Chemistry 2 Flashcards
1
Q
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
A
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
Q
Specific heat capacity definition
A
describes energy absorption for ONE individual substance
2
Q
∆Hvaporization definition
A
enthalpy value associated with phase change from liquid to gas(therefore, ∆Hcondensation is the same value, just with a reverse sign)
3
Q
Rate Law equation:X^Y=Z What do X, Y, and Z represent?
A
X=factor by which [reac] changedY=order of reactantZ=factor by which the rate changed (across the same 2 trials)
3
Q
When is heat ,q, added exactly proportional to the increase in temperature? Why?
A
When system is at ~VNo PV work gets done–it 100% of energy added goes towards increasing temperature
3
Q
Define “Entropy (∆S)”
- What are its units?
A
ENTROPY
a measure of DISORDER in a system
units=Joules/K
3
Q
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?
A
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
- Energy is REQUIRED
- If a bond is FORMED during the reaction:
- Energy is RELEASED
- ∴ the BDE should be given a NEGATIVE sign
- Energy is RELEASED
Once again, multiply all bond energy values by their coefficients in the balanced equation
4
Q
Entropy, ∆S
How would the following affect ENERGY being released/required?
- As a rxn proceeds, if ∆S DECREASES
- i.e., it becomes more ORDERED
A
- 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
Q
The ln of and positive (+) numer LESS than 1 is?
A
negative (-)
5
Q
Anything to the zeroth power is equal to?
A
1
5
Q
Catalyst definition
A
increases rxn rate w/o being consumed in the process
5
Q
How do you solve for ΔH using a BOMB Calorimeter?
- What DOES and DOESNT solving for Bomb Calorimeters give us?
A
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
Q
Rate Order Graphs:Will only be linear when? (2)
A
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
Q
What is a Black Body Radiator?
A
Theoretically perfect body that absorbs ALL energy that hits it, and then re-emits 100% of the energy absorbed
8
Q
Calculating E˚ for electrochemical rxns
A
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
Q
∆G (+) means?
A
Energy is available and the system CAN do work
8
Q
What formula relates the Keq to Gibbs Free energy? (2 versions)
A
∆G˚=-RTlnKeqORKeq=e^(-∆G/RT)
9
Q
Slow step=?
A
rate-determining step
9
Q
How to write rate law for a catalyzed rxn
A
write rate law in same way as normal, w/ [catalyst] added in as a reactant
9
Q
When heat enters into a system, if the system is capable of volume change, heat can either…
A
Do PV workIncrease temperature (Av KE of molecules)or Both
9
Q
∆G (-) means?
A
energy must be added to rxn (i.e. heat must be added to system) to make it proceed
10
Q
Define the ZEROTH LAW of Thermodynamics
A
- 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!
11
Q
Calculating ∆Hrxn using BDE’s
A
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
Q
Acid-Base Chemistry
- Describe a Bronsted-Lowry Acid & Base
HINT: Black L.A.P.D
A
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
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?
## Footnote
**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?
## Footnote
**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**
## Footnote
*is **more** likely to?*
Happen **SPONTANEOUSLY!**
30
How will the following affect rxn rate?
## Footnote
**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**
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=**S**tandard **T**emperature & **P**ressure
****
****
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)**kB_T_**
"OSD"
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
43
When you see "**ENTHALPY (ΔH)**,"
## Footnote
*THINK: ?*
The **ENERGY CONTAINED**
## Footnote
**_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 **DIS**ordered
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 **dis**ordered)
* 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**"*
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
65
Entropy, ∆S
a **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)
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?
## Footnote
**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 **UN**stable 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 **E°** 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
## Footnote
**∆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?
## Footnote
**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
* **Accept** a **PAIR 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**
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 hea**t along a **current**
* ex: pot of boiling water
* Heat conduction is analogous to*
***current flow through a wire***
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 _____ \_\_\_\_\_"*
**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^^**
**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**
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**
## Footnote
(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, **H2O + H2O ⇔ 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-**
## Footnote
(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 **K**b 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**
## Footnote
* 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
* therefore**THE _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 **10-7**M
_**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 **exo**thermic **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
