Chemistry: Lecture 3: Thermodynamics

Front 1

State functions

Back 1

-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.

Front 2

2 properties that describe macroscopic state of a system

Back 2

1) extensive - proportional to size of system, e.g. V and n.
2) intensive - independent of size, e.g. P and T.

Front 3

Thermodynamics: 3 systems

Back 3

1) open - exchange both mass and energy w/ surroundings
2) closed - exchange only energy
3) isolated - exchange neither

Front 4

Path functions

Back 4

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.

Front 5

Two ways to transfer energy between systems

Back 5

1) heat (q)
2) work (w)

Front 6

Three forms of heat

Back 6

1) conduction - thermal energy transfer via molecular collisions
2) convection - thermal energy transfer via fluid movements
3) radiation - thermal energy transfer via electromagnetic waves

Front 7

Conduction

Back 7

-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.

Front 8

Thermal Conductivity

Back 8

An object's ability to conduct heat (k).

Q/t = kA[(Th-Tc)/L] = heat current I.

Front 9

Convection

Back 9

differences in pressure or density drive warm fluid in the direction of cooler fluid (EX: ocean and air currents)

Front 10

Radiation

Back 10

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

Front 11

Newton's law of cooling

Back 11

states that the rate of cooling of a body is appr. proportional to the temperature difference btwn the body and its environment.

Front 12

PV work

Back 12

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.

Front 13

First law of thermodynamics

Back 13

the energy of the system and surroundings is always conserved

dE = q + w (for work ON the system is positive)

Front 14

Second law of thermodynamics

Back 14

-heat cannot be changed completely into work in a cyclical process

Front 15

Heat engine

Back 15

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

Front 16

The seven state functions

Back 16

1) internal energy - U
2) temperature - T
3) pressure - P
4) volume - V
5) enthalpy - H
6) entropy - S
7) Gibbs energy - G

Front 17

Zeroth Law of Thermodynamics

Back 17

-states that temperature exists

Front 18

The average kinetic energy of a single molecule in any fluid is given by:

Back 18

KEavg = (3/2)kT

Front 19

What's so special about temperature?

Back 19

Virtually all physical properties change w/ temperature.

Front 20

Pressure of an ideal gas

Back 20

-the random translational kinetic energy per volume
-the greater the random translational kinetic energy of gas molecules per volume, the greater the pressure

Front 21

Enthalpy

Back 21

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)

Front 22

Change in Enthalpy (constant pressure)

Back 22

dH = dU + PdV (constant P)

Front 23

Standard State

Back 23

-An element in its standard state at 25 degrees C is randomly assigned an enthalpy value of O J/mol.

Front 24

Standard Enthalpy of Formation(dHof)

Back 24

-the change in enthalpy for a reaction that creates one mole of that compound from its raw elements in their standard state.

Front 25

For reactions involving no change in pressure, the change in enthalpy is equal to:

Back 25

-Heat: dH = q (constant P, closed system at rest, PV work only)

Front 26

If gas is not part of the reaction (such as liquid/sold chem reactions in the lab), what is enthalpy change equal to?

Back 26

-Heat, which, in the absence of work, is equal to a change in energy. Can be described by the heat of reaction.

Front 27

Heat of Reaction

Back 27

dHo(rxn) = dHof(products) - dHof(reactants)

Front 28

Hess' Law

Back 28

-When you add reactions, you can add enthalpies.

Front 29

Transition State

Back 29

-Peak of the energy hill in rxn vs energy graph; the old bonds are breaking and new bonds are forming.

Front 30

Intermediates

Back 30

-Products of the first step in a two step reaction. The intermediates exist in the trough btwn the two humps.

Front 31

Does a catalyst affect the enthalpy change in a reaction?

Back 31

No, only the rate.

Front 32

Entropy (S)

Back 32

-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.

Front 33

Second Law of Thermodynamics

Back 33

-The entropy of an isolated system will NEVER decrease. So the entropy of the universe increases for ANY reaction.

Front 34

The Universe

Back 34

-It is an isolated system. The system AND the surroundings together make up the entire universe.

Front 35

When can chemists call a reaction "irreversible"?

Back 35

-When the activation energy of the reverse direction is REALLY high.

Front 36

What dictates the direction of a reaction?

Back 36

-Entropy, NOT energy. So a reaction can still proceed even though it is unfavorable for energy and/or enthalpy.

Front 37

Equilibrium (in terms of Entropy)

Back 37

-The point in a reaction where the universe has gained maximum entropy.

Front 38

The Third Law of Thermodynamics

Back 38

-A perfect crystal at zero kelvin is assigned an entropy value of zero. All other substances and all temperatures have a positive entropy value.

Front 39

Gibbs free energy (G)

Back 39

-dG = dH - TdS (all variables refer to SYSTEM)
-extensive property and state function
-negative dG indicates a spontaneous reaction.

Front 40

If the signs of both enthalpy and entropy are the same for a reaction, the spontaneity of the reaction will depend on?

Back 40

-Temperature. A higher temperature will favor the direction favored by entropy.