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
- -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.
- 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
- What's so special about temperature?
- 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
- 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
- -When you add reactions, you can add enthalpies.
- Transition State
- -Peak of the energy hill in rxn vs energy graph; the old bonds are breaking and new bonds are forming.
- -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.
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