Spontaneous Reactions

Proceed forward on their own without
outside or external cause.

Certain conditions can help us predict
whether a reaction will occur
spontaneously or not.
Factors that Determine Spontaneity
Change in Enthalpy (∆H)
Exothermic (- ∆H)
reactions are more likely to
be spontaneous.
A + B → C + D + energy

Why?
 Release energy, producing lower energy more
stable products.
 Require less activation energy, so are easier to
start
Factors that Determine Spontaneity
Change in Entropy (∆S)
 Reactions with an INCREASE in Entropy are
more likely to be spontaneous.

Entropy is a measure of the disorder,
randomness or lack of organization in a system.

+ ∆S =
increase entropy
or disorder

Entropy deals with the Second
Law of Thermodynamics, which
basically states that all systems
in the universe progress to a
state of higher disorder.
+∆S
-∆S
Entropy =
Chaos and
Disorder
http://www.youtube.com/watch?
v=CgppGozbFd4&safe=active
http://www.youtube.com/watch?
v=uQSoaiubuA0&safe=active
How to Determine the Change in Entropy
+∆S entropy increases
 - ∆S entropy decreases


Changes to Phases of Matter
Solids
Liquids
Gases
Most ordered
Least ordered
Ex:
H2O(s) → H2O (l)
+∆S
Ex:
H2O(l) → H2O (s)
-∆S
Gas particles are highly disbursed with random
motion and high entropy.
 Liquids have much lower entropies
 Solids are most ordered and lowest entropy.

If Phases Stay the Same
Look at:
Is there an increase in number of moles of gas? +∆S
More highly disorganized gas = more chaos
Is the substance breaking apart or dissolving? +∆S
More pieces, more disbursed = more chaos
What is the Sign of +∆S?
Reaction
Ag+(aq) + Cl-(aq) → AgCl(s)
Sign
of ∆S
-∆S
H2O(l) → 2 H2(g) + O2(g)
+∆S
CO(g) + 3 H2(g) → CH4(g) + H2O(g)
-∆S
2 NaHCO3(s) ---> Na2CO3(s) + H2O(g) + CO2(g) +∆S

Given the balanced equation representing a
phase change:
C6H4Cl2(s) + energy → C6H4Cl2(g)
Which statement describes this change?
(1) It is endothermic, and entropy decreases.
(2) It is endothermic, and entropy increases.
(3) It is exothermic, and entropy decreases.
(4) It is exothermic, and entropy increases.

The entropy of a sample of H2O increases
as the sample changes from a
(1) gas to a liquid
(2) gas to a solid
(3) liquid to a gas
(4) liquid to a solid

Which 1-mole sample has the least
entropy?
(1) Br2(s) at 266 K
(2) Br2(l) at 266 K
(3) Br2(l) at 332 K
(4) Br2(g) at 332 K

In terms of energy and entropy, systems in
nature tend to undergo changes toward
(1) higher energy and higher entropy
(2) higher energy and lower entropy
(3) lower energy and higher entropy
(4) lower energy and lower entropy

Which list of the phases of H2O is
arranged in order of increasing entropy?
(1) ice, steam, and liquid water
(2) ice, liquid water, and steam
(3) steam, liquid water, and ice
(4) steam, ice, and liquid water
Given the balanced equation:
I2(s) + energy → I2(g)
As a sample of I2(s) sublimes to I2(g), the entropy
(1) increases because the particles are less
randomly arranged
(2) increases because the particles are more
randomly arranged
(3) decreases because the particles are less
randomly arranged
(4) decreases because the particles are more
randomly arranged

At STP, a sample of which element has
the highest entropy?
(1) Na(s)
(2) Hg(l)
(3) Br2(l)
(4) F2(g)

Which of these changes produces the
greatest increase in entropy?
(1) CaCO3(s) → CaO(s) + CO2(g)
(2) 2 Mg(s) + O2(g) → 2 MgO(s)
(3) H2O(g) → H2O(l)
(4) CO2(g) → CO2(s)

Even though the process is endothermic,
snow can sublime. Which tendency in
nature accounts for this phase change?
(1) a tendency toward greater entropy
(2) a tendency toward greater energy
(3) a tendency toward less entropy
(4) a tendency toward less energy

Which process is accompanied by a
decrease in entropy?
(1) boiling of water
(2) condensing of water vapor
(3) subliming of iodine
(4) melting of ice

Answer: A

Answer: 2

Answer: 1
Is Reaction Spontaneous?

A reaction will always be spontaneous if:



- ∆H
+ ∆S
A reaction will never be spontaneous if:


+ ∆H
- ∆S
Sometimes Spontaneous


+ ∆H
+ ∆S



- ∆H
- ∆S



More likely at higher temperatures
More likely at lower temperatures
These situations depend on the temperature
conditions.
You would have to do a calculation to determine.
Gibbs Free Energy
Gibbs Free Energy (∆G)
A portion of energy from a spontaneous reaction that can
perform useful work.

Ex: battery, glowstick
Spontaneous Reactions are - ∆G
They give off free energy
Nonspontaneous Reactions are + ∆G
They require the input of free energy to occur

When a rxn is no longer spontaneous and producing free
energy the system is at equilibrium and ∆G = 0
(- ∆H) (+ ∆S) Always Spontaneous so - ∆G
(+ ∆H) (- ∆S) Never Spontaneous so + ∆G
What about sometimes spontaneous?
How would you know if it is or not?
Use the Gibbs Free Energy Equation
∆G = ∆H - T ∆S
Kelvin Temperature
Gibbs Free Energy of Formation
(∆Gf) (Honors Only)

Quantity of free energy involved in the
formation of one mole of a compound from
it’s elements.

Way to determine overall ∆G of a reaction
∆Grxn = Σ (∆Gf Products) - Σ (∆Gf Reactants)
This should look familiar!!! Same as ∆Hf problems from last test.
An Endothermic reaction may proceed
spontaneously if there is an increase in
A. Potential Energy
B. Order
C. Concentration
D. Entropy
A reaction must be spontaneous if its
occurrence is
A. exothermic with an decrease in entropy
B. exothermic with an increase in entropy
C. endothermic with an decrease in entropy
D. endothermic with an increase in entropy
Above 0°C, ice changes spontaneously to water
according to the following equation:
H2O(s) + heat --> H2O(l)
The change in H2O(s) involve
A. an absorption of heat and a decrease in entropy
B. a release of heat and a decrease in entropy
C. an absorption of heat and a increase in entropy
D. a release of heat and a increase in entropy
The ΔG of a chemical reaction refers to the
change in
A. entropy
B. free energy
C. state
D. activation energy
The change of the reactants into products
will always be spontaneous if the products,
compared to the reactants, have
A. lower enthalpy and higher entropy
B. higher enthalpy and higher entropy
C. higher enthalpy and lower entropy
D. lower enthalpy and lower entropy