Question

Classify each process as exothermic or endothermic. (a) gasoline burning in a car (b) isopropyl alcohol evaporating from skin (c) water condensing as dew during the night

Short answer

Process (a) is exothermic. Process (b) is endothermic. Process (c) is exothermic.

Step-by-step solution

Identify the Nature of Process (a)

The process of gasoline burning in a car is characterized by the release of heat energy as the gasoline combusts. Since energy is released to the surroundings, this process is exothermic.

Identify the Nature of Process (b)

The process where isopropyl alcohol evaporates from the skin requires the absorption of heat energy to transition from a liquid to a gas. Thus, this is an endothermic process.

Identify the Nature of Process (c)

Water condensing as dew during the night releases heat to the surroundings as it changes from a gas to a liquid state. Consequently, this process is exothermic.

Key concepts

Thermochemistry

Thermochemistry is the study of the heat energy involved in chemical reactions and phase changes. Understanding the flow of heat during these processes is crucial in classifying them as exothermic or endothermic.

In an exothermic process, the system releases heat to the surroundings. This release often occurs because the total energy of the products is less than that of the reactants, meaning that energy is given off during the reaction, as seen when gasoline combusts in a car engine. The energy is often released in the form of heat or light, making the surroundings warmer or brighter.

On the other hand, an endothermic process requires the absorption of heat from the surroundings, leading to a decrease in temperature around the system. When isopropyl alcohol evaporates, it absorbs the necessary heat energy to overcome the intermolecular forces that hold the liquid molecules together, enabling them to disperse as a gas.

Understanding these concepts is not just about labeling a reaction, but also about comprehending the energy transformation that occurs - and thermochemistry provides us with the framework to analyze and predict these energetic changes.

Phase Transitions

Phase transitions are the transformations from one state of matter to another, such as solid to liquid, liquid to gas, or vice versa. These transitions come with enthalpy changes, which can be either absorbed or released depending on the nature of the transition.

When a substance like isopropyl alcohol on our skin evaporates, it undergoes a phase transition from liquid to gas. This transition requires absorbing energy, to allow the molecules to spread out and move freely as a gas - indicative of an endothermic transition.

Conversely, during the night when water condenses to form dew, the transition from gas to liquid releases heat to the surroundings, because the gas molecules lose energy and move closer together. This is an excellent example of an exothermic phase transition. Each phase change involves a fundamental rearrangement of molecules and a significant energy exchange, key factors in understanding chemical and physical behaviors.

Energy in Chemical Reactions

The role of energy in chemical reactions is central to our understanding of how substances interact and transform. The burning of gasoline in a car engine illustrates how chemical potential energy stored within the molecular bonds of a substance can be released as kinetic energy - in this case, heat and light - during a chemical reaction.

Each chemical bond has a certain amount of energy associated with it, and the restructuring of these bonds during a reaction leads to either the release or absorption of energy. Exothermic reactions, like the combustion of gasoline, release more energy than is required to break the bonds of the reactants, leading to an overall release of energy.

In contrast, endothermic reactions, such as the evaporation of a liquid, require an input of energy to proceed. This energy is used to overcome the existing bonds and allow the formation of new ones, often resulting in products with higher potential energy than the reactants. This integral exchange of energy is what drives chemical reactions and is a key concept in the study of chemistry.