Problem 98 - Chemistry and Chemical Reactivity

Question

Define "bond dissociation energy." Does the enthalpy change for a bond- breaking reaction [e.g., \(\mathrm{C}-\mathrm{H}(\mathrm{g}) \longrightarrow \mathrm{C}(\mathrm{g})+\mathrm{H}(\mathrm{g})]\) always have a positive sign, always have a negative sign, or vary? Explain briefly.

Step-by-Step Solution

Answer

The enthalpy change for a bond-breaking reaction is always positive, as energy is absorbed.

Step 1: Understanding Bond Dissociation Energy

Bond dissociation energy is defined as the amount of energy required to break a specific chemical bond in a molecule in the gaseous state, producing separate atoms or radicals. This energy is usually expressed in kilojoules per mole (kJ/mol).

Step 2: Analyzing the Reaction Enthalpy

In a bond-breaking reaction such as \(\mathrm{C}-\mathrm{H}(\mathrm{g}) \longrightarrow \mathrm{C}(\mathrm{g})+\mathrm{H}(\mathrm{g})\), energy is absorbed to break the bond between the carbon and hydrogen atoms. This implies that the process of breaking a chemical bond results in the intake of energy from the surroundings.

Step 3: Determining the Sign of Enthalpy Change

Because energy is consumed during the bond-breaking process, the enthalpy change \(\Delta H\) for such a reaction is positive. In thermodynamic terms, an input of energy or an endothermic process is represented by a positive \(\Delta H\).

Step 4: Conclusion on Enthalpy Signage

For any bond-breaking reaction, such as the one involving \(\mathrm{C}-\mathrm{H}\), the enthalpy change will always have a positive sign. This is because breaking bonds requires an input of energy.

Key Concepts

Understanding Enthalpy Change

Enthalpy change is a key concept in chemistry that measures the heat content change in a system during a reaction. It is represented by the symbol \( \Delta H \). It reveals whether a reaction absorbs or releases energy.
While reactions proceed, reactants are converted into products, and this transformation involves energy changes. The enthalpy change tells us how much energy is needed or released during this process, as it depends on the breaking of bonds and forming new ones.

• For reactions like a bond-breaking reaction, it requires energy to be taken in from the surroundings.
• The amount of energy can be calculated in terms of enthalpy change.

This is crucial because if \( \Delta H \) is positive, the reaction is endothermic, meaning it absorbs energy from its environment. If \( \Delta H \) is negative, the reaction is exothermic, releasing energy into its surroundings.

Bond-Breaking Reaction

A bond-breaking reaction involves the splitting of chemical bonds between atoms. This is an essential part of many chemical reactions.
When a bond like the carbon-hydrogen bond is broken, as in the equation: \[ \mathrm{C}-\mathrm{H}(\mathrm{g}) \rightarrow \mathrm{C}(\mathrm{g}) + \mathrm{H}(\mathrm{g}), \]it requires a specific amount of energy known as bond dissociation energy.
Here, the input of energy is necessary to overcome the forces holding the atoms together.

• Bond-breaking is often analyzed in isolated gaseous molecules.
• The reaction process involves absorbing energy, which disrupts the bond and results in separate atoms.

This energy absorbed is reflected by the positive enthalpy change, as it indicates an endothermic process.

Understanding Endothermic Processes

Endothermic processes are those that absorb energy from their surroundings, leading to a positive enthalpy change. This is typical of reactions where bonds are broken, like the bond-breaking reactions discussed earlier.
In these scenarios, the system gains heat from its environment, resulting in an increase in internal energy. This absorption of energy is essential for overcoming the potential energy barrier needed to break bonds.

• Endothermic reactions are identified by a positive \( \Delta H \).
• Common examples include dissolving certain salts in water and photosynthesis.

Understanding the nature of endothermic processes helps in predicting the behavior of chemical reactions and the energy requirements involved.