4th Lecture on Chemical Thermodynamics Chemistry Part I12th Standard by Rizwana Mohammed

1. The Malegaon High School & Jr. College
Malegaon, (Nashik), 423203
4th Lecture on Chemical
Thermodynamics
Chemistry Part I, 12th Science
By
Rizwana Mohammad

2. Spontaneous (irreversible) process:
Spontaneous process have a natural tendency to occur and do
not require any external influence for their occurrence. ”The
spontaneous (natural) processes tend to occur in a direction that
leads to equilibrium.”
Energy and spontaneity:
The spontaneous reaction takes place in a direction in which
energy of the system is lowered. It is accompanied by release of
energy. The reaction between NaOH and HCl is exothermic
(ΔrH˚ = -57 kJ) and is spontaneous.
On the other hand:
i. Ice melts spontaneously by absorbing heat from surrounding.
ii. NaCl dissolves spontaneously by absorption of heat
These are endothermic and spontaneous.
Therefore, exothermicity is not the sufficient criterion for
spontaneity. There must be another factor to describe spontaneity.

3. Entropy:
Consider following figure
During melting of ice or the vaporization of liquid water the disorder or
randomness increases. The disorder or randomness is measured by
entropy, denoted by S.
During above two processes, entropy change
ΔS > 0
e.g.
i. Dissolution of solid I2 in water
I2(s) + aq. → I2(aq); ΔS is positive
ordered state disordered state
ii. Dissociation of H2 molecule into atoms
H2(g) → 2H(g); ΔS is positive
disordered state highly disordered state

4. Quantitative definition of entropy:
Entropy change of a system is equal to the amount of heat transferred
(Qrev) to it in a reversible manner divided by the temperature in Kelvin.
Thus, ΔS =
𝑄 𝑟𝑒𝑣
𝑇
ΔS is expressed in JK-1
i. ΔS is directly proportional to Qrev
ii. ΔS is inversely proportional to T
Entropy and spontaneity:
Look at following examples:
i. The entropy increases when ice melts above 0°c and water vaporizes at
100°c. Both are so spontaneous.
ii. Consider the spontaneous reaction at room temperature
2H2O2(l) → 2H2O(l) + O2(g); ΔS = +126 JK-1
In the above examples, entropy of the system increases in the
spontaneous process.
Consider the reaction
2H2(g) + O2(g) → 2H20(l); ΔS = - 327 JK-1
The entropy of the system decreases, yet the reaction is spontaneous.

5. Second law of thermodynamics:
"The second law of thermodynamics states that total entropy of a system
and its surroundings increases in a spontaneous process”
i.e. ΔStotal = ΔSsys + ΔSsurr > 0
Consider,
2H2(g) + O2(g) → 2H₂O(l) at 298 k.
ΔS = -327 JK-1, ΔH = -572 kJ
ΔSsurr =
𝑄 𝑟𝑒𝑣
𝑇
=
572 ∗103
𝐽
298 𝑘
= 1919 Jk-1
Δstotal = ΔSsys + ΔSsurr
= -327 + 1919
= +1592 Jk-1
therefore ΔStotal > 0
The reaction is thus spontaneous. It follows that to decide spontaneity
of reactions, we need to consider the entropy of system and its surrounding.
Thus,
i. ΔStotal > 0, process is spontaneous,
ii. ΔStotal < 0, process is nonspontaneous,
iii. ΔStotal = 0, process is at equilibrium.

6. Gibbs energy:
The Gibbs energy is defined as
G = H –TS …1
ΔG = ΔH – TΔS …2
Gibbs energy and spontaneity:
The total entropy change is given by
ΔStotal = ΔSsys + ΔSsurr
= ΔS + ΔSsurr …3
Relation between ΔG and ΔStotal:
According to second law of thermodynamics for a process to be spontaneous,
ΔStotal > 0
If ΔH is enthalpy change of system, -ΔH is enthalpy change of surrounding.
ΔSsurr = −
Δ 𝐻
𝑇
From eqn 3
ΔStotal = ΔS -
Δ𝐻
𝑇
…4
-TΔStotal = ΔH – TΔS …5
Comparing eqn 2 & 5
ΔG = -TΔStotal
Thus, i. ΔStotal > 0 and ΔG < 0, process is spontaneous,
ii. ΔStotal < 0, ΔG > 0, process is non spontaneous,
iii. ΔStotal = 0, ΔG = 0, process is at equilibrium.

7. From ΔG = ΔH - T ΔS (at constant T and
P).
The temperature term determines
relative contributions of ΔH and ΔS to
ΔG.
1. ΔH and ΔS are both negative then
ΔG will be negative only when ΔH is
more negative than TΔS. This is
possible at low temperatures only.
2. ΔH and ΔS both positive ΔG will be
negative only if TΔS > ΔH. This is
possible only at high temperatures.
3. For ΔH negative and ΔS is positive it
follows that ΔG is negative
regardless of temperature.
4. For ΔH positive and ΔS is negative
then ΔG is positive regardless of
temperature. Such reactions are
nonspontaneous at all
temperatures.
ΔH ΔS ΔG Spontaneity of reactions
-
(exothermic)
+ - Reactions are spontaneous at all temperatures.
-
(exothermic)
- -
Reactions become spontaneous at low temperatures
when | T. ΔS | < |ΔH|
+
(endothermic)
+ -
Reactions become spontaneous at high temperatures
when T. ΔS > ΔH
+
(endothermic)
- + Reactions are nonspontaneous at all temperatures.
Spontaneity and ΔH or ΔS:

8. Temperature of equilibrium:
ΔG = ΔH – TΔS
For equilibrium, ΔG = 0
T =
ΔH
ΔS
T is the temperature at which the change over from spontaneous to non
spontaneous behaviour occurs.
Gibbs function and equilibrium constant:
Gibbs energy change for a chemical reaction is given by
ΔG = ΔG˚ + RT lnQ
ΔG˚ = standard Gibbs energy change,
Q = reaction quotient.
Consider
aA + bB ⇌ cC + dD
ΔG = ΔG˚ + RT lnQc
= ΔG˚ + RT ln
𝐶 𝑐
𝐷 𝑑
𝐴 𝑎
𝐵 𝑏
Or ΔG = ΔG˚ + RT lnQp
= ΔG˚ + RT ln
𝑃 𝑐
𝐶
∗𝑃 𝑑
𝐷
𝑃 𝑎
𝐴
∗𝑃 𝑏
𝐵

9. When the reaction reaches equilibrium, ΔG=0
Qc = Kc, Qp = Kp
0 = ΔG˚ + RT lnKc and
0 = ΔG˚ + RT lnKp
Or
ΔG˚ = -RT lnkc and ΔG˚ = -RT lnkp
Or
ΔG˚ = -2.303 RT log10Kc and
ΔG˚ = -2.303 RT log10Kp