Entropy and Gibbs free energy
1. Spontaneous Change
A spontaneous change is one that occurs
without a continuous input of energy from
outside the system.
All chemical processes require energy (activation energy) to
take place, but once a spontaneous process has begun, no
further input of energy is needed.
2
A nonspontaneous change occurs only if the surroundings
continuously supply energy to the system.
2H2(g) + O2(g) → 2H2O(l)
2H2O(l)→2H2(g) + O2(g)
If a change is spontaneous in one direction, it will be
nonspontaneous in the reverse direction.
3
DH Does Not Predict Spontaneous Change
Ba(OH)2 ·8H2O(s) + 2NH4NO3(s) → Ba2+(aq) + 2NO3-(aq) + 2NH3(aq) + 10H2O(l)
DHθ = +62.3 kJ
water
This reaction occurs spontaneously when the solids are mixed. The
reaction mixture absorbs heat from the surroundings so quickly that
the beaker freezes to a wet block.
4
反应总是在一定条件下进行的。要讨论反应的自发性，也要指明反应条件。如果没有
交代反应条件，只讲“某某反应不自发”“某某反应自发”，实际上是指在常温常压
下进行的反应。“一定条件”指“一定的温度一定压强”。
例如，反应 CaCO3== CaO + CO2↑ 在常温常压是非自发的，而在高温下则是自发发
生的。
“外界帮助”这句话中，指的是为反应体系提供能量。“帮助”并不是指提供反应
发生的温度和压强等反应条件，也不是点燃、使用催化剂等等。一定的温度、一定
的压强是反应所需要的条件。很多场合下,提高温度,往往是为了加速反应。氢气氧
气化合成水的反应在常温下也是自发的。常温下将H2与O2混合，不能观察到有明显
现象，在氢氧燃料电池中，反应在常温下就能进行。在钯粉催化下H2与O2的混合气
体也能快速生成水。点燃氢气氧气，能迅速化合成水，也只是加速了反应速率。
“外界帮助”指的是对体系做“有用功”。例如，水的分解在常温常压下是不自发的，
但通电，为体系提供电能，可以使水分解成H2和O2.
5
Q1: Which of the following changes are likely to be spontaneous?
i sugar dissolving in water
spontaneous (because there are more ways of spreading out the sugar
molecules when they dissolve)
ii the smell from an open bottle of aqueous ammonia diffusing throughout a room
spontaneous (because there are more ways of spreading out the ammonia
molecules when they mix with the air molecules)
iii water turning to ice at 10℃
not spontaneous (because ice forms at 0 ℃ and the energy transferred from the
surroundings at 10℃ gives the molecules enough energy to move semi-independently of
each other and so there are a greater number of ways of arranging the energy in the
liquid than in the solid)
iv ethanol vaporising at 20℃
spontaneous (because there is enough energy given to the molecules at 20 ℃for
some of them to escape, thus allowing more spreading out of molecules in the vapour
than in the liquid)
v
water mixing completely with cooking oil
not spontaneous (because the strength of the bonding between water molecules alone
is greater than the strength between water and oil molecules; so the oil molecules
cannot mix and spread between the water molecules)
vi limestone decomposing at room temperature
not spontaneous (because the temperature is not high enough to break bonds in the
limestone (calcium carbonate); the ions in the calcium carbonate remain ordered)
6
law of thermodynamics
First law of thermoldynamics
Energy can be transformed from one form to another, but can be
neither created nor destroyed.
The total energy of the universe is constant:
DEsys = -DEsurr or DEsys + DEsurr = DEuniv = 0
Second law of thermoldynamics
A reaction is spontaneous if entropy change for the universe
or total is positive.
7
DH=0
When the stopcock opens, the
number of microstates is 2n,
where n is the number of particles.
8
2. Entropy
Entropy
is a measure of the dispersal of energy at a specific temperature, either from
systerm to the surroundings or from the surroundings to the system.
Entropy can also be thought of as a measure of the randomness or disorder of
a system. The higher randomness or disorder, ther greater entropy of the
system.
9
2.1 Comparing entropy values
Entropy can also be thought of as a measure of the randomness or disorder of a system.
To make any comparison of entropy values fair, we must use standard conditions.
n a pressure of 105Pa
n a temperature of 298K
n each substance involved in the
reaction is in its normal physical
state(solid, liquid or gas) at
105Pa and 298K
10