Reaction Spontaneity

How can a fire start on its own? (without a spark)

Spontaneous Reaction

• Occurs naturally and favors formation of products

4Fe(s) + 3O2(g) 2Fe2O3(s) ΔH = -1625 kJ

CH4(g) + 2O2(g) CO2(g) + 2H2O(l) ΔH = -891 kJ

Nonspontaneous Reaction

Does not favor formation of products

2Fe2O3(s) 4Fe(s) + 3O2(g) ΔH = 1625 kJCO2(g) + 2H2O(l) CH4(g) + 2O2(g) ΔH = 891 kJ

H2O(s) H2O(l) ΔH = 6.01 kJ

Some endothermic reactions are SPONTANEOUS!

ENTROPY (S)

• The measure of disorder or randomness of particles in a system

LAW OF DISORDERSpontaneous processes always increase the entropy of a system

Law of Disorder

Changes in Entropy (ΔS)

•Δssystem = Sproducts – Sreactants

1. Changes in State

2. The dissolving of a gas in a solvent always results in a decrease in entropy.

CO2(g) CO2(aq) Δssystem < 0

3. Assuming no change in physical state, the entropy increases when: # of product particles > # of reactant particles.

2SO3(g) 2SO2(g) + O2(g) Δssystem > 0

4. when a solid or a liquid dissolves to form a solution.

NaCl(s) Na+(aq) + Cl-(aq) Δssystem > 0

5. increase in the temperature increase in entropy

Does entropy increase or decrease?

CaCO3 (s) CaO (s) + CO2 (g)

NH3 (g) + HCl (g) NH4Cl (s)

How ΔHsystem and ΔSsystem

affect reaction spontaneity

ΔH ΔS Spontaneous?

- + yes+ + Depends on T- - Depends on T+ - No

Gibb’s Free Energy!!

ΔGsystem = ΔHsystem – TΔSsystem

The energy that is available to do work (useful energy)

ΔGsystem and reaction spontaneity

Type of Reaction or Process

ΔGsystem ΔSuniverse

Spontaneous - +Nonspontaneous + -

Is the following reaction spontaneous?

N2(g) + 3H2(g) 2NH3(g) ΔH°system= -91.8 kJ

ΔS°system= -197 J/K

ΔGsystem = ΔHsystem – TΔSsystem

For a process, ΔHsystem=145 kJ and ΔSsystem=322J/K. Is the reaction spontaneous at 382 K?