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

Analyze combustion of gasoline in a car engine

The combustion of gasoline in a car engine is a chemical reaction that occurs when gasoline is burned. This reaction produces heat and releases energy, which is used to power the car. As a result, this process is exothermic since energy is released as heat to the surroundings.

Analyze water condensing on a cold pipe

In this process, water vapor molecules lose energy when they come into contact with a cold pipe and change from the gas phase to the liquid phase. This energy loss is manifested as a release of heat, as the molecules come together and result in a lower energy state. Since energy is released to the surroundings during this process, it is an exothermic process.

Analyze \(\mathrm{CO}_{2}(s) \longrightarrow \mathrm{CO}_{2}(g)\)

The given process describes the sublimation of solid carbon dioxide (CO2) into gaseous CO2. In this process, CO2 molecules in the solid phase gain energy to break the bonds holding them together, allowing them to enter the gas phase. Because energy is absorbed from the surroundings to facilitate this phase change, the process is endothermic.

Analyze \(\mathrm{F}_{2}(g) \longrightarrow 2 \mathrm{~F}(g)\)

This process represents the dissociation of fluorine gas (F2) into individual fluorine atoms (F). To break the bond between the two fluorine atoms in the F2 molecule, energy must be absorbed from the surroundings. As a result, this process is endothermic. To summarize: a. Exothermic b. Exothermic c. Endothermic d. Endothermic

Key concepts

Chemical Reactions

Chemical reactions are processes where substances undergo changes to form new substances. During these reactions, bonds between atoms are broken and new bonds are formed. This process involves energy changes that can either release energy to the surroundings or absorb energy from them. This is where the distinction between exothermic and endothermic reactions comes into play.

• In exothermic reactions, the energy required to break bonds is less than the energy released when new bonds are formed. As a result, there is a net release of energy.
• In endothermic reactions, more energy is absorbed to break the bonds than is released in forming new ones, leading to an overall absorption of energy.

A classic example is the combustion of gasoline, which is exothermic. It releases energy that powers engines, making it an essential chemical reaction in numerous mechanical processes.

Phase Changes

Phase changes refer to transformations between different states of matter: solid, liquid, and gas. These changes can be either exothermic or endothermic based on whether energy is released or absorbed.

• Common exothermic phase changes include condensation and freezing. During these changes, molecules release energy as they transition to a more organized state.
• Endothermic phase changes, such as sublimation and melting, require energy input to allow molecules to break free from their organized structures.

For example, water condensing on a cold pipe is exothermic because energy is released as water vapor turns into liquid. On the other hand, sublimation of dry ice, turning solid CO₂ directly into gas, absorbs heat, classifying it as endothermic.

Energy Absorption and Release

The processes of energy absorption and release are central to understanding endothermic and exothermic reactions. The direction of energy flow determines whether the reaction provides energy to the environment or draws energy from it.

Exothermic Processes

Exothermic processes release energy to the surroundings, often in the form of heat. This makes the surroundings warmer.
Examples include combustion and condensation, as seen in the problem where gasoline combustion powers an engine by releasing heat.

Endothermic Processes

Endothermic processes absorb energy, causing the surroundings to feel cooler. They require energy for the process to proceed and include reactions like photosynthesis and phase changes like sublimation of CO₂, which needs heat absorption to occur.

Molecular Bond Dissociation

Molecular bond dissociation is a fundamental concept in chemistry, involving breaking bonds within molecules to form atoms or simpler molecules. Breaking bonds requires energy input because atoms in a molecule attract each other; the stronger the bond, the more energy needed. This energy requirement is what makes certain reactions endothermic.

In the example of dissociation of fluorine gas (F₂), energy must be absorbed to break the bond between the two fluorine atoms, enabling them to exist as individual atoms. This shows the endothermic nature of bond dissociation.

Understanding these concepts helps clarify why certain reactions and processes absorb or release energy, providing a deeper insight into chemical behavior.