Determine whether each process is exothermic or endothermic and indicate the sign of \(\Delta H\). a. dry ice evaporating b. a sparkler burning c. the reaction that occurs in a chemical cold pack used to ice athletic injuries

An endothermic process requires the absorption of energy from the surroundings to proceed. For example, the evaporation of dry ice to form carbon dioxide gas is a typical endothermic reaction. In this process, heat energy is taken from the surrounding environment to overcome the bonds holding the molecules in solid form, allowing them to disperse as gas. During such transformations, it is common to observe a decrease in temperature around the system where the reaction is taking place.

Therefore, it's of paramount importance for students to understand that in an endothermic process, the energy flow is into the system, not out. When we consider the enthalpy change, or \( \Delta H \), this is positive in endothermic reactions indicating a gain in energy within the system. The application of this concept can be seen in items like chemical cold packs, which absorb thermal energy, thus providing a cooling effect when activated.

In contrast to endothermic processes, exothermic processes release energy, typically in the form of heat or light, into the surrounding environment. The burning of a sparkler can be used as a classic example of an exothermic reaction. As the sparkler combusts, it converts chemical potential energy from the reactants into light and thermal energy, which is then emitted outwards, often raising the temperature of the nearby area.

Students should comprehend that exothermic processes are characterized by the release of energy, resulting in a negative enthalpy change, \( \Delta H \), which signifies that the system has lost energy. Exothermic reactions are integral in everyday life, supporting phenomena such as combustion engines, warming hand packs, and even in biological processes like cellular respiration.

Enthalpy change, denoted as \( \Delta H \), quantifies the heat exchange at a constant pressure during a chemical or physical transformation within a system. The positive or negative sign of \( \Delta H \), indicates whether a process is endothermic or exothermic respectively. A positive value shows that the system absorbed heat, whereas a negative value indicates it released heat.

Understanding \( \Delta H \), is essential for grasping how different reactions require or release energy, affecting the overall energy balance in various systems. It's also noteworthy that enthalpy change is a state function, which means it depends only on the initial and final states of a system, not on the path taken to get from one to the other. This concept is pivotal in thermochemistry and allows students to theoretically predict the thermal outcome of reactions.

Differentiating between physical and chemical changes is crucial in understanding various processes in chemistry. A physical change involves a change in the form of matter but not in its chemical identity, such as ice melting into water. On the contrary, a chemical change results in new substances with different properties by rearranging atoms through chemical reactions.

For instance, the evaporation of dry ice is a physical change where solid carbon dioxide turns into its gaseous form, whereas the burning of a sparkler constitutes a chemical change where the sparkler’s composition chemically reacts with oxygen to form new substances. Recognizing this distinction helps students to properly classify processes and predict energy changes within those processes according to the principles of thermochemistry.