2012 15 3 and 15 4

LESSON

Entropy

Spontaneity

IB Chemistry Power Points

Topic 15

Energetics

www.pedagogics.ca

ENTROPYEntropy, S is a measure of the DISORDER or randomness of a system.

A single coin can have 2 possible configurations.

A system of 4 coins can have 16 (more disorder)

The greater the number of configurations (or microstates) in a particular system, the greater the entropy (disorder) of the system

Entropy on the Molecular Scale

Implications to a chemical system (MUST KNOW):

• more particles -> more states -> more

entropy

• higher T-> more energy states -> more

entropy

• less structure (gas vs solid) -> more states -> more

entropy

Entropy changes (dissolving)

ΔS = Sfinal

– Sinitial

Entropy changes (heating)

ΔS = Sfinal

– Sinitial

Entropy changes (change of state)

ΔS = Sfinal

– Sinitial

Standard entropy values: Sѳ

standard entropy is an absolute value

The standard entropy value for a substance is defined as the entropy increase of the substance when heated from 0 K to 298 K (standard conditions)

ΔSѳ = ΣSѳproducts

–

ΣSѳreactants

The standard entropy change for a reaction can be determined by:

Consider the following reaction

Describe the change in entropy in this reaction.

Calculate the change in entropy in this reaction.

Spontaneous Processes

• Spontaneous processes are those that can proceed without any outside intervention.

• The gas in vessel B will spontaneously effuse into vessel A, but once the gas is in both vessels, returning to the original state is not spontaneous

Spontaneous Processes

Processes that are spontaneous in one direction are non spontaneous in the reverse direction.

Spontaneous Processes• Processes that are spontaneous at one

temperature may be nonspontaneous at other temperatures.

• Above 0C it is spontaneous for ice to melt.

• Below 0C the reverse process is spontaneous.

18.4Spontaneous reactions produce substantial amounts of products at equilibrium and release free energy.

Free energy is energy that is available to do work

The Gibbs free energy change, G is the maximum amount of free energy that can be extracted to do work.

Gibbs Free Energy Change(some things to know)

1. If the G (for a particular reaction under a specific set of conditions) is negative, the forward reaction is spontaneous. (or vice versa)

2. The G for a reaction can be determined from standard free energies of formation, Gf values available on Data Tables. This is analogous to using standard enthalpies of formation, Hf values.

The key equation to predict spontaneity:

This equation shows how G changes with temperature.

(We assume S & H values are not affected by T.)

G H T S

Free Energy and Temperature

• There are two parts to the free energy equation: H the enthalpy term TS the entropy term

• The temperature dependence of spontaneity comes from the entropy term.

By knowing the sign (+ or -) of S and H, we can get the sign of G and determine if a reaction is spontaneous.

endothermic so ΔH must be positive

entropy increasing so ΔS must be positive

3

1

1

176 298 284

9136

0

8

91.4

1

G H T S

Jmol

kJmol

not spontaneous

3

1

1

176 298 284

9136

0

8

91.4

1

G H T S

Jmol

kJmol

increase in temperature increases spontaneity

3100 176 284

620

346

o

G H T S

T

T K

C