Question

Explain why the bond enthalpy of a molecule is usually defined in terms of a gas-phase reaction. Why are bondbreaking processes always endothermic and bondforming processes always exothermic?

Short answer

Bond enthalpy is defined in the gas phase for accuracy; bond breaking is endothermic as it requires energy, while bond formation is exothermic as it releases energy.

Step-by-step solution

Understanding Bond Enthalpy

Bond enthalpy, also known as bond dissociation energy, is the energy required to break a bond in a molecule into its individual atoms when it is in the gas-phase. It's defined in the gas phase to ensure that intermolecular forces, such as hydrogen bonding, do not influence the energy measurement. In the gaseous state, molecules are sufficiently separated, allowing for more accurate measurement of the energy involved in breaking the bonds.

Energy Requirement of Bond Breaking

Bond breaking is always endothermic because it requires energy input to overcome the attraction between the atoms that are bonded together. Energy must be supplied to the system in order to break these stable interactions and separate the bonded atoms.

Energy Release in Bond Formation

Bond formation is exothermic because energy is released when new bonds are formed. When atoms bond together, they enter a more stable, lower-energy state, releasing the excess energy in the process. This energy release is a fundamental aspect of chemical reactions, where new bonds form stronger or more stable configurations.

Key concepts

Gas-Phase Reaction

In chemistry, reactions involving the breaking or forming of bonds are often studied in the gas phase. The main reason for this is to avoid the impact of intermolecular forces, such as those seen in liquids and solids.

These forces can alter the amount of energy required or released during a reaction. In the gaseous state, molecules are far apart. This means they interact less with each other compared to when they're in a condensed state.

By examining reactions in the gas phase, chemists can get a clearer understanding of the intrinsic bond energies. These energies are considered pure, as they're not influenced by additional intermolecular forces.

• Allows measurement without interference of external forces.
• Provides more accurate bond energy values.
• Reflects the true energy changes in reactions.

Endothermic Process

An endothermic process is one where energy is absorbed from the surroundings. In the context of bond enthalpy, breaking a bond is always endothermic.

Imagine a situation where two atoms are happily bonded together; separating them requires energy. This energy is needed to overcome the attractive forces holding them together.

As a result, energy enters the system and usually leads to an increase in enthalpy or heat content.

• Energy input is essential to break stable bonds.
• Leads to an increase in system enthalpy.
• Always absorbs surrounding energy.

Exothermic Process

Contrary to an endothermic process, an exothermic process occurs when energy is released. This happens during bond formation.

When two atoms combine to form a bond, they shift from a higher energy state to a lower, more stable one. This shift releases energy into the surroundings.

The release of energy makes the products less energic and more stable compared to the reactants—forming the base for many reactions in chemistry.

• Energy is released to surroundings.
• Results in a lower energy, more stable state.
• Drives many natural and industrial reactions.

Bond Dissociation Energy

Bond dissociation energy is a concept that quantifies the strength of a chemical bond. It represents the amount of energy needed to break a bond into separate atoms within a molecule.

Determining this energy provides insights into the stability of a molecule. Usually, the greater the bond dissociation energy, the stronger and more stable the bond.

It is crucial for predicting the behavior of a molecule during chemical reactions.

• Measures strength of chemical bonds.
• Vital for understanding reaction mechanisms.
• Essential in energy-related calculations in reactions.