Entropy and Gibbs Free Energy Flashcards
(16 cards)
Thermodynamics
branch of physics that typically measures heat and
predicts whether a process will occur under the given conditions, i.e.,
spontaneity.
* Processes that will occur on their own without being forced are called
spontaneous.
* Nonspontaneous processes require energy input to go
Entropy (S)
is a thermodynamic function that measures disorder. S univ +
means that a process is spontaneous.
* Entropy (S) has units of J/mol. There are several equations used to solve for
entropy. Below is the Boltzmann equation. We will use this equation very
rarely.
* S = k ln W
* k = Boltzmann constant = 1.38 × 10–23 J/K
* W is the number of energetically equivalent ways a system can exist.
* Random systems require less energy than ordered systems.
* W=N!/n1!n2!n3!….
* N is the total number of particles
* is the total number of particles in compartment 1
* is the total number of particles in compartment 2, etc.
Laws of Thermodynamics
The first law of thermodynamics says that energy
cannot be created or destroyed (The total energy of the universe cannot change, but energy
can be transferred from one place to another)
ΔEuniv = 0 = ΔEsys + ΔEsurr
The second law of thermodynamics says that the total entropy
change of the universe must be positive for a process to be
spontaneous. It equals zero for a process in equilibrium.
ΔSuniv = ΔSsys + ΔSsurr
The third law of thermodynamics states that the entropy of a perfect
crystalline substance is zero at absolute (Kelvin scale) zero. It is believed that
we will never reach absolute zero
Entropy increases in a
substance as
temperature increases
from absolute zero.
* The 5th state of matter
was discovered by
scientists reaching
temperatures very
close to absolute 0
Changes in Entropy, ΔS
ΔS = Sfinal – Sinitial
* ΔS positive means that a process is thermodynamically favored. Change
in entropy will only predict spontaneity if is change in entropy of the
universe.
* Changes that increase the entropy are as follows:
– Reactions whose products are in a more random state
* Solid more ordered than liquid; liquid more ordered
than gas
– Reactions that have larger numbers of product molecules than
reactant molecules
– Increase in temperature
– Solids dissociating into ions upon dissolving
Entropy Changes in a System: Calculating ΔSsub
sys
Ssys = Sfinal - Sinitial
or
Ssys = nRln(Vfinal / Vinitial)
Change in Entropy and Chemical Reactions
For a process in which the final condition is more random than
the initial condition:
– ΔSsys is positive; and
– entropy change is favorable for the process to be spontaneous.
* For a process in which the final condition is more orderly than
the initial condition:
– ΔSsys is negative; and
– entropy change is unfavorable for the process to be spontaneous.
The equation below is used when a problem mainly gives you only
a chemical reaction and asks you to calculate entropy.
ΔSsys = ΔSrxn = n(S°products) – n(S°reactants)
Standard Entropy, S°
In addition to translational motion, molecules exhibit vibrations
and rotations
Conceptual Standard Entropy, S° trends
There are several important trends in entropy:
* Sliquid > Ssolid
* Sgas > Sliquid
* S° increases with molar mass.
* S° increases with molecular complexity.
* S° increases with the mobility of a phase (for an element with
two or more allotropes- examples of allotropes are
diamond/graphite/buckyball).
18
© McGraw Hill LLC
Qualitatively Predicting the Sign of ΔS°sub
sys
Several processes that lead to an increase in entropy are:
* Melting.
* Vaporization or sublimation.
* Temperature increase.
* Reaction resulting in a greater number of gas molecules
Quantifying Entropy Changes in the Surroundings
The entropy change in the surroundings is proportional to the amount
of heat gained or lost.
qsurr = –qsys
* The entropy change in the surroundings is also inversely proportional
to the temperature.
* At constant pressure and temperature, the overall relationship is as
follows. This is one of the main entropy equations that you will use
ΔSsurr = –qsys / Temperature (T) = (–) ΔHsys / T
Gibbs Free Energy, ΔG
A process will be spontaneous when ΔG is negative.
* ΔG will be negative under the following conditions:
– ΔH is negative and ΔS is positive.
* Exothermic and more random
– ΔH is negative and large and ΔS is negative but small.
– ΔH is positive but small and ΔS is positive and large.
* Or high temperature
* ΔG will be positive under the following conditions:
– ΔH is negative and small ΔS is negative and large.
* Or high temperature
– ΔH is positive and ΔS is positive and small.
* Or low temperature
– ΔH is positive and ΔS is negative.
* Never spontaneous at any temperature
* When ΔG = 0, the reaction is at equilibrium
Gibbs Free Energy and Spontaneity
It can be shown that –TΔSuniv = ΔHsys – TΔSsys.
* The Gibbs free energy, G, is the maximum amount of work energy that can be
released to the surroundings by a system for a constant temperature and pressure
system.
* Gibbs free energy is often called chemical potential because it is analogous to the
storing of energy in a mechanical system.
ΔGsys = ΔHsys – TΔSsys
* Because ΔSuniv determines whether a process is spontaneous, ΔG also
determines spontaneity.
ΔSuniv is positive when spontaneous, so ΔG is negative
© McGraw Hill LLC
Standard Conditions
The standard state is the state of a material at a
defined set of conditions.
* Gas = pure gas at exactly 1 atm pressure
* Solid or Liquid = pure solid or liquid in its most stable
form at exactly 1 atm pressure and temperature of
interest
* Usually 25 °C
* Solution = substance in a solution with concentration 1
M
What’s “Free” about Free Energy?
The free energy is the maximum amount of energy released
from a system that is available to do work on the surroundings.
* For many exothermic reactions, some of the heat released as a
result of the enthalpy change goes into increasing the entropy of
the surroundings, so it is not available to do work.
* Even some of this free energy is generally lost to heating up the
surroundings.
* In order to calculate the temperature at which a process will be
spontaneous, use H/S
Thermodynamics of Living Systems
Many biological reactions have positive ΔG° value, making the
reaction nonspontaneous.
Nonspontaneous reactions can be coupled with spontaneous
reactions in order to drive a process forward
Many biological reactions have positive ΔG° value, making the
reaction nonspontaneous
