MCAT General Chemistry Review 3

Thermodynamics

Study of energy and its relationship to macroscopic properties of chemical systems.

Only valid for systems composed of large numbers of molecules.

Open system
Exchange both mass and energy with surroundings.
Closed System
Exchange energy but not mass with the surroundings
Isolated system
Exchange neither mass nor energy with surroundings.
State Functions

Describe extensive and intensive properties.

Pathway independent.

Extensive Properties
Proportional to the size of the system
Intensive
Independent of the size of the system
Path function

Do not describe the state of a system, but depend upon the pathway used to achieve any state.

(Ex: Work and Heat)

How to transfer energy between systems
Heat and Work
Conduction
Transfer of heat through molecular collisions
Convection
Thermal energy transfer via fluid movements
Radiation
Thermal Energy transfer via EM waves.
PV Work

At constant pressure it is equal to the product of pressure and the change in volume.

w= PΔV

If volume remains constant, no PV work is done at all.

The First Law of Thermodynamics

Energy of a system and its surrounds are always conserved.

ΔE = q + w (work on a system)

ΔE = q – w (work done by system)

Second Law of Thermodynamics
Heat cannot be changed completely into work in a cylindrical process.
Internal Energy, U

Collective energy of molecules measured on a microscopic scale (includes vibrational , rotational, translational, electronic, intermolecular potential, and rest mass energy) 

All the possible forms of energy on a molecular scale.

Average Kinetic Energy of a single molecule in any fluid:
KEavg=(3/2)kT
Virtually all physical properties change with…
Temperature
Enthalpy

Extra capacity to do PV work [in Joules].

H≡U+PV

 

 

Change in Enthalpy, ΔH

ΔH=ΔU+PΔV

Enthalpy values vary depending on the standard state.

Standard state for enthalpy change

An element in SS is arbitrarily assigned an enthalpy value of 0J/mol

(at 25oC and 1 atm/750torr/105Pascals)

Standard Enthalpy of Formation, ΔHof
Change in enthalpy for a reaction that creates one mol of that compound from its raw elements in standard state.
Heat of reaction

The change in enthalpy from reactants to products

ΔHoreaction=ΔHfoproducts – ΔHforeactants

Hess’s law
The sum of enthalpy changes for each step is equal to the total enthalpy change regardless of the path chosen.
Exothermic Reaction
Release heat, making reaction system hot (-;H)
Endothermic Reaction
Absorb heat, making the system cold (+;H)
Entropy, S

Nature’s tendancy towards disorder.

Degree of randomness.

 

Second Law of Thermodynamics

Entropy of a system will never decrease.

Entropy is the driving force that will determine whether or not a reaction proceeds.

If entropy of a system can decrease only if the surroundings of that system increase by an amount greater than or equal to the decrease.

Entropy increases with…
number, volume and temperature.
Third Law of Thermodynamics
Zero entropy is assigned to any pure substance at absolute zero.
Gibbs Free Energy, ΔG

ΔG=ΔH-TΔS

Only good for constant temperature reactions.

ΔG<0

Spontaneous Reaction

ΔG>0

Nonspontaneous Reaction
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