Chemistry 100 Chapter 19

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