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44 Cards in this Set

  • Front
  • Back
Thermodynamics
Study of energy and its relationship to macroscopic properties of chemical systems

divides universe into 2:
1. surroundings
2. system
System
macroscopic body under study

3 types:
1. open
2. closed
3. isolated
Surroundings
everything other than macroscopic body under study (system)
Open system
exchange both mass and energy with surroundings
Close system
exchange energy but do not exchange mass with surroundings
Isolated system
doesn't exchange mass or energy with surroundings
Extensive properties
chance with amount

Examples:
volume (V) and # of moles (n)

describes macroscopic state of a system

proportional to size of system
Intensive properties
describes macroscopic state of a system

independent of size of system

examples:
Pressure (P) and temperature (T)
State functions
properties that describe the state of a system

pathway independent

state properties describe the state of a system

change in a state property going from one state to another is the same regardless of the process via which the system was changed

3 state properties describe the state of a system (one being extensive property) unambiguously
2 ways to transfer energy between systems
1. Heat (Q)
2. Work (W)
Heat (Q)
Natural transfer of energy from a warmer body to a cooler body

3 forms:
1. conduction
2. convection
3. radiation

movement of energy via conduction, convection or radiation; always from hot to cold
Work (W)
any energy transfer between systems that is not heat
Conduction
thermal energy transfer via molecular collisions

requires physical contact

higher energy molecules of one system transfer some of their energy to lower energy molecules of other system via molecular collisions
Energy flow
heat flow:
change in T = IR
T: temperature
I: heat current (Q/t)
R: resistance to heat flow

Current flow:
V = IR
V: voltage
I: electrical current
R: resistance to electrical flow

Fluid flow:
change in P = QR
P: pressure
Q: heat
R: resistance to flow

thicker conduits = greater flow
longer conduits = less flow
Convection
thermal energy transfer via fluid movements

differences in pressure or density drive warm fluid in direction of cooler fluid

examples:
ocean and air currents
Radiation
thermal energy transfer via electromagnetic waves

only type of heat that transfer though a vacuum
PV work
system at rest, no gravitational potential energy and no kinetic energy may still be able to do PV work

at constant pressure, work is equal to pressure multiplied by change in volume

W = P(change in V)
[at constant pressure]

No PV work is done if volume is constant

PV work takes place when a gas expands against a force regardless of whether or not the pressure is constant
1st Law of Thermodynamics
energy of system and surroundings is always conserved

any energy change to a system must equal heat flow into system + work done by system

Change in E = Q + W

Work done on the system is positive and work done by the system is negative
2nd Law of Thermodynamics
Heat cannot be changed completely into work in a cyclical process

Qh = W + Qc
Qh: heat entering engine
W: work
Qc: heat leaving engine
Carnot's efficiency
e = 1 - (Tc/Th)

e: efficiency
Tc: temperature of cold reservoir
Th: temperature of hot reservoir
7 state functions
1. internal energy (U)
2. temperature (T)
3. pressure (P)
4. volume (V)
5. enthalpy (H)
6. entropy (S)
7. Gibbs free energy (G)
Internal energy (U)
all possible forms of energy imaginable on a molecular scale

examples:
vibrational, rotational, translational, electronic, intermolecular potential and rest mass energy

heat energy, thermal energy and or heat
Temperature
thermodynamic property described by zeroth law:

2 systems in thermal equilibrium with 3rd system are in thermal equilibrium with each other

any increase in thermal energy, increases temperature

KE = (3/2)kT
KE: kinetic energy
T: temperature
k: boltzmann constant (1.38e-23 J/K)

measurement of how fast molecules are moving or vibrating
Enthalpy (H)
H = U + PV
H: enthalpy (joules)
U: internal energy
PV: Pressure x Volume

not conserved, constantly changing

Enthalpy is a state function and en extensive property

Depends only on temperature

Change in H = (change in U) + P(change in V)
[constant pressure]
Standard State
do not confuse with STP (standard temperature and pressure)

an element at its standard state at 25 degrees Celsius is arbitrarily assigned an enthalpy value of 0 J/mol

Any chosen temperature

1 bar of pressure = 750 torr = 10^5 Pascals
Reference form
standard state for a pure solid or liquid

any chosen temperature

1 bar of pressure = 750 torr = 10^5 Pascals

form that is most stable at the values
Standard enthalpy of formation (change in Hf)
Change in enthalpy for a reaction that creates 1 mole of that compound from its raw elements in their standard state
Change in enthalpy at constant pressure
Change in H = Q
constant pressure, closed system at rest, PV work only
H: enthalpy
Q: heat
Heat of reaction
change in enthalpy from reactants to products

Change in Hreaction = (Change in Hfproducts) - (change in Hfreactants)
Hess' Law
Sum of enthalpy changes for each step is equal to total enthalpy change regardless of path chosen

when you add reactions, you can add their enthalpies

because enthalpy is a state function
Endothermic
if enthalpy change is positive

absorbs heat, making reaction system cold

at constant P, where change in H = Q, endothermic reaction produces heat flow to system
Exothermic
if enthalpy change is negative

release heat, making reaction system hot

at constant P, where change in H = Q, exothermic reaction produces heat flow to surroundings
Activation energy
initial increase in energy
Transition state
peal of energy hill represents molecules in transition state

old bonds are breaking and new bonds are forming

occurs during reaction collision

do not confuse with intermediates (products of 1st step in 2 step reaction)
Catalyst
lowers activation energy of forward and reverse reactions

equilibrium and enthalpy change is unaffected

Affects the rate of reaction
Entropy (S)
Nature's tendency to create the most probably situation that can occur within a system

nature's tendency toward disorder

state function, which means that entropy change of forward reaction is equal to negative entropy change of reverse reaction
2nd law of thermodynamics
entropy of an isolated system will never decrease without some outside intervention (work)
Entropy change of universe
Change Ssystem + change Ssurroundings = Change Suniverse > or = 0

Sum of entropy changes of any system and its surroundings equals entropy change of universe, which is always equal to or greater than zero (positive)

entropy of system can decrease, only if, at the same time, entropy of surroundings increases by greater or equal magnitude
Reversible
Only ideal reactions are reversible because only ideal reactions create zero change in entropy of universe

on a microscopic scale, all real chemical reactions are reversible
Irreversible
all reactions are irreversible on a macroscopic scale
what drives the direction of a reaction
it is entropy, not energy, that drives the direction of a reaction

entropy is nature's effort to spread energy evenly between system, from high to low

entropy increases with number, volume and temperature

reaction must increase the entropy of the universe in order to proceed
Equilibrium
point in reaction where universe has achieved maximum entropy
3rd law of thermodynamics
assigns by convention a zero entropy value to any pure substance (either element or compound) at absolute zero and in internal equilibrium
Gibbs Free energy (G)
change G = (change H) = T(change S)
G: gibbs free energy
H: enthalpy
T: temperature (constant)
S: entropy

All variables refer to the system and not surroundings

only good for constant T & P reactions

negative change G = spontaneous reactions

extensive property and state function

not conserved, an isolated system can change its gibbs free energy

Maximum non-PV work available from a reaction


positive change in H and negative change in S, can never equal negative change in G, which means can never be spontaneous

negative change in H and positive change in S = negative change in G = spontaneous

if both change in H and S are same sign, than change in G depends on T