6.04
A
spontaneous change is a change in a
system that proceeds without a net input of
energy from an outside source.



Needs some activation energy to get started
It may be a fast reaction, or slow reaction
Once it is started, it takes place without any
continuous outside influence such as heating,
cooling, stirring, etc..
 Spontaneous
super fast!
does not mean instantaneous or
 Exothermic
reactions can be spontaneous.
These reactions have a lower energy
state than reactants

Negative enthalpy change
 However,
Endothermic reactions can
also be spontaneous! These reactions will
have a higher energy state than the
reactants. 

Positive enthalpy change
 Enthalpy
is represented by ΔH
Entropy is a measure of the disorder, or
molecular randomness, of a system.
 It is natural for things to become more
disordered. If you throw a deck of cards on the
floor, they are going to be randomly out of
place. It will take effort to sort them and put
them into increasing order of the deck.
 Entropy is represented by ΔS
 Entropy increases when ‘randomness’ increases.
For example- solids are more ordered, and gases
are more random. Therefore, gases have an
increase in entropy compared to solids!

Freezing of water- Decrease in entropy
 Dissolving salt in water- Increase in entropy
because it increases the disorder!
 Evaporating rubbing alcohol- Increase in entropy
because it is turning into a gas and becoming
more disordered!
 CO2 + CaO  CaCO3 – Decrease in entropy. As a
general rule, the side of the equation with more
moles of gas has more entropy. Also, when the
total number of moles decrease, entropy
typically decreases as well!
 2KClO3  2KCl + 3O2- Increase in entropy. There
are more moles in the product as well as a gas
produced! Both signs of an increase in entropy!

 Free
Energy is represented by ΔG
 A reaction, at a constant temperature and
pressure, is spontaneous in the direction that
gives a negative change in fee energy (-ΔG)
 Temperature, enthalpy, and entropy all have
an effect on the spontaneity of a chemical
reaction.
 Equation to calculate these values...
ΔG = ΔH – (T x ΔS)
The units for temperature used in the equation
should ALWAYS be in Kelvin!


 The
melting of ice has an enthalpy value of
ΔH = +333 kJ/kg. What is true about the
spontaneity of this process?




A. Spontaneous at all temperatures
B. Spontaneous at low temperatures
C. Spontaneous at high temperatures
D. Not spontaneous at any temperature
C-
Spontaneous at high
temperatures
 The
melting of ice has a positive
entropy change (melting from a
solid to a liquid is always a positive
entropy change) and a positive
enthalpy change (ΔH = +333 kJ/kg).
For this combination of entropy
and enthalpy, the reaction will be
spontaneous (-ΔG) at high
temperatures.
 The
combustion of ethane gas has a negative
enthalpy change and a positive entropy
change.




A. Spontaneous at all temperatures
B. Spontaneous at low temperatures
C. Spontaneous at high temperatures
D. Not spontaneous at any temperature
A-
Spontaneous at all
temperatures
 A negative enthalpy change and
a positive entropy change are
both “preferred” by nature, so
this process is spontaneous at
any temperature.
 The
process of freezing methane (CH4 (l) →
CH4 (s)) has a negative enthalpy change (ΔH
= -58.4 kJ/kg).




A. Spontaneous at all temperatures
B. Spontaneous at low temperatures
C. Spontaneous at high temperatures
D. Not spontaneous at any temperature
B-
Spontaneous at low
temperatures.
 The
freezing of methane has a
negative entropy change (freezing
from a liquid to a solid is always a
negative entropy change) and a
negative enthalpy change (ΔH = 58.4 kJ/kg). For this combination of
entropy and enthalpy, the reaction
will be spontaneous (-ΔG) at low
temperatures.