, ∆Hcondensation is the same value, just with a reverse sign)

3

Rate Law equation:X^Y=Z What do X, Y, and Z represent?

X=factor by which [reac] changedY=order of reactantZ=factor by which the rate changed (across the same 2 trials)

3

When is heat ,q, added exactly proportional to the increase in temperature? Why?

When system is at ~VNo PV work gets done--it 100% of energy added goes towards increasing temperature

3

Define "Entropy (∆S)"

  • What are its units?

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ENTROPY

a measure of DISORDER in a system

 

units=Joules/K

3

Common forms of ΔH

  • Calculating for "ΔHrxn"

...Using Bond Dissociation Energies (BDE's)

  • How do you solve for ΔHrxn?
    • Wrt signs (+/-), what do you do if a bond is BROKEN? If a bond is FORMED?
    • What last thing do you need to remember to do before you're done solving for ΔHrxn?

Calculating ΔHRXN Using Bond Energies:

This is not quite as common on the MCAT as the method of adding reactions (but it is a bit more intuitive in our opinion)

STEPS:

To do so, simply add up the BDE's of all of the products and reactants

  • If a bond is BROKEN during the reaction:
    • Energy is REQUIRED
      • ∴ the BDE should be given a POSITIVE sign
  • If a bond is FORMED during the reaction:
    • Energy is RELEASED
      • ∴ the BDE should be given a NEGATIVE sign

Once again, multiply all bond energy values by their coefficients in the balanced equation

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4

Entropy, ∆S

How would the following affect ENERGY being released/required?

  • As a rxn proceeds, if ∆S DECREASES 
    • i.e., it becomes more ORDERED

  • Energy would be REQUIRED
    • to create the increased orderliness

...and there'll be exactly that much LESS energy available to do work! 

In short: it's UNFAVORABLE

4

The ln of and positive (+) numer LESS than 1 is?

negative (-)

5

Anything to the zeroth power is equal to?

1

5

Catalyst definition

increases rxn rate w/o being consumed in the process

5

How do you solve for ΔH using a BOMB Calorimeter?

  • What DOES and DOESNT solving for Bomb Calorimeters give us?

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Solve using q=mc ∆T

DOES give:

  •  change in internal energy (∆U or ∆E)

DOESNT give:

  • a negative ∆H

bomb calorimeter is at ~V, in a sealed steel container

6

Rate Order Graphs:Will only be linear when? (2)

1) When rxn only has 1 reactant, or2) when it is part of a multiple reactant rxn where rate is independent of ALL other reactants (basically, when the other reactant is zeroth order or is in excess)

7

What is a Black Body Radiator?

Theoretically perfect body that absorbs ALL energy that hits it, and then re-emits 100% of the energy absorbed

8

Calculating E˚ for electrochemical rxns

add together 1/2 rxns, but when calculating E˚, do NOT multiply by coefficients in balanced rxn (UNlike when you calculate ∆Hrxn by adding then multiplying BDE values by the coefficients)

8

∆G (+) means?

Energy is available and the system CAN do work

8

What formula relates the Keq to Gibbs Free energy? (2 versions)

∆G˚=-RTlnKeqORKeq=e^(-∆G/RT)

9

Slow step=?

rate-determining step

9

How to write rate law for a catalyzed rxn

write rate law in same way as normal, w/ [catalyst] added in as a reactant

9

When heat enters into a system, if the system is capable of volume change, heat can either...

Do PV workIncrease temperature (Av KE of molecules)or Both

9

∆G (-) means?

energy must be added to rxn (i.e. heat must be added to system) to make it proceed

10

Define the ZEROTH LAW of Thermodynamics

  • If object A is in thermal equilibrium with object B
  • And object C is in thermal equilib with object B

...then object A and object C are ALSO in thermal equilibrium!

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11

Calculating ∆Hrxn using BDE's

add up BDE's in products & reactants-bond broken during rxn= energy REQUIRED, BDE has (+) sign-bond formed as a result of rxn= energy RELEASED, BDE has a (-) sign

12

Acid-Base Chemistry

  • Describe a Bronsted-Lowry Acid & Base

HINT: Black L.A.P.D

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Acids

  • DONATE protons (H + )

Bases

  • ACCEPT protons (H + )

 

"B.L.A.P.D"

BLACK L.A.P.D 

 

13

If Keq > 1, rxn will be...?(if we are starting at standard conditions)

SPONTANEOUS!

14

Heat capacity formula

C=q/∆T

14

On a pressure vs. volume graph:

  • Where is PV work?

The area UNDER the curve

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15

How to calculate ORDER of each reactant (using experimental data)

1) Find 2 steps where [reac] in question changed, but ALL OTHERS DID NOT2) Note the factor by which [reac] changed3) note the factor by which RATE changed across those same 2 trials4) Use this to figure out X^Y=Z

16

Thermodynamics

  • How is "WORK" defined in CHEMISTRY?

Work=

A change in VOLUME

  • ...at a constant ~P

17

How will the following affect rxn rate?

increasing energy of transition state

DECREASE rate

because it increases Ea

18

Bond broken during rxn=energy ___

required

20

How will the following affect rxn rate?

increase [ reactants ]

increase rate

21

Differentiate heat capacity "C" and specific heat capacity "c"

Heat capacity is for a system (ex: solution, container holding sol'n, etc.)Specific heat capacity describes energy absorption for ONE individual substance ONLY!

22

Do catalysts increase % yield?

NOPE!!

22

What should you think of (and mentally replace!) whenever you see the word "temperature?"

temperature=av. KE of molecules

23

PV Work

  • Formula=?
  • What do you need for in order to see PV work?
  • What is a dead giveaway PV work has been done?

PV Work =P∆V

Needs ~P

Change in Volume is a dead giveaway!!

24

if slow step is 2nd, what happens wrt the rate law?

rate law=rate law of the slow stepslow step will include an intermediate as of the reactants

25

Bond formed as result of rxn=energy ___

released

26

How will the following affect rxn rate?

increase [products]

NO effect

26

∆Hfusion definition

enthalpy value associated with phase change from liquid to solid (sign reverses for ∆Hmelting)

27

If 2 objects are in thermal equilibrium, by definition they have what?

The SAME TEMP!

27

Entropy, ∆S

POSITIVE ∆S means?

  • _______ randomness, ____ energy available to do work

  • INCREASED randomness
  • MORE energy available to do work

28

A reaction with a HIGHER ∆S 

is more likely to?

Happen SPONTANEOUSLY!

30

How will the following affect rxn rate?

increasing [catalyst]

increase rate

31

Acid-Base Chemistry

  • What are 2 COMMON examples of Lewis Acids? (LAEPA)

THINK: What 2 things are very likely to accept an electron pair? 

AlCl3

BF3

32

the word "decomposition" tells us what?

we are going from ONE species to 2+ species (∆S is positive)

32

What is the SPECIFIC HEAT of H2O? 

Specific Heat of Water=

1.0 cal/g˚C

OR 

4.18 J/g˚C

33

Define the FIRST LAW of thermodynamics

  • Energy cannot be created nor destroyed

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34

Thermodynamic Formula

∆G=∆H-T∆S

35

∆Hrxn definitionHow do you solve for it? (1 step, 2 things to remember to do)

enthalpy change for a rxn. -Add together enthalpy change values from the table (given) that, when added, produce net rxn for which you are calculating ∆Hrxn-REMEMBER: if rxn proceeds in opposite direction as it would in the net rxn, change its sign! -REMEMBER: You need to multiply the value given in the table by the coefficient in the balanced net rxn (like...if ∆H value from table for H20 is 1.5 but you have 2 moles of water, multiply 1.5 x 2=3)

36

Enthalpy

  • Define "Standard State"
    • What values do you need to have memorized for Standard State?
    • What should you be careful not to mix it up with? 

​STANDARD STATE

is a set of conditions set as a

REFERENCE POINT for measuring:

∆H, ∆G, and ∆S

  • Do NOT memorize ANY values for Standard State!
    • because they can be different for different tables

Don't mix Standard State up with ​STP! 

STP=Standard Temperature & Pressure

36

When comparing trials to write a rate law for a reactant, what should you look for?

Find 2 trials where [ ]'s of reactant A changes, but everything else (like [ ] of reactant B and temperature) stays the same. Something with the same conditions!

37

Rate Order graphs: first order

ln [A] vs time is linear with slope= -k

38

Define heat capacity

amount of energy (in joules or calories) a system must absorb to give a unit change in temperature

40

Relationship b/t enzymes & catalysts

Enzyme=biological catalystALL enzymes are catalysts, but not all catalysts are enzymes

41

ZEROTH Law of Thermodynamics (A=B, B=C, ∴ A=C)

  • What is the equation that shows the direct relationship b/t TEMP and KE?

HINT: It uses Boltzmann's Constant, "kB"

 

KE=(3/2)kBT

"OSD"

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42

The First Law of Thermodynamics

2 ways to define: 

  1. The total energy of an isolated system is always constant
  2. The energy change in a closed system is equal to the heat absorbed by that system plus any work done on that system by its surroundings
    • ∆E = q + w

​How do both of these definitions of the First Law essentially say the SAME THING?

BASICALLY:

What we consider to be "The System" & "The Surroundings" is all RELATIVE!! 

  • An isolated system is a system for which neither mass nor energy can be exchanged with the surroundings
  • A closed system is a system that can exchange energy with its surroundings but not mass. Definition 2 treats the system and its surroundings separately
  • It is overtly stated that energy transfer is occurring between the system and the surroundings (i.e., work is being done on the system by the surroundings and/or heat is being added to the system from its surroundings)
  • The total energy change of the system in case 2 would therefore be the sum of the energy absorbed and the work done on the system (∆E = q + w)
  • However, if we defined "the system" as BOTH the system receiving the heat/work AND its surroundings, we would then have an isolated system
  • The entire isolated system would not change in energy—which is exactly what definition 1 implies
  • What were formerly considered the “system” and the “surroundings” in definition 2 are now just parts of the “system” as defined for definition 1
  • Therefore, heat or work can be transferred back and forth between parts of the system and the total energy of the entire isolated system would not change
  • To further illustrate, let’s say that 10J of heat were transferred to the system and 10J of work were also done on the system by the surroundings
  • According to definition 2, ∆E = 10J + 10J = 20J
  • According to definition 1, ∆Esystem = 10J + 10J = 20J and ∆Esurroundings = -10J + (-10J) = -20J
  • Therefore ∆Eisolated system = 20J – 20J = 0

43

Convection, Conduction, & Radiation

  • A pan of water is placed upon an electric heating element on a stove
  • Describe all types of heat exchange expected to occur in this scenario

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43

When you see "ENTHALPY (ΔH)," 

THINK: ?

The ENERGY CONTAINED

WITHIN CHEMICAL BONDS

44

∆Hsol'n definition

enthalpy value associated with dissolution of a species into a solution

46

What do exponents in a rate law represent?

the "order" of each reactant

47

High (+) BDE means the cpd is very ___

stable

48

Entropy, ∆S

  • How would the following affect ENERGY?

As a reaction proceeds, its ∆S INCREASES

  • i.e., becomes more DISordered

 

Energy will be RELEASED

  • MORE energy will be available to do work!

In short: It's FAVORABLE

50

What are the 3 types of heat exchange?

1) Convection2) Radiation3) Conduction

51

Think of ∆H as...units?

energy contained w/in chemical bondsunit= Joules

53

When writing RATE LAWS, what 2 things must you assume

1) Assume reactions only proceed FORWARD

  • ignore reverse rxn

2) Only consider the first few seconds of the reaction

  • when theres a high [ ] of:
    • each reactant and
    • catalysts (enzymes) present

53

Entropy, ΔS

INCREASES when there is an INCREASE IN:

(4 things can increase entropy)

One of these 4 carries a CAVEAT

HINT: gas

Entropy increases with:

1) Increased number of:

  • items/particles/etc.

Caveat:

The number of moles of GAS TRUMPS the number of moles of species in any other phase!

  • Thus, even if a reaction turns 2 moles of reactants into 1 mole of product, if that one mole of product is a GAS and the reactants are NOT:
    • Entropy has increased
    • ΔS will therefore be positive

2) Increased VOLUME

3) Increased TEMPERATURE

4) Increased DISORDER or COMPEXITY

  • Increased "Order
    • a crystal (highly ordered) is changed to an amorphous material (highly disordered)
  • Increased "Complexity
    • S is greater for C2H6 than for CH4

54

Large (-) ∆Hformation means what? (2)

Energy released when bonds are formed is very highcompound is very stable

55

More unstable cpds: -∆Hcombustion?

have a HIGHER ∆Hcombustion, release MORE energy when reacted with O2

55

If a graph for a reactant is linear at:[A] vs. time, slope=-kit must be? What if it isnt linear?

Zeroth order not linear=not zeroth order, but it could be first or second, etc

57

Light colors radiate and absorb ___dark colors radiate and absorb ___

Light: absorb lessDark: absorb more

57

Isobaric definition

~P

58

[OH][H]What increases in [ ] when you INCREASE pH?

[OH]

59

  • Name the 2 kinds of calorimeters
    • What are their distinguishable properties?
    • HINT: What is constant in each type? 

  1. COFFEE CUP Calorimeters
    • ~P
  2. BOMB calorimeters
    • ~V

59

Think of Gibbs Free Energy, ∆G, as...

the amount of "free" or "useful" energy available to do work (**Note**doesnt include pv work)

61

Calorimeters

  • Which type of calorimeter allows for PV WORK?
    • Why?

COFFEE CUP Calorimeters!

  • because it's constant pressure 

Top of cup is covered by insulated cork "stopper"

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63

When values are given for standard state conditions, what thingy is added?

superscript is added --> ∆H˚ "not"

64

Heat Exchange

  • Define RADIATION 

Give an example

Electromagnetic waves emitted from hot body into surrounding environment

  • ex: a black car on a hot day emits heat

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65

Entropy, ∆S

NEGATIVE ∆S means?

  • _______ randomness, ____ energy available to do work

  • LESS randomness
  • LESS energy available to do work

65

When a solute is mixed in solution, ∆S is always ___. Why?

is always (+)something becomes more disordered dissolved in a solution

67

Give an example of ~V heat capacity

system confined by rigid walls

68

Which thermodynamic elements are "NON-SPONTANEOUS" or "UNFAVORABLE?"

∆G is (+)∆S is (-)∆H is (+)

70

∆Hformation definitionExothermic=?Endothermic=?

enthalpy value for formation of a cpd from its elements in their natural stateExothermic is (-)Endothermic is (+)

72

Heat Exchange

  • Define CONVECTION

What are 2 common examples?

Hotter sections of fluid rise, cooler portions sink

Examples

  1. Air currents
  2. Convection currents (El Nino)

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73

Rate Order graphs: Zeroth order

[A] vs time is linear with slope= -k

74

If a graph for a reactant is linear at:ln[A] vs. time, slope=-kit must be? What if it isnt linear?

must be 1st ordernot linear= not first order. could still be 0th or 2nd order

75

Define "kinetics"

 How is it measured?

study of reaction RATE

  • Measured in terms of how fast REACTANTS DISAPPEAR
    • by tracking changes in [ ] of reactants

77

What DO catalysts affect? What DONT they affect?

DO affect rate (by lowering Ea)DONT affect Keq, ∆H, ∆S, ∆G, or any other thermodynamic properties

78

in a 2-step rxn, does the yield of the first rxn affect the Keq of the second?

NOPERS

79

Entropy increases with increasing...

number of moleculestemperaturevolumemoles of gas

81

Collisions cause?

reactions!

83

How will the following affect rxn rate?

increase temp

increase rate

84

If a graph for a reactant is linear at:1/[A] vs time, slope=-kit must be? what if it isnt linear?

it must be 2nd ordernot linear=not 2nd order. could still be 1st or 0th

85

Isothermal definition

~T (~av. KE of molecules)aka NO HEAT EXCHANGE

87

is heat capacity greater for ~V or ~P?

For ~P, because it CAN do pv work as well as temperature work (therefore has more "capacity" that a system with ~V)

88

How will the following affect rxn rate?

increasing energy of reactants

increase rate

  • increasing energy of reactants brings it closer to the Ea
    • makes it easier to get over the "hill"

89

When calculating ∆Hrxn, what do you need to remember with regards to signs?

bonds FORMED= energy released (-)bonds BROKEN= energy req'd (+)

90

If, during a rxn, something happens and the Temp increases as a result, what must ∆H be?

(-) aka exothermic

91

Common forms of ΔH

  • Define "ΔHcombustion"
    • What does a HIGH heat of combustion mean wrt a molecule's stability?

ΔHcombustion

  • The enthalpy value for the combustion of a compound with O2
    • ...to form CO2 and water

____+ O2⇒CO2+ H2O

A HIGH heat of combustion is associated with:

  • an UNstable molecule

∴ a LOW heat of combustion means

the molecule is more stable

93

How is the sign of work determined in CHEMISTRY?

Work done ON system= positiveWork done BY system=Negative

93

pH scale

  • All equilibrium constants (Keq, Ka, Kb. Kw or Ksp) are written via the ___ of ____ ______, with ____s and ____s OMITTED

All equilibrium constants (Keq, Ka, Kb Kw or Ksp):

  • are written via the LAW of MASS ACTION

with PURE liquids (l) and solids (s) omitted

95

What should you remember for the MCAT about thermal equilibrium?

Everything TENDS TO move towards thermal equilibrium with everything else. -objects with higher temperatures will always equilibrate over time with their surroundings, including other objects with which they are in contact

96

1˚ C= ___ Kelvin

274 K

97

Give an example of ~P heat capacity

water in a beaker (open to the atmosphere)

98

Common forms of ΔH

  • Calculating for "ΔHrxn"
    • What 2 things do you ALWAYS need to remember to look out for when solving for ΔHrxn?

Which of these things should you IGNORE when calculating for  in electrochemical reactions? 

When calculating for ΔHrxn, always be cognizant of:

  1. Multiply by the coefficients in balanced equation
  2. Change the sign! (Depending on direction of the reaction)

Solving for ΔHrxn and E° is done in almost an IDENTICAL way! (Adding things together)

BUT!!!

When calculating E°:

  • You DO NOT multiply by the coefficients in the balanced equation

(You DO change the sign if the

half-reaction is reversed, however)

100

∆E=?

q+W

101

The First Law of Thermodynamics

  • What are the two common ways to DEFINE the First Law? 

HINT: Has to do with energy in both isolated & closed systems

  1. The total energy of an isolated system:
    • is always constant
  2. The energy change in a closed system:
    • is equal to the heat absorbed BY that system plus
    • any work done ON that system by its surroundings

∆E = q + w

102

ZEROTH Law of Thermodynamics (A=B, B=C, ∴ A=C)

  • If two objects are in THERMAL equilibrium, by definition they have the same ______

  • Same TEMPERATURE
  •  

103

Thermodynamics

  • How is "WORK" defined in PHYSICS?

Work=

Energy transfer via a force

104

When the MCAT tests LEWIS acids or bases, they always specifically label them as such

  • Otherwise, when you see “acid” on the MCAT, think of ____ _____definition
    • In other words, think of....?
  • What about the "Arrhenius" definition?

  • Otherwise, when you see “acid” on the MCAT, think of the Bronsted-Lowry definition
  • In other words, think of species that donate an acidic hydrogen

 

  • ​The Arrhenius definition is oversimplified and rarely seen on the MCAT

106

Thermodynamics includes what 4 things?

  1. ΔG
  2. ΔH
  3. ΔS
  4. Keq

107

Rate Order graphs: Second order

1/[A] vs time is linear, slope = k

108

∆G=-RTlnKeqWhat do you need to remember about this formula?

This formula helps find sign of ∆Gremember: ln of a (+) that is 1 is positive**remember the (-) sign in front of RT! If (lnKeq) is positive, that will turn it negative. Negative x positive=negative, therefore ∆G will be (-), & vice versa

110

How will the following affect rxn rate?

increase Ea

decrease rate

111

If slow step is 1st, what can you do wrt the rate law?

the rate law can be written as if it were only 1 step

112

What are 2 ways to define the first law of thermodynamics?

1) total energy of an isolated system is constant2) Total energy in a closed system (energy can leave, mass can't) is equal to the heat,q, absorbed by that system PLUS and work done on that system by its surroundings

113

Acid-Base Chemistry

  • Describe a LEWIS Acid & Base

"La E.P.A"

(Environmental Protection Agency en español)

 

Acids

  • AcceptPAIR OF ELECTRONS

L.A.E.P.A

Bases

  • Donate a pair of electrons

 

 

 

114

How is the sign (+/-) defined in PHYSICS when it comes to WORK? 

HINT: Think of the formula for work

  • When force & displacement are in same direction (ie lifting a box)
    • work is (+)
  • When force & displacement are in opposite direction (ie lowering a box)
    • work is (-)

115

If Keq

NON-spontaneous∆G˚ is (-)

116

What does Keq=1 mean?

rxn is at equilibrium∆G˚=0

117

Elements in their elemental state have ∆H˚formation of?

ZERO(no ∆ in H needed to create THEMSELVES)

117

For ∆H rxn, if you're given an ∆H value of +1.5, but your water is a reactant instead of a product, what do you do?

Reverse the sign to (-)

118

Heat Capacity, C=q / ΔT

  • For the SAME SYSTEM, which heat capacity will be greater?
    • The constant volume heat capacity or the constant pressure heat capacity?

CONSTANT PRESSURE

If the VOLUME is held constant:

  • 100% of the energy added will go toward an increase in temperature

If the PRESSURE is held constant:

  • the volume can still change and therefore some of the added heat will go toward pv work
  • If we think of heat capacity as “the amount of energy we can add before the system increases by one temperature unit,” it is fairly easy to see that the system capable of pv work will be able to absorb MORE heat before increasing by one degree Celsius or Kelvin

REAL-LIFE EXAMPLE:

  • It is much like asking how many gallons of water can be added to Tank A vs. Tank B?
  • Tank A and Tank B are both 5-gallon tanks
    • ...but Tank B is connected via a hose to a reserve tank that holds 2 gallons
  • So, you can add 5 gallons to Tank A before it is “full” (analogous to a one unit increase in temperature)
  • However, you can add 7 gallons to Tank B before it is full (reserve tank = pv work)
  • We would therefore say that Tank B has the higher “water capacity” in terms of our analogy

This indicates that the constant PRESSURE heat capacity (allows for pv work; i.e., includes the 2-gallon reserve tank) will always be more than the constant volume heat capacity (does not allow for pv work; i.e., no reserve tank) for the same system

119

Watts is the same as?

Joules per second

121

Define the THIRD LAW of Thermodynamics 

Pure crystalline substances at absolute zero

...have a ∆S of 0

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123

Think of "RATE" as...(3 parts)

  1. a function of how fast the reactant molecules are moving
  2. how much KE they have
  3. height of Ea "hill"
    • that must be overcome to form products

124

∆S increases with? (4, plus 1 caveat)

1) increased # of items/particles/molecules**CAVEAT: # moles gas trumps # moles in other phases (cause gas is VERY disordered)...2 mol react with 1 mol (g) product=Positive ∆S2) Increased Volume3) Increased temp (av. KE of molecules)4) Increased DISORDER (ex: amorphous struc more disordered than crystalline) and COMPLEXITY (ex: C2H6 has greated ∆S than CH4)

126

Heat Exchange

  • Define CONDUCTION

Give an example

What thing is heat conduction analogous to?

CONDUCTION:

Molecular collisions carry heat along a current

  • ex: pot of boiling water

 

Heat conduction is analogous to

current flow through a wire

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127

What do you do to find the "overall order" of a rxn?

Add together the exponents in the rate law

128

ZEROTH Law of Thermodynamics (A=B, B=C, ∴ A=C)

For the MCAT, just remember this: 

"_______ tends to move towards ____ ______

with _____ _____"

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EVERYTHING tends to move towards THERMAL EQUILIBRIUM with EVERYTHING ELSE

129

∆Hcombustion definitionHigh ∆Hcombustion means what?

enthalpy value for combustion of a cpd w/ O2 to form water and CO2High ∆Hcomb. value=UNSTABLE molecule(low ∆Hcomb.value=stable)

131

What thermodynamic properties are"SPONTANEOUS?" or "FAVORABLE?"

∆G is (-)∆S is (+)∆H is (-)

133

Difference b/t spectator ion and catalyst

-spectator ion: no effect, do NOT participate in rxn-catalyst: decreases Ea, increases rate. NOT consumed in rxn (makes it through the rxn without changing. Same among products as it is among reactants)

134

Common forms of ΔH

  • Define ΔHrxn 
  • How do you usually calculate a ΔHrxn?
    • What do you add to get the ΔHrxn?
    • What 2 things must you remember to watch before adding?
      • How does the direction of the rxn affect these things? 
    • ^^EXAMPLE PROVIDED^^

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ΔHrxn

is the enthalpy change for a reaction

TO CALCULATE:

  • ΔHrxn usually calculated by adding reactions (and their associated enthalpy changes) from a table
  • You must select the reactions from the table that— when added together—will produce the net reaction for which you are calculating ∆HRXN
  • To calculate ∆HRXN you will add all of the values given for each of the reactions you use

Paying careful attention to ​SIGNS and STOICHIOMETRY

  • If the reaction proceeds in the SAME direction as it would in the net reaction
    • use the value given DIRECTLY
  • If it proceeds in the OPPOSITE direction
    • CHANGE THE SIGN of the value given

You MUST multiply the number given in the table by the coefficient in the balanced net reaction

  •  For example, suppose one of the reactions given was the formation of liquid water from the elements hydrogen and oxygen:

½O2 + H2⇒H2O

∆H° f = -285 kJ/mol

  • ...but in your net reaction water is a reactant
    • (rather than a product)
  • You would need to:
    1. CHANGE the sign of ∆H° to be positive, and
    2. Combine it with the enthalpy changes from other reactions used

If your balanced net reaction contained TWO moles of water, you would need to DOUBLE the value given, and so forth

135

2 L container of water and 4 L container of water Compare their heat capacities and specific heat capacities

Heat capacity of 4L is twice as much as other, b/c theres more water available to absorbSpecific heat capacities are the same in both

136

As far as the MCAT is concerned, ___ is the only thing that changes K

temperature

137

KMnO4What is oxidation state of Mn? Why?

+7

138

For a reaction to occur, what 2 things have to happen with regards to collisions?

1) Reactants must collide with enough energy to OVERCOME Ea "hill"

2) Reactants must be in correct SPATIAL orientation

139

Define the SECOND LAW of Thermodynamics

(has 2 parts!)

  1. Heat cannot be changed COMPLETELY into work in a cyclical process
  2. ∆S in an isolated system can never decrease

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140

Don't confuse standard state with?

STP values!

141

∆G=∆H-T∆S (+) (+)What is ∆G?

∆H is unfavorable∆S is favorable∆G is dependent on Temp! Will be favorable (-) only if Temp is high enough

142

Heat Exchange

  • What happens when heat ENTERS a system?
  • Does the temperature ALWAYS increase?
  • Is any increase in temperature always EXACTLY proportional to the heat absorbed by that system?

Hint:

Think of adding energy to a sealed steel container vs. adding energy to a balloon

  • remember that temperature is the average KE of the molecules, but an increase in temperature is NOT the only “place” where added energy can go

PV work is the work necessary

to produce an increase in VOLUME

  • For example, when a sealed balloon is heated, the gases inside the balloon will expand and must do work on the rubber walls of the balloon and the air around it to accomplish this expansion
  • Because some of the heat energy added to the balloon was used for pv work, only the REMAINING portion of the heat will go toward increasing the average KE of the molecules (i.e., temperature)

So, when heat enters a system, if the system is capable of volume change, heat can go to either:

  • pv work
  • increased temperature (av. KE)
  • or BOTH

For this reason, the addition of a certain amount of heat will NOT necessarily be EXACTLY proportional to the resultant

increase in temperature

If the system is NOT capable of changing volume:

  • NO pv work can be done
  • ∴ ALL of the added heat will go toward an increase in temperature

143

What is a CALORIMETER?

  • What do we assume about "q?"

CALORIMETER: a device used to calculate ∆H

  • assume q = ∆H
    • which is true at ~P

144

Acid-Base Equilibria

You should think of Ka or Kb just as you do ___?

  • A large Ka (or a small pKa) indicates WHAT?
  • A large Kb (or a small pKb) indicates WHAT?

You should think of Ka or Kb just as you do Keq

A large Ka (or a small pKa) indicates that at equilibrium:

  • more products reactants
    • For an acid dissociation, this would mean a lot of dissociation
      • (i.e., a lot of H+ formed)
    • And thus a very strong acid

Similarly, a large Kb (or a small pKb) indicates a very strong base

  • i.e., a lot of -OH FORMED, either from:
    1. Dissociation of a hydroxide base
      • i.e., NaOH
    2. ...or from deprotonating water 

145

Acid-Base Equilibria

  • Give the formula for: 

IONIZATION OF WATER

H2O + H2O ⇔ H3O+ + OH-

(H3O+ is the same as H+ )

146

  • Acid-Base Equilibria

Ionization of Water, H2O + H2O ⇔ H3O+ + OH- 

  • What is Kwater (or just "Kw") at 25°C?
  • What is pKwater equal to? (HINT: __+__= #)

Show how the equation for pKw is DERIVED FROM Kw

Kwater = [H3O+][OH- ] = 10-14

(at 25° C)

Remember: H3O+ is the same as H+

This caveat should make sense

  • because you learned earlier that TEMPERATURE  is the one thing that DOES change Keq
    • (​This is why 25C IS included here)

pKwater = pH + pOH = 14

DERIVATION, 

Starting with:

 Kw = [H3O+ ][OH- ] = 10-14

First, we take the negative log of all terms, yielding:

logKw = -log[H3O+] + -log][OH- ] = -log(10-14)

The middle two terms come from the log rule that states:

  • logAB = logA + logB

The first term can be replaced with pKa by definition

The second term with pH by definition

The third term with pOH by definition

The fourth term by 14

  • because 14 IS the –log of 10-14!

This leaves:

pKw = pH + pOH = 14

147

  • Acid-Base Equilibria

Ionization of Water, H2H2O ⇔ H3O+ + OH- 

  • pKa + pKb= ?

14

148

Acid-Base Equilibria

  • Acid Dissociation
    • Give the equation that represents Acid Dissociation
    • Ka=? 

​HINT: "HA"

 

HA + H2O ⇔ H3O+ A-

 (H3O+ is the same as H+)

  • Ka = [H+][A-] / [HA]

 

 

149

Acid-Base Equilibria

  • Acid Dissociation, HA + H2O ⇔ H3O+ A-
    • ​Acid H-X dissociates 80% in water

Would you expect its Ka to be greater than,

less than, or equal to ONE?

HINT: Ka=  [H+][A-] / [HA]

Because the acid almost fully dissociates:

  • we know that the ratio of products over reactants would have to be GREATER than one

An acid is considered “STRONG” when:

  • Ka > 1, or a
  • pKa < 0

...so this acid would clearly qualify as a strong acid

150

Acid-Base Equilibria

  • Acid Dissociation
    • With regards to Ka and pKa values, what makes an acid "STRONG?"

An acid is considered “STRONG” when:

  • Ka > 1

​OR

  • pKa < 0

151

Acid-Base Equilibria

  • Base Dissociation
    • Give the formula that represents a base dissociating in water
    • What is Kb equal to?
    • How do Ka and Kb COMBINE
      • to create Kw?

HINT: "HA" is to ACID Dissociation as "A-" is to BASE Dissociation

Base Dissociation:

A-+H2O ⇔ OH- + HA

Kb = [OH- ][HA] / [A- ]

(Kacid ) x( Kbase) = Kwater = 10-14

(at 25°C)

  • because ([H+ ][A- ]/[HA])*([OH- ][HA]/[A- ]) = [H+ ][OH- ] = Kw

152

Acid-Base Equilibria

  • Base Dissociation
    • T/F? An aqueous solution with a pH of 8 is BASIC, and therefore by definition it does NOT contain ANY unreacted H+ ions

FALSE

  • An aqueous solution with a pH of 8 IS basic
    • but that does NOT mean that it does not contain any hydrogen ions

In fact, the presence of hydrogen ions is easily verified

  • by solving the formula pH = -log[H+] for [H+]
  • There are 1.0 x 10-8 moles of hydrogen ions per liter of this solution

It is classified as "basic" because:

  • it has fewer hydrogen ions than are found in neutral water
  • and more hydroxide ions than are found in neutral water

153

Acid-Base Equilibria

  • When you see ACID / BASE:

THINK?

Balance between:

[OH-] and [H+] ions! 

154

Acid-Base Equilibria

  • Base Dissociation
    • Acid A has a Kb of 1.0 x 10-9
    • Acid B has a Kb of 1.0 x 10-10

Which acid will create the largest DECREASE in pH when added in equimolar amounts to pure water?

  • aka, will Acid A or Acid B make the water more acidic?

Acid B will give the largest drop in pH

THE SMALLER Kb REPRESENTS THE STRONGER ACID

  • The largest decrease in pH will be caused by addition of the most acidic of the two species

You could simply recognize from the equation: 

Kw = Ka*Kb

  • ...that Kb and Ka are inversely related
    • thereforeTHE SMALLER Kb REPRESENTS THE STRONGER ACID
      • because Ka and acid strength are directly related

155

Acid-Base Equilibria

SUMMARY:

  • How do:
    1. Ionization of Water
    2. Acid Dissociation, and
    3. Base Dissociation

...RELATE to one another?

  • Ionization of water is often called the “autoionization of water”
    • because it happens automatically in ALL water
  • So, when an acid or a base is added to water the autoionization equilibrium is ALREADY ONGOING in that solution—
    • a process we can describe with an equilibrium constant, Kw
  • If we are at 25°C, we know that:
    1. Kw will be 10-14 and
    2. The concentrations of [H+] ions and [OH] ions will both be 10-7M

After adding an acid or a base we suddenly have TWO equilibriums present in the same solution:

  • 1) The ionization of water, and
  • 2) The equilibrium for the acid or the base we added

Just as we described the ionization of water with an equilibrium constant, Kw,

...We can also describe the dissociation of the acid or base we added with Ka or Kb

156

Acid-Base Equilibria

SUMMARY:

An important key many students can't visualize is this: 

  • When we add an acid or base to water, the EQUILIBRIUM of that acid or base will directly impact WHAT?
    • Explain yo'self! 

HINT: Use H2O + H2O ⇔ H3O+ + OH-

When we add an acid or base to water, the equilibrium of that acid or base will directly impact the equilibrium for the ionization of water

  • according to Le Chatelier’s Principle

Looking at the formula, H2O + H2O ⇔ H3O+ + OH-

  • We can see that adding an acid will shift the equilibrium to the left
    • This will use up hydroxide ions (OH-)
  • Each hydroxide ion that reacts will also use up
    • ONE hydronium ion (H3O+)

But remember that we just added EXTRA hydronium ions in the form of the acid

  • The net result will be more hydronium ions relative to hydroxide ions
    • and therefore a LOWER pH

(Remember that the [OH- ] equaled the [H+ ]

before we added the acid)​

157

NEGATIVE ΔG=?

SPONTANEOUS!

Free energy IS available to do work!

158

POSITIVE ΔG=?

NON-SPONTANEOUS!

Energy is NOT available to do work! :(

159

WATER has a VERY HIGH heat capacity because...?

You should know that water has a very high heat capacity... 

due to hydrogen bonding

160

  • For an exothermic decomposition reaction, which of the following properties is(are) known?
    • I. the sign of ∆S
    • II. the sign of ∆H
    • III. if K is > or < 1
    • IV. the sign of ∆G
  • A. I only
  • B. I & II
  • C. I, II & III
  • D. I, II, III & IV

 D

  • The word “decomposition” tells us that we are going from one species to two or more species, meaning entropy must be positive
  • The word “exothermic” tells us that the enthalpy must be negative
  • Finally, from ∆G = ∆H - T∆S we know that if we plug in these two values Gibbs free energy must be negative
  • Because Gibbs free energy is negative, we also know that K must be greater than one from the equation: ∆G = -RTlnK
  • Thus, we know all of the items listed and the correct answer is D