Entropy

State Function (°)

Property with a specific value only influenced by a system’s present condition

Only dependent on the initial and final states, NOT on the path taken to reach the condition Initial state Final state

** State functions go back to initial values in opposite process and system returns to initial state**

Ex. Enthalpy (H), Entropy (S) , Free Energy (G)

Spontaneous vs. Nonspontaneous

1)Spontaneous Process Occurs WITHOUT help outside of the system,

natural Many are exothermic—favors energy release to

create an energy reduction after a chemical reaction Ex. Rusting iron with O2 and H2O, cold coffee in a

mug

Some are endothermic Ex. Evaporation of water/boiling,dissolving in

water

Spontaneous vs. Nonspontaneous

2) Nonspontaneous Process REQUIRES help outside system to perform chemical

reaction, gets aid from environment

Ex. Water cannot freeze at standard conditions (25°C, 1atm), cannot boil at 25°C

**Chemical processes that are spontaneous have a nonspontaneous process in reverse **

Entropy (S)

Measure of a system’s disorder The degree of randomness associated with particles (molecules,

etc.)

Disorder is more favorable than order ΔS = S(products) - S(reactants)

ΔS is (+) with increased disorder ΔS is (-) with decreased disorder

State function Only dependent on initial and final states of a reaction

Ex. Evaporation, dissolving, dirty house

When does a system become MORE disordered from a

chemical reaction? (ΔS > 0)1) Melting 2) Vaporization 3) More particles present in the products than

the reactants 4C3H5N3O9 (l) 6N2 (g) + 12CO2 (g) + 10H2O (g) +

O2 (g)

4) Solution formation with liquids and solids 5) Addition of heat, increasing temperature

Thermodynamic Laws 1st Law of Thermodynamics

Energy cannot be created or destroyed

2nd Law of Thermodynamics The entropy of the universe is always

increasing. Naturally favors a disordered state

3rd Law of Thermodynamics

The entropy (ΔS) of a perfect crystal is 0 at a temperature of absolute zero (0°K). No particle motion at all in crystal structure All motion stops

How do we determine if a

chemical reaction is spontaneous?

1)Change in entropy (ΔS)

2) Gibbs Free Energy (ΔG)

Gibbs Free Energy

Gibbs Free Energy (G) Balances the relationship between enthalpy (ΔH)

and entropy (ΔS) State function Enthalpy of system minus the product of

temperature times entropy of system

G = H – TS

Maximum amount of energy available to do work, “free”

Change in Gibbs Free

Energy (ΔG) ΔG = ΔH – TΔS Relates enthalpy and entropy to determine

which has more importance in determining whether a reaction is spontaneous

Combines energy transfer as heat (ΔH) and energy released to contribute to disorder (ΔS)

DG is the change in Gibbs free energy.

DG can be calculated asDGo = DHo - TDSo

The term DH represents enthalpy or heat energy which is available to do work.

The term DS represents entropy or random motion which is not available to do work.

Gibbs Free Energy

Example 2:

Find ΔG for a chemical reaction given ΔH = -218 kJ and ΔS = -765 J/K at 32°C.

Change in Gibbs Free

Energy (ΔG) ΔG = ΔH – TΔS ΔG < 0 , spontaneous reaction, reaction occurs as

written Energy available to do work

ΔG > 0, nonspontaneous reaction, reaction will NOT occur as written Energy deficiency, no leftover energy and not enough

energy for reaction

** All reactions want to move toward low or minimal ΔG

A spontaneous reaction is NOT necessarily fast!!!! Reaction rate involves kinetics ! !

Entropy(ΔS) > 0, POSITIVE

Reaction creates more disorder

Free Energy (ΔG) < 0, NEGATIVE

What makes a reaction

spontaneous?

Read pp. 546-549

p. 550 #2-4, 5

Homework