Free Energy and Reaction Types: Thermodynamics Explained
Free Energy and Types of Reactions
We can consider four types of reactions:
-
Exothermic Reactions (ΔH < 0) with increasing disorder (ΔS > 0) for example:
2H2O(l) -> 2H2(g) + O2(g)
The terms ΔH and -TΔS are both negative, so that ΔG is always negative at all temperatures. Consequently, these reactions are always spontaneous. -
Endothermic Reactions (ΔH > 0) with increasing disorder (ΔS > 0) for example:
SO3(g) -> SO2(g) + 1/2 O2(g)
The spontaneity of these reactions is favored by the entropic term (-TΔS < 0) but disfavored by the enthalpy term (ΔH > 0). These reactions are spontaneous at temperatures high enough so that the negative contribution of the term -TΔS dominates over the contribution of ΔH. -
Exothermic Reactions (ΔH < 0) with increasing order (ΔS < 0) for example:
C(s) + 2H2(g) -> CH4(g)
Now the enthalpy term (ΔH < 0) favors spontaneity, while the entropic term (-TΔS > 0) disfavors it. Therefore, these reactions are spontaneous at temperatures low enough so that the negative contribution of ΔH in the expression of ΔG dominates over the positive contribution of the term -TΔS. -
Endothermic Reactions (ΔH > 0) with an increase of order (ΔS < 0) for example:
CO(g) -> C(s) + 1/2 O2(g)
The terms enthalpy (ΔH > 0) and entropic (-TΔS > 0) contribute positively to the value of ΔG. Therefore, these reactions are not spontaneous at all temperatures, as always ΔG > 0.
Thermodynamic Systems
- System: It is a material part of the universe that is isolated from the rest to observe. For example, a test tube with a chemical precipitation.
- Environment: The rest of the universe that may or may not be related to the system.
Work
A mechanism of energy transfer between bodies based on the use of force. In thermodynamics, work relates to the expansion or compression of a gas in a closed cylinder with a movable piston.
1st Principle of Thermodynamics
It is based on the law of conservation of energy, which states that energy in an isolated system is conserved, i.e., energy can neither be created nor destroyed, only transformed. ΔU = Q + W
Equilibrium Temperature
Teq = ΔH / ΔS
Enthalpy Diagram
These diagrams represent the enthalpy changes that occur during a chemical reaction.
- Exothermic Reactions: The enthalpy of the reactants is greater than that of the products.
- Endothermic Reactions: The enthalpy of the reactants is less than that of the products.
3rd Principle of Thermodynamics
The entropy of a perfectly ordered crystalline solid at 0 K (absolute zero) is 0 (highest order). Solid crystalline -> T = 0 K = -273 °C
Accordingly, all entropies are positive.
Hess’s Law
As enthalpy is a state function, the enthalpy change of any process depends only on the initial and final states but not the thermodynamic path followed, i.e., the intermediate steps. When we apply this general rule to chemical reactions, we obtain Hess’s Law: “The reaction enthalpy depends only on the initial and final substances, and its value is the same whether the process occurs in one or more steps.”