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## Glossary of Chemistry: Lecture 3: Thermodynamics

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State functions
-State functions are pathway independent.
-The macroscopic state of any one-component fluid system in equilibrium can be described by just 3 properties: of which at least one is extensive. (EX: if, for a gas in eq, PVT are known, then all other properties that describe the state of that gas must have a specific value.
2 properties that describe macroscopic state of a system
1) extensive - proportional to size of system, e.g. V and n.
2) intensive - independent of size, e.g. P and T.
Thermodynamics: 3 systems
1) open - exchange both mass and energy w/ surroundings
2) closed - exchange only energy
3) isolated - exchange neither
Path functions
Properties that do not describe the state of a system, but depend upon the pathway used to achieve any state, e.g. work and heat.
Two ways to transfer energy between systems
1) heat (q)
2) work (w)
Three forms of heat
1) conduction - thermal energy transfer via molecular collisions
2) convection - thermal energy transfer via fluid movements
3) radiation - thermal energy transfer via electromagnetic waves
Conduction
-Requires direct physical contact
-Higher energy molecules of one system transfer transfer some of their energy to the lower energy molecules through a single object.
Thermal Conductivity
An object's ability to conduct heat (k).

Q/t = kA[(Th-Tc)/L] = heat current I.
Convection
differences in pressure or density drive warm fluid in the direction of cooler fluid (EX: ocean and air currents)
The rate at which an object radiates EM radiation depends upon its T, surface area, and is given by the Stefan-Boltzmann law:

P = oeAt^4
Newton's law of cooling
states that the rate of cooling of a body is appr. proportional to the temperature difference btwn the body and its environment.
PV work
w = P * dV (constant pressure)

-PV work takes place when a gas expands against a force regardless of whether or not the pressure is constant
-If the volume is constant, no PV work is done.
First law of thermodynamics
the energy of the system and surroundings is always conserved

dE = q + w (for work ON the system is positive)
Second law of thermodynamics
-heat cannot be changed completely into work in a cyclical process
Heat engine
Via conservation of energy, the heat entering the engine (qh) must equal the net work done by the engine (w) plus the heat leaving the engine (qc):

qh = w + qc
The seven state functions
1) internal energy - U
2) temperature - T
3) pressure - P
4) volume - V
5) enthalpy - H
6) entropy - S
7) Gibbs energy - G
Zeroth Law of Thermodynamics
-states that temperature exists
The average kinetic energy of a single molecule in any fluid is given by:
KEavg = (3/2)kT
Virtually all physical properties change w/ temperature.
Pressure of an ideal gas
-the random translational kinetic energy per volume
-the greater the random translational kinetic energy of gas molecules per volume, the greater the pressure
Enthalpy
H = U + PV

-measured in units of energy, but it is not conserved (enthalpy in the universe is NOT constant). Also, it is an extensive state function (depends only on T)
Change in Enthalpy (constant pressure)
dH = dU + PdV (constant P)
Standard State
-An element in its standard state at 25 degrees C is randomly assigned an enthalpy value of O J/mol.
Standard Enthalpy of Formation(dHof)
-the change in enthalpy for a reaction that creates one mole of that compound from its raw elements in their standard state.
For reactions involving no change in pressure, the change in enthalpy is equal to:
-Heat: dH = q (constant P, closed system at rest, PV work only)
If gas is not part of the reaction (such as liquid/sold chem reactions in the lab), what is enthalpy change equal to?
-Heat, which, in the absence of work, is equal to a change in energy. Can be described by the heat of reaction.
Heat of Reaction
dHo(rxn) = dHof(products) - dHof(reactants)
Hess' Law
Transition State
-Peak of the energy hill in rxn vs energy graph; the old bonds are breaking and new bonds are forming.
Intermediates
-Products of the first step in a two step reaction. The intermediates exist in the trough btwn the two humps.
Does a catalyst affect the enthalpy change in a reaction?
No, only the rate.
Entropy (S)
-Nature's tendency to create the most probable situation that can occur within a system.
-dS(system) + dS(surroundings) = dS(universe) >= 0
-Extensive property, so it increases with number, volume, and temperature.
The Universe
-It is an isolated system. The system AND the surroundings together make up the entire universe.
When can chemists call a reaction "irreversible"?
-When the activation energy of the reverse direction is REALLY high.
What dictates the direction of a reaction?
-Entropy, NOT energy. So a reaction can still proceed even though it is unfavorable for energy and/or enthalpy.
Equilibrium (in terms of Entropy)
-The point in a reaction where the universe has gained maximum entropy.
The Third Law of Thermodynamics
-A perfect crystal at zero kelvin is assigned an entropy value of zero. All other substances and all temperatures have a positive entropy value.
Gibbs free energy (G)
-dG = dH - TdS (all variables refer to SYSTEM)
-extensive property and state function
-negative dG indicates a spontaneous reaction.
If the signs of both enthalpy and entropy are the same for a reaction, the spontaneity of the reaction will depend on?
-Temperature. A higher temperature will favor the direction favored by entropy.