Chemistry 100 Chapter 19 Spontaneity of Chemical and Physical Processes: Thermodynamics
Feb 24, 2016
Chemistry 100 Chapter 19
Spontaneity of Chemical and Physical Processes: Thermodynamics
What Is Thermodynamics?
Study of the energy changes that accompany chemical and physical processes.
Based on a set of laws. In chemistry, a primary application
of thermodynamics is as a tool to predict the spontaneous directions of a chemical reaction.
What Is Spontaneity?
Spontaneity refers to the ability of a process to occur on its own!
Can the Niagara Falls suddenly reverse?
“Ice will melt, water will boil,” Neil Finn, Tim Finn of Crowded House/Plant ‘It’s Only Natural’.
Water spontaneously freezes on a cold winter day!
The First Law of Thermodynamics
The First Law deals with the conservation of energy changes.
E = q + w The First Law tells us nothing
about the spontaneous direction of a process.
Entropy and Spontaneity
Need to examine the entropy change of the process as well
as its enthalpy change (heat flow). Entropy – the degree of randomness
of a system. Solids – highly ordered low entropy. Gases – very disordered high entropy. Liquids – entropy is variable between that
of a solid and a gas.
Entropy Is a State Variable
Changes in entropy are state functions
S = Sf – Si Sf = the entropy of the final stateSi = the entropy of the initial state
Entropy Changes for Different Processes
S > 0 entropy increases (melting ice or making steam)
S < 0 entropy decreases (examples freezing water or condensing
steam)
The Solution Process For the dissolution of NaCl (s) in water
NaCl (s) Na+(aq) + Cl-(aq)
Highly ordered – low entropy
Disordered or random state – high entropy
The formation of a solution is always accompanied by an increase in the
entropy of the system!
The Entropy Change in a Chemical Reaction
Burning ethane! C2H6 (g) + 7/2O2 (g) 2CO2 (g) + 3H2O (l)
The entropy change rS np S (products) - nr S (reactants)
np and nr represent the number of moles of products and reactants, respectively.
Finding S Values
Appendix C in your textbook has entropy values for a wide variety of species.
Units for entropy values J / (K mole)
Temperature and pressure for the tabulated values are 298.2 K and 1.00 atm.
Finding S Values
Note – entropy values are absolute!
Note – the elements have NON-ZERO entropy values!
e.g., for H2 (g) fH = 0 kJ/mole (by def’n)
S = 130.58 J/(K mole)
Some Generalizations
For any gaseous reaction (or a reaction involving gases).
ng > 0, rS > 0 J/(K mole).ng < 0, rS < 0 J/(K mole).ng = 0, rS 0 J/(K mole).
For reactions involving only solids and liquids – depends on the entropy values of the substances.
The Second Law of Thermodynamics
The entropy of the universe (univS) increases in a spontaneous process. univS unchanged in an equilibrium
process
What is univS?
univS = sysS + surrSsysS = the entropy change of the
system.surrS = the entropy change of the
surroundings.
How Do We Obtain univS?
We need to obtain estimates for both the sysS and the surrS.
Look at the following chemical reaction.
C(s) + 2H2 (g) CH4(g) The entropy change for the systems is
the reaction entropy change, rS. How do we calculate surrS?
Calculating surrS Note that for an exothermic process,
an amount of thermal energy is released to the surroundings!
Heat
Insulation
surroundings System
Calculating surrS
Note that for an endothermic process, thermal energy is absorbed from the surroundings!
Heat
surroundings System
Connecting surrS to sysH
For a constant pressure process qp = H
surrS surrH = -sysH surrS = -sysH / T
For a chemical reactionsysH = rH
surrS = -rH/ T
The Use of univS to Determine Spontaneity
Calculation of TunivS two system parameters rS rH
Define a system parameter that determines if a given process will be spontaneous?
The Definition of the Gibbs Energy
The Gibbs energy of the systemG = H – TS
For a spontaneous processsysG = Gf – G i
Gf = the Gibbs energy of the final stateGi = the Gibbs energy of the initial state
Gibbs Energy and Spontaneity
sysG < 0 - spontaneous processsysG > 0 - non-spontaneous process
(note that this process would be spontaneous in the reverse
direction)sysG = 0 - system is in equilibrium
Note that these are the Gibbs energies of the system under non-
standard conditions
Standard Gibbs Energy Changes
The Gibbs energy change for a chemical reaction?
Combustion of methane. CH4 (g) + 2 O2 (g) CO2 (g) + 2 H2O (l)
Define rG = np fG (products) - nr fG
(reactants) fG = the formation Gibbs energy of the
substance
Gibbs Energy Changes
fG (elements) = 0 kJ / mole. Use tabulated values of the Gibbs
formation energies to calculate the Gibbs energy changes for chemical reactions.
The Third Law of Thermodynamics
Entropy is related to the degree of randomness of a substance.
Entropy is directly proportional to the absolute temperature.
Cooling the system decreases the disorder.
The Third Law of Thermodynamics
The Third Law - the entropy of any perfect crystal is 0 J /(K mole) at 0 K (absolute 0!)
Due to the Third Law, we are able to calculate absolute entropy values.
At a very low temperature, the disorder decreases to 0 (i.e., 0 J/(K mole) value for S).
The most ordered arrangement of any substance is a perfect crystal!
Applications of the Gibbs Energy
The Gibbs energy is used to determine the spontaneous direction of a process.
Two contributions to the Gibbs energy change (G) Entropy (S) Enthalpy (H)
G = H - TS
Spontaneity and Temperature
H S G
+ + < 0 at high temperatures
+ - > 0 at all temperatures
- + < 0 at all temperatures
- - < 0 at low temperatures
Gibbs Energies and Equilibrium Constants
rG < 0 - spontaneous under standard conditions
rG > 0 - non-spontaneous under standard conditions
The Reaction Quotient
Relationship between QJ and Keq
Q < Keq
- reaction moves in the forward directionQ > Keq
- reaction moves in the reverse directionQ = Keq
- reaction is at equilibrium
rG° refers to standard conditions only!
For non-standard conditions - rG rG < 0 - reaction moves in the
forward directionrG > 0 - reaction moves in the
reverse directionrG = 0 - reaction is at equilibrium
Relating Keq to rG
rG = rG +RT ln QrG = 0 system is at equilibrium
rG = -RT ln Qeq
rG = -RT ln Keq
Phase Equilibria
At the transition (phase-change) temperature only - trG = 0 kJ
tr = transition type (melting, vapourization, etc.)
trS = trH / Ttr