Question

Are the following processes exothermic or endothermic? a. the combustion of gasoline in a car engine b. water condensing on a cold pipe c. \(\mathrm{CO}_{2}(s) \longrightarrow \mathrm{CO}_{2}(g)\) d. \(\mathrm{F}_{2}(g) \longrightarrow 2 \mathrm{F}(g)\)

Short answer

a. Exothermic b. Exothermic c. Endothermic d. Endothermic

Step-by-step solution

Process a: Combustion of gasoline in a car engine

The combustion of gasoline in a car engine involves the reaction between gasoline and oxygen, producing heat and work. The heat produced is released into the surroundings, making this an exothermic process because heat is exiting the system.

Process b: Water condensing on a cold pipe

When water condenses from a gas to a liquid, it releases energy in the form of heat. The fact that the pipe is cold suggests that heat is being transferred from the water to the pipe, making this an exothermic process (heat is leaving the system).

Process c: \(\mathrm{CO}_{2}(s) \longrightarrow \mathrm{CO}_{2}(g)\)

This process represents the sublimation of solid carbon dioxide (dry ice) into gaseous carbon dioxide. Sublimation is an endothermic process because it requires the addition of energy to overcome the attractive forces between the molecules in the solid phase. Therefore, this process is endothermic (energy is entering the system).

Process d: \(\mathrm{F}_{2}(g) \longrightarrow 2 \mathrm{F}(g)\)

This process represents the dissociation of diatomic fluorine gas into two single fluorine atoms. The bond between the fluorine atoms in the diatomic molecule must be broken, and breaking bonds requires energy. Thus, this process is endothermic as energy is needed to transform the diatomic fluorine into two individual atoms (energy is entering the system).

Key concepts

Thermodynamics in Chemistry

Thermodynamics is a fundamental concept in chemistry that deals with the study of energy changes in chemical processes. It's critical to understand that during any chemical reaction, energy is either absorbed from or released into the surrounding environment. An endothermic process is one that absorbs energy, typically in the form of heat, making the surroundings feel colder. On the other hand, an exothermic process releases energy, warming up the surroundings.

The principles of thermodynamics also help us understand what drives a reaction and whether it will occur spontaneously. When you're approaching a problem, it's important to recognize the direction of energy flow to classify the reaction correctly. For example, the sublimation of dry ice into carbon dioxide gas requires energy input, making it an endothermic process. In contrast, the condensation of water is exothermic because it releases heat to the surroundings.

Energy Transfer in Reactions

Every reaction involves energy transfer between the system (the reactants and products) and the surroundings. The laws of thermodynamics teach us that energy can neither be created nor destroyed, only transferred or changed in form.

In the context of the exercises, the combustion of gasoline is a classic example of energy release. During the reaction, chemical potential energy stored in the fuel is converted to thermal energy, which can then do work or provide heat. Similarly, water condensing on a pipe liberates heat as the water molecules shift from a high-energy gaseous state to a lower-energy liquid state. Both processes are exothermic, as they release energy to the surroundings. Recognizing these energy flows is critical for understanding the nature of chemical processes.

Combustion Reactions

A combustion reaction is a type of exothermic reaction where a substance, typically a hydrocarbon like gasoline, reacts with oxygen to produce carbon dioxide, water, and substantial amounts of heat and light. These reactions are what makes our cars go and our furnaces heat our homes.

A crucial aspect of combustion is its utility in power generation. For students examining such reactions, it's essential to comprehend both the chemical process and the resulting energy transfer. Whether you’re discussing a campfire or a car engine, combustion is a quintessential example of exothermic energy transfer that's easily observed in daily life.

Phase Changes

Phase changes, such as melting, freezing, vaporization, and condensation, involve a transformation between different states of matter: solid, liquid, and gas. Each phase change entails a specific energy exchange.

Melting and vaporization are endothermic; the substances absorb energy to transition to a more disorderly state (from solid to liquid or liquid to gas, respectively). Freezing and condensation are the reverse processes, exothermic in nature, where energy is released as the substances shift to a more ordered state.

In the given exercise, the sublimation of carbon dioxide, where a solid turns directly into a gas, is another significant endothermic phase change, absorbing energy and embodying the intricate interplay between thermodynamic principles and physical transformations.