Thermodynamics Flashcards
system and surroundings
system = section of universe under study (open, closed, and isolated) surroundings = remainder of universe outside system
extensive vs. intensive properties
extensive - change with amount, proportional to size of system (V and n)
intensive - do not change with amount, independent of size of system (P and T)
state function
properties that describe the state of a system
- pathway independent
- change in a state property going from one state to another is the same regardless of the process used to change it
path function
depend upon pathway used to achieve state (work and heat)
2 ways to transfer energy between systems
heat (natural transfer of energy from a warmer body to a cooler body by conduction, convection, or radiation) and work
conduction
thermal energy transfer via molecular collisions, requires direct physical contact
convection
thermal energy transfer via fluid movements, differences in pressure or density drive warm fluid in the direction or cooler fluid
radiation
thermal energy transfer via electromagnetic waves
PV work
at constant pressure, it is equal to the product of the pressure and the change in volume
w = P(delta V) = work done by the gas
work done on the gas is (-)
1st Law of Thermodynamics
energy of the system and surroundings is always conserved
delta E = q + w
heat flow between two systems that _____ is an irreversible process
heat flow between 2 systems that are not at the same temperature is an irreversible process
2nd Law of Thermodynamics
- work is required to transfer energy from a cold reservoir to a hot reservoir
- heat cannot be completely converted to work in a cyclical process
- the entropy of an isolated system will never decrease
- could be violated, but unlikely
temperature
- measure of the thermal energy per mol of molecules
- thermodynamic property described by the zeroth law
- measurement of how fast the molecules are moving or vibrating
- Increase in thermal energy increases temperature
the greater the random translational kinetic energy of gas molecules per volume, the great the ___
pressure
enthalpy
not conserved like energy, enthalpy of the universe does not remain constant
*a state function (since U, P, and V are state functions)
enthalpy and internal energy depend only on ___
temperature (enthalpy increase with temperature)
change in enthalpy
delta H = delta U + P (delta V)
U is internal energy
enthalpy change for only PV work at constant pressure
delta H = q (constant pressure, closed system at rest, PV work only)
Standard state
scientists assigned enthalpy values to compounds based on their standard states
- the reference form of a substance at any chosen temperature and a pressure of 1 bar (about 750 torr, 105 Pa)
- assume 1 bar (1 atm)
reference form
the from that is most stable at 1 bar and a given temperature
standard enthalpy of formation
change in enthalpy for a reaction that creates one mole of that compound from its raw elements in their standard state.
-can be found by experiment
heat of reaction
delta H reaction = standard enthalpy of formation of products - standard enthalpy of formation of reactants
Hess’s law
the sum of the enthalpy changes for each step is equal to the total enthalpy change regardless of the path chosen (enthalpy is a state function)
-forward reaction has exactly the opposite change in enthalpy as the reverse
endothermic
if enthalpy change is positive, heat flow to the system
exothermic
if enthalpy change is negative, heat to surroundings
transition state
old bonds breaking and new bonds forming, during reaction collision (not intermediates)
-at peak of activation energy hill
catalyst and enthalpy change
catalyst does not affect enthalpy change
knowing what delta H is - page 77
page 77
entropy
S, nature’s tendency toward disorder, tendency to create the most probable situation that can occur within a system, spread energy evenly throughout the universe
- state function, extensive property
- dictates direction of a reaction
2nd law of thermo. entropy
delta S system + delta S surroundings = delta S universe >= 0
*the of a system can decrease only if the entropy of the surroundings increases by a greater or equal magnitude
3rd law of thermodynamics
conventionally assigns a zero entropy value to any pure substance at absolute zero and in internal equilibrium.
all other substances have a positive entropy value
since entropy in an isolated system cannot decrease, the universal entropy change for any reversible reaction must be ____
must be zero in both directions
equilibrium is point of ____ entropy
maximum universal entropy
Gibbs free energy
delta G = delta H - T(delta S)
refers to changes in system
delta G = 0
equilibrium
negative delta G
spontaneous reaction
