5.6.1. Is Decrease in Enthalpy a Criterion for Spontaneity ?

If we examine the phenomenon like flow of water down hill or fall of a stone on to the ground, we find that there is a net decrease in potential energy in the direction of change. By analogy, we may be tempted to state that a chemical reaction is spontaneous in a given direction, because decrease in energy has taken place, as in the case of exothermic reactions. For example: 1/2 N2(g) + H2(g) = NH3(g); ∆r H  = – 46.1 kJ mol–1 1/2 H2(g) + 1/2 Cl2(g) = HCl (g); ∆r H  = – 92.32 kJ mol–1 H2(g) + 1/2 O2(g) → H2O(l) ; ∆r H  = –285.8 kJ mol–1.

This chunk discusses the concept of spontaneity in chemical reactions and draws an analogy between natural physical processes and chemical reactions. Spontaneous reactions are those that occur without additional energy input, similar to how water flows downhill due to gravity, resulting in a decrease of potential energy. The examples provided showcase exothermic reactions where the enthalpy change (∆H) is negative, indicating a release of energy, reinforcing the idea that a decrease in energy might correlate with spontaneous reactions.

Imagine rolling a ball down a hill; it moves spontaneously due to gravity without any push. Similarly, in chemical reactions like combustion, reactants such as gasoline or wood release energy (in the form of heat) as they transform into products, making the process spontaneous. This resemblance between gravity's role in physical movement and the energy change in exothermic chemical reactions helps us grasp why certain reactions occur naturally and others do not.

Now let us examine the following reactions: 1/2 N2(g) + O2(g) → NO2(g); ∆r H  = +33.2 kJ mol–1 C(graphite, s) + 2 S(l) → CS2(l); ∆r H  = +128.5 kJ mol–1 These reactions though endothermic, are spontaneous.

In this chunk, we observe that certain spontaneous reactions have positive enthalpy changes, meaning they absorb energy. This contradicts the idea that decreased enthalpy is always necessary for spontaneity. The reactions listed illustrate that despite needing heat to proceed, they can still occur spontaneously under the right conditions. This reveals the complexity in predicting spontaneity based solely on enthalpy.

Consider a cold pack used for injuries. When activated, it absorbs heat from the environment, making its surroundings cooler, but this process happens spontaneously. Just as the pack absorbs energy, these endothermic reactions absorb heat, yet they drive the reaction forward, much like how we can willingly jump into a pool despite the chill, because the experience is refreshing regardless of the cold water.

Then, what drives the spontaneous process in a given direction? Let us examine such a case in which ∆ H = 0 i.e., there is no change in enthalpy, but still the process is spontaneous. Let us consider diffusion of two gases into each other in a closed container. ... This leads us to entropy, denoted as S.

The chunk transitions to discussing entropy (S) as a key concept in understanding spontaneity. Even when enthalpy does not change, processes can still occur spontaneously, as evidenced in the diffusion of gases. Here, two gases mixing represent a move towards a more disordered state, which aligns with the second law of thermodynamics, stating that the total entropy of an isolated system tends to increase.

Think about how perfume diffuses in a room. Initially, the scent is concentrated in one area, but over time, the fragrance spreads throughout the space. The entropic idea here is that the perfume's molecules are transitioning from an ordered state (concentrated) to a more disordered state (evenly dispersed), a process that occurs spontaneously without any push, similar to spontaneous chemical reactions where energy disperses and order increases.

Ultimately, it becomes obvious that while decrease in enthalpy may be a contributory factor for spontaneity, but it is not true for all cases.

This final chunk summarizes the essence of spontaneous processes, asserting that while a decrease in enthalpy can contribute to spontaneity, it is not a definitive criterion across all reactions. The relationship between enthalpy, entropy, and spontaneity requires a nuanced understanding, emphasizing that entropy plays a significant role in these processes.

To cement this idea, consider a hot cup of coffee left on a counter. It will cool down (decrease in energy) and become mixed with the cooler air around it, illustrating enthalpy change. However, the coffee cooling and air mixing do not always define if a reaction or process will happen spontaneously, much like how ice melts in a warm room without external force, where heat flow and entropy contributions dictate the direction of change.

--