In order to predict whether a reaction is spontaneous, we need to apply the Second Law of Thermodynamics which states that the entropy of the universe increases for a spontaneous change. Technically, we should consider the entropy change that takes places in both the system and the surrounding. This is given as:
ΔSuniverse = ΔSsystem + ΔSsurrounding
In Chemistry, the focus is always on the system. Thus, to simplify discussion, it is more convenient if ΔSsystem + ΔSsurrounding are combined together in a single function known as the Change in Gibbs Free Energy, ΔG.
Change in Gibbs Free Energy, ΔG can be expressed in terms of Enthalpy Change, ΔH and Entropy Change, ΔS of the system.
ΔG = ΔH – TΔS
Do note the importance of the units involved in the three different thermodynamics terms.
- Units of ΔG and ΔH are kJmol-1 or Jmol-1.
- Units of ΔS are kJmol-1K-1 or Jmol-1K-1.
- Units of T is K.
We can use the sign of ΔG to predict if a reaction is spontaneous.
Three Scenarios for ΔG:
1. ΔG < 0
The reaction is feasible and takes place spontaneously. Sometimes, we said that the reaction is exergonic.
2. ΔG = 0
The system is said to be at equilibrium. There is no net reaction in the forward or reverse direction. ΔG = 0 is observed in all physical state changes such as melting, boiling, condensation, freezing, etc.
3. ΔG > 0
The reaction is said to be not feasible and cannot take place spontaneously. It is spontaneous in the reverse direction. Sometimes, we said that the reaction is endergonic.
Temperature Dependence of ΔG:
For a reaction to be spontaneous, ΔG must be negative.
The sign of ΔG depends on the signs and magnitudes of both ΔH and ΔS, as well as the temperature, T. We observe that most exothermic reactions (ΔH=-ve) are spontaneous as the enthalpy, ΔH contribution to ΔG is much more than the entropy, ΔS contribution. Hence, a highly negative ΔH is likely to make ΔG negative.
However, the temperature of a reaction does influence the magnitude of the term TΔS and hence, the spontaneity of many reactions depends on the temperature, T. We can use the equation ΔG = ΔH – TΔS to predict the sign of ΔG when temperature, T varies.
Four Possible Scenarios of ΔG:
|ΔH||ΔS||ΔG = ΔH – TΔS||Remarks|
|negative||positive||always negative||spontaneous at ALL temp.|
|positive||negative||always positive||not spontaneous at ALL temp.|
|negative||negative||negative if |ΔH| > |TΔS|||spontaneous at LOW temp.|
|positive||positive||negative if ΔH < TΔS||spontaneous at HIGH temp.|
Scenario 1: ∆H is negative & ∆S is positive
These reactions are feasible at all temperature.
Reactants are metastable under all conditions and only exist because all the activation energy of the reaction is so high i.e. kinetically slow.
- Few decomposition reactions N2O(g) –> N2(g) + ½ O2(g)
- Organic combustion rxns C8H18(g) + 12 ½ O2(g) –> 8CO2(g) + 9H2O(g)
- Explosives 2C3H5N3O9 –> 3N2 + 5H2O + 6CO2 + ½ O2
Scenario 2: ∆H is positive & ∆S is negative
Reactions of this type are not spontaneously feasible and have to be driven.
- Photosynthesis 6CO2(g) + 6H2O(l) –> C6H12O6(s) + 6O2(g)
Scenario 3: ∆H & ∆S both negative
These exothermic reactions (∆H < 0) are thermodynamically feasible at low temperature.
- Condensation & Freezing
- Addition & Combination reaction
Scenario 4: ∆H & ∆S both positive
These endothermic reactions (∆H > 0), which may not be feasible at room temperature, become feasible if the temperature is raised.
- Melting & Boiling
- Decomposition process
Limitations in the use of ∆G
o to predict spontaneity of a reaction:
1. Non-standard conditions
o can only be used to predict the spontaneity of a reaction under standard conditions. Under non-standard conditions, ∆G must be calculated.
2. Kinetics considerations
It is important to note that just because a reaction is spontaneous does not mean that it will occur at an observable rate. The term spontaneous (thermodynamics term) does not mean instantaneous (kinetics term).
While the Gibbs Free Energy change, ∆G can be used to determine the spontaneity of a reaction, it does not take into account the kinetics of the reaction i.e. rate of reaction. There might be a large energy barrier (Ea) which the reacting species have to overcome before a reaction can occur.
Some reactions are energetically feasible (also known as thermodynamically feasible) since ∆G is negative, but kinetically not feasible since it just occurs too slowly. Such reactions are spontaneous but very slow.
I hope you find the content easy for your understanding and if you have any questions, leave me a comment below. Feel free to share this blog post with your friends.
Do stay tuned to the upcoming posts as we will be looking at some GCE A-Level H2 Chemistry examination questions with regards to Enthalpy Change, Entropy Change and Gibbs Free Energy Change.
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