Classify each process as exothermic or endothermic and indicate the sign of \(\Delta H_{\mathrm{rxn}}\). (a) butane gas burning in a lighter (b) the reaction that occurs in the chemical cold packs used to ice athletic injuries (c) the burning of wax in a candle

When discussing chemical reactions, it's critical to understand the concept of exothermic reactions. These are reactions that release heat, causing the temperature of the surrounding environment to increase. They are generally characterized by a warm or hot sensation, which is a telltale sign that an exothermic reaction is occurring. Examples of exothermic reactions include combustion, such as the burning of butane gas in a lighter or wax in a candle.

From a chemical thermodynamics perspective, the energy released usually comes from the breaking and forming of bonds. During an exothermic reaction, more energy is released when product bonds form than is consumed when reactant bonds break. This surplus of energy is then emitted as heat. Mathematically, we express the energy change of a reaction with the enthalpy change, symbolized as ΔH_{mathrm{rxn}}. For exothermic reactions, this value is negative because the system loses heat to its surroundings.

In contrast to exothermic reactions, endothermic reactions absorb heat from their surroundings, often resulting in a cooling effect. This type of reaction requires energy to proceed, which is taken from the environment, often feeling cold to the touch. A practical example of an endothermic reaction is the process within chemical cold packs used to treat athletic injuries.

These reactions require energy for the reactant bonds to break, more than the energy released when the product bonds form. The overall energy absorbed is reflected as a positive value in the enthalpy change ΔH_{mathrm{rxn}}. This value is fundamental in understanding whether heat was absorbed or released during a reaction.

Chemical thermodynamics is the study of the interrelation of heat with chemical reactions or with physical changes of state within the confines of the laws of thermodynamics. This branch of science deals primarily with energy changes, particularly the energy exchange between a system and its surroundings. One key concept in this study is the conservation of energy, which states that in an isolated system, energy can be transformed from one type to another but cannot be created or destroyed.

Within this framework, the concepts of endothermic and exothermic reactions play a significant role. By understanding how energy is exchanged, chemists can predict reaction behavior, manipulate conditions to steer reactions, and harness energy changes for practical applications, like power generation or creating refrigerants.

Enthalpy, represented as ΔH_{mathrm{rxn}}, is a measure of the total heat content in a chemical system, including both internal energy and the energy required to make room for it by displacing its environment. Enthalpy change is the difference in enthalpy of a system from the start of a reaction to the end. It's a central part in chemical thermodynamics as it helps predict whether a process will release or absorb heat.

In the context of exothermic and endothermic reactions, enthalpy change provides quantitative insight: a negative ΔH_{mathrm{rxn}} indicates an exothermic reaction, releasing heat, and a positive ΔH_{mathrm{rxn}} signals an endothermic reaction, absorbing heat. Understanding this concept helps scientists and engineers calculate the energy needs or outputs of various chemical processes, enabling efficient industrial and laboratory operations.