Question

Indicate the direction of heat transfer between the system and the surroundings, classify the following processes as endo- or exothermic, and give the sign of \(\Delta H^{\circ}\). (a) Evaporating rubbing alcohol from your skin. (b) Solidifying molten gold into a gold bar. (c) Melting solid gallium metal with the heat from your hand.

Short answer

(a) Endothermic, \( \Delta H^{\circ} > 0 \); (b) Exothermic, \( \Delta H^{\circ} < 0 \); (c) Endothermic, \( \Delta H^{\circ} > 0 \).

Step-by-step solution

Analyze Process (a)

Evaporating rubbing alcohol from your skin involves the alcohol transitioning from a liquid to a gas. This process requires the absorption of heat from the surroundings (your skin) to enable the phase change. Therefore, the direction of heat transfer is from the skin (surroundings) to the rubbing alcohol (system). This indicates an endothermic process, as energy is absorbed. Consequently, the sign of \( \Delta H^{\circ} \) is positive (+).

Analyze Process (b)

Solidifying molten gold into a bar involves the transition from a liquid to a solid. During solidification, heat is released into the surroundings as the gold loses energy while forming solid bonds. Thus, the heat transfer direction is from the system (molten gold) to the surroundings. This makes it an exothermic process, with \( \Delta H^{\circ} \) being negative (-).

Analyze Process (c)

Melting solid gallium metal with the heat from your hand means the gallium is absorbing heat to transition from a solid to a liquid. The heat source is your hand, making the direction of heat transfer from the surroundings (your hand) to the system (gallium). This is an endothermic process, characterized by a positive \( \Delta H^{\circ} \).

Key concepts

Heat Transfer in Chemistry

Heat transfer in chemistry refers to how thermal energy moves from one object or substance to another. This process is essential for understanding many chemical reactions and processes.
Different types of heat transfer can occur, including:

• Conduction: Direct heat flow through a substance or between substances in contact with each other.
• Convection: Movement of heat through fluids (liquids and gases) due to the movement of the fluid itself.
• Radiation: Transfer of heat through electromagnetic waves without involving direct contact or fluid movement.

In chemistry, tracking heat transfer helps determine whether a process is endothermic or exothermic. Heat transfer's direction and magnitude influence the reactants and products, impacting chemical equilibrium and reaction rates. For example, if a system absorbs heat from its surroundings, it becomes an endothermic process. Conversely, if the system releases heat to the surroundings, it is exothermic. Understanding these concepts is vital for predicting the behavior of chemical reactions.

Phase Changes

Phase changes involve transitions between different states of matter: solid, liquid, and gas. These changes occur due to energy, typically in the form of heat, added to or removed from a substance.
Common phase changes include:

• Melting: Transition from solid to liquid when heat is absorbed.
• Freezing: Transition from liquid to solid when heat is released.
• Evaporation: Transition from liquid to gas, involving heat absorption.
• Condensation: Transition from gas to liquid, involving heat release.
• Sublimation: Direct change from solid to gas, skipping the liquid phase, also requiring heat absorption.
• Deposition: Direct change from gas to solid, releasing heat.

Each of these changes occurs at a specific temperature and pressure unique to each substance. During a phase change, energy is used to alter the structure of the substance, but not its temperature, until the change is complete. This is why melting ice does not increase in temperature until all of it liquifies. Recognizing phase changes explains many natural and industrial processes, such as weather patterns and refrigeration.

Enthalpy Change

Enthalpy change, represented by the symbol \( \Delta H \), is a measure of the total heat content in a thermodynamic system that is available or absorbed during a process. It is a key concept for determining whether a reaction is endothermic or exothermic.
An positive \( \Delta H^{\circ} \) indicates that the process requires energy input from the surroundings, characteristic of endothermic processes. Examples include:

• Evaporation: As seen with the evaporation of rubbing alcohol, the system absorbs heat, making \( \Delta H^{\circ} \) positive.
• Melting: Melting gallium in your hand involves absorbing heat, resulting in a positive \( \Delta H^{\circ} \).

Whereas, a negative \( \Delta H^{\circ} \) signifies that heat is released to the surroundings, typical for exothermic reactions. For instance:

• Solidification: When molten gold solidifies, it releases heat, making \( \Delta H^{\circ} \) negative.

Calculating enthalpy change is crucial in predicting the energy changes occurring in chemical reactions. Understanding \( \Delta H \) helps chemists design and control reactions to maximize the desired energy exchange, ensuring better efficiency and sustainability in chemical processes.