The Gibbs energy change of a reaction can be used to assess (a) how much heat is absorbed from the surroundings; (b) how much work the system does on the surroundings; (c) the net direction in which the reaction occurs to reach equilibrium; (d) the proportion of the heat evolved in an exothermic reaction that can be converted to various forms of work.

Enthalpy change, represented by the symbol \( \Delta H \), is a measure of the total heat content change in a chemical reaction. It tells us how much energy, in the form of heat, is either absorbed from or released into the surroundings during the reaction. An exothermic reaction, where \( \Delta H < 0 \), releases heat to the surroundings, making it feel warm to the touch. On the contrary, an endothermic reaction absorbs heat, signified by \( \Delta H > 0 \), making the surroundings feel cooler.

Understanding enthalpy change is crucial because it helps predict whether a reaction will be product-favored under constant pressure. Furthermore, knowing \( \Delta H \) allows us to calculate the energy change in a reaction, which is important from the perspective of energy resources and requirements in chemical processes.

Entropy change, denoted as \( \Delta S \), is a measure of the disorder or randomness within a system. The second law of thermodynamics states that the total entropy of an isolated system always increases over time. This means that systems naturally progress from a state of order to a state of disorder.

When \( \Delta S > 0 \) there's an increase in entropy, indicating a more disordered system after the reaction. Conversely, \( \Delta S < 0 \) implies a decrease in entropy, representing a more ordered system. Entropy change plays a pivotal role in determining reaction spontaneity through the Gibbs energy equation, as a positive entropy change can drive a reaction to be spontaneous even if it's endothermic.

The spontaneity of a chemical reaction refers to whether a reaction can proceed without any external input of energy. Spontaneity is governed by the Gibbs free energy change \( \Delta G \), which incorporates both enthalpy and entropy changes according to the relationship \( \Delta G = \Delta H - T\Delta S \).

A negative \( \Delta G \) indicates a spontaneous reaction, as the process can happen on its own under certain conditions. Positive \( \Delta G \) means the reaction is non-spontaneous and requires external energy to proceed. It's essential to note that spontaneity doesn't imply the rate at which a reaction will occur—some spontaneous reactions can be incredibly slow.

An exothermic reaction is one where energy, primarily in the form of heat, is released to the surroundings. It is characterized by a negative enthalpy change \( \Delta H < 0 \). These reactions often feel hot as they transfer thermal energy to the environment. Exothermic reactions are common in everyday life, including combustion processes like burning wood or fossil fuels, and metabolic reactions in our bodies.

Moreover, exothermic reactions can be spontaneous when also accompanied by an increase in entropy \( \Delta S > 0 \), leading to \( \Delta G \) being negative. However, the ability to do work, such as electrical or mechanical work, depends on both the heat released and the change in entropy during the reaction.