1 Enthaply vs. entropy in chemical reaction: S solid < S liquid - - PDF document

SLIDE 1

1

Topic 9.4 Enthalpy, entropy and spontaneity

There are two fundamental tendencies in nature that help determine whether a reaction will occur:

• 1. Enthalpy
• Energy change during reaction

On the basis of energy alone, we expect reactions to go in the exothermic direction

= heat of reaction (ΔH)

• Reactions tend toward lower energy state

= exothermic

• 2. Entropy

spontaneous

Entropy tends to ↑

nonspontaneous

Entropy tends to ↓

• A measure of disorder
• Symbolized by ‘S’

+ΔS means ↑ in disorder

• ΔS means ↓ in disorder
• 2. Entropy

On the basis of entropy alone, we expect reactions to go in the direction of greater randomness

SLIDE 2

2

Ssolid < Sliquid << Sgas

A(g) A(s) A(s) A(l)

reaction ↓ or ↑ entropy Increases +ΔS Decrease

• ΔS

A(g) + B(g) C(g)

Decrease

• ΔS

2A(g) + 3B(s) 3C(g) + 2D(s)

Increases +ΔS

NaCl(s) Na+

(aq) + Cl (aq) Increases +ΔS

Enthaply vs. entropy in chemical reaction:

• Spontaneous reactions favored by:
• ↑ Entropy =

Four different scenarios:

• 1. ΔH is (-):

ΔS is (+): Exothermic, favorable More disordered, favorable

• 2. ΔH is (+):

ΔS is (-): Endothermic, unfavorable Less disordered, unfavorable

• 3. ΔH is (-):

ΔS is (-): favorable unfavorable

spontaneous nonspontaneous ?

• 4. ΔH is (+):

ΔS is (+): unfavorable favorable

?

More disordered

• ↓ Enthalpy = Heat released from reaction

If ΔG is (–), spontaneous If ΔG is (+), nonspontaneous If ΔG is = 0, reaction is at equilibrium

ΔG = ΔH − TΔS

Free E change Heat of Reaction Temp. in K Entropy For situations 3 and 4 use Gibb’s Free Energy (ΔG ): a measure of how spontaneous a reaction is Will the following reactions be spontaneous or nonspontaneous?

1)

2X(g) Y(g) + 2Z (g)

H =

2 to 3 moles gas

S =

exothermic

ΔG = ΔH − TΔS

– –(+) –

S = + H = -

spontaneous

2)

X(g) + Z (g) Y(g) +U (s)

H =

2 gas to 1 gas and 1 solid

S =

endothermic

ΔG = ΔH − TΔS

+ –(–) +

= -

= +

nonspontaneous

+

3)

Y(s) A

• (aq) +B

+ (aq)

1 solid to 2 ions endothermic

ΔG = ΔH − TΔS

+ –(+)

S = H =

–

ΔG =

At low T: At high T:

ΔG = (+), nonspontaneous ΔG = (–), spontaneous

• or + depending on T

H = + S = +

H > TS TS > H

SLIDE 3

3

4)

X(g) + X (g) X 2(g)

H =

2 to 1 moles gas

S =

exothermic

ΔG = ΔH − TΔS

– –(–)

Spontaneous at low T Nonpontaneous at high T

= -

• +

Bond forming is always exothermic

The stability of compounds can be determined by comparing heats of formation values C2H2 C2H4 C2H6 ΔHf +227 +52

• 84

Most stable Most unstable

• The more endothermic the heat of formation, the

more unstable a compound is.

• The more exothermic, the more stable