Chemistry 2 Flashcards
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
Specific heat capacity definition
describes energy absorption for ONE individual substance
∆Hvaporization definition
enthalpy value associated with phase change from liquid to gas(therefore, ∆Hcondensation is the same value, just with a reverse sign)
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)
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
Define “Entropy (∆S)”
- What are its units?
ENTROPY
a measure of DISORDER in a system
units=Joules/K
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
- 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
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_
The ln of and positive (+) numer LESS than 1 is?
negative (-)
Anything to the zeroth power is equal to?
1
Catalyst definition
increases rxn rate w/o being consumed in the process
How do you solve for ΔH using a BOMB Calorimeter?
- What DOES and DOESNT solving for Bomb Calorimeters give us?
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*
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)
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
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)
∆G (+) means?
Energy is available and the system CAN do work
What formula relates the Keq to Gibbs Free energy? (2 versions)
∆G˚=-RTlnKeqORKeq=e^(-∆G/RT)
Slow step=?
rate-determining step
How to write rate law for a catalyzed rxn
write rate law in same way as normal, w/ [catalyst] added in as a reactant
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
∆G (-) means?
energy must be added to rxn (i.e. heat must be added to system) to make it proceed
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!
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
Acid-Base Chemistry
- Describe a Bronsted-Lowry Acid & Base
HINT: Black L.A.P.D
Acids
- DONATE protons (H + )
Bases
- ACCEPT protons (H + )
“B.L.A.P.D”
BLACK L.A.P.D
If Keq > 1, rxn will be…?(if we are starting at standard conditions)
SPONTANEOUS!
Heat capacity formula
C=q/∆T
On a pressure vs. volume graph:
- Where is PV work?
The area UNDER the curve
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
Thermodynamics
- How is “WORK”defined inCHEMISTRY?
Work=
A change in VOLUME
- …at a constant ~P
How will the following affect rxn rate?
increasing energy of transition state
DECREASE rate
because it increases Ea
Bond broken during rxn=energy ___
required
How will the following affect rxn rate?
increase [reactants]
increase rate
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!
Do catalysts increase % yield?
NOPE!!
What should you think of (and mentally replace!) whenever you see the word “temperature?”
temperature=av. KE of molecules
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!!
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
Bond formed as result of rxn=energy ___
released
How will the following affect rxn rate?
increase [products]
NO effect
∆Hfusion definition
enthalpy value associated with phase change from liquid to solid (sign reverses for ∆Hmelting)
If 2 objects are in thermal equilibrium, by definition they have what?
The SAME TEMP!
Entropy, ∆S
POSITIVE ∆S means?
- _______ randomness, ____ energy available to do work
- INCREASED randomness
- MORE energy available to do work
A reaction with a HIGHER ∆S
is more likely to?
Happen SPONTANEOUSLY!
How will the following affect rxn rate?
increasing [catalyst]
increase rate
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
the word “decomposition” tells us what?
we are going from ONE species to 2+ species (∆S is positive)
What is the SPECIFIC HEAT of H2O?
Specific Heat of Water=
1.0 cal/g˚C
OR
4.18 J/g˚C
Define the FIRST LAW of thermodynamics
- Energy cannot be created nor destroyed
Thermodynamic Formula
∆G=∆H-T∆S
∆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)
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
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!
Rate Order graphs: first order
ln [A] vs time is linear with slope= -k
Define heat capacity
amount of energy (in joules or calories) a system must absorb to give a unit change in temperature
Relationship b/t enzymes & catalysts
Enzyme=biological catalystALL enzymes are catalysts, but not all catalysts are enzymes
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”
The First Law of Thermodynamics
2 ways to define:
- The total energy of an isolated system is always constant
- 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
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
When you see “ENTHALPY (ΔH),”
THINK: ?
The ENERGY CONTAINED
WITHIN CHEMICAL BONDS
∆Hsol’n definition
enthalpy value associated with dissolution of a species into a solution
What do exponents in a rate law represent?
the “order” of each reactant
High (+) BDE means the cpd is very ___
stable
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*
What are the 3 types of heat exchange?
1) Convection2) Radiation3) Conduction
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”*
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
Heat Exchange
- Define CONVECTION
What are 2 common examples?
Hotter** sections of fluid **rise**, **cooler** portions **sink
Examples:
- Air currents
- Convection currents (El Nino)
Define the THIRD LAW of Thermodynamics
Pure crystalline substances at absolute zero
…have a ∆S of 0
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
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
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:
- CHANGE the sign of ∆H° to be positive, and
- 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
Define the SECOND LAW of Thermodynamics
(has 2 parts!)
- Heat cannot be changed COMPLETELY into work in a cyclical process
- ∆S in an isolated system can never decrease
