Recall that \(\Delta H\) is positive for the melting of ice. In general, does \(\Delta \mathrm{H}>0\) tend to be the case for naturally occurring processes?

Imagine a cold block of ice on a warm day. Gradually, the ice absorbs heat from the surroundings and transitions to water. This is a classic example of an endothermic reaction, a process that requires an input of energy, specifically heat, from its environment. In scientific terms, we describe this phenomenon by stating that the enthalpy change ((Delta H)) is positive, as the system's internal energy increases.

• The melting of ice, evaporation of water, and photosynthesis in plants are all endothermic reactions.
• Endothermicity isn't an indicator of a reaction being spontaneous or non-spontaneous.
• Many endothermic reactions require an external source of energy, like heat or light, to proceed.

Endothermic processes are vital in various industrial applications such as endothermic gas generators used in heat treatment processes. These reactions provide foundational understanding in both the classroom and lab, where students observe heat absorption and its effects on chemical reactions.

A spontaneous process is like water flowing downhill; it just naturally occurs given the opportunity. There's no need to push it; gravity does all the work. Similarly, in chemistry, a spontaneous reaction or process takes place without external intervention and has a tendency to occur under specific conditions.

• Rusting of iron and the diffusion of perfume in a room are examples of spontaneous processes.
• They do not necessarily happen quickly; some spontaneous processes, such as diamond turning to graphite, are so slow they seem static.
• The concept of spontaneity is not directly tied to the energy changes involved in a process.

A spontaneous process can have either a positive or negative enthalpy change, but what decisively determines spontaneity is the concept of Gibbs free energy, which we'll explore next.

The Gibbs free energy ((Delta G)), named after the scientist Josiah Willard Gibbs, is the supreme judge in the realm of spontaneous reactions. It's a thermodynamic quantity that combines the system's enthalpy ((Delta H)) and entropy ((Delta S)) changes to predict the favorability of a process at constant pressure and temperature.

• When (Delta G) is negative, the process is spontaneous, indicating that it's energetically favorable.
• A positive (Delta G) suggests a non-spontaneous process that won't occur on its own at the given conditions.
• (Delta G) is affected by temperature, and under certain conditions, can shift a reaction from non-spontaneous to spontaneous or vice versa.

The equation (Delta G = Delta H - TDelta S)), where T is the temperature in Kelvins, elegantly demonstrates the interdependent relationship between these variables. Understanding Gibbs free energy unlocks the ability to predict the 'will it, won't it' of chemical reactions and processes, making it a cornerstone concept in understanding the spontaneity of reactions.