Problem 34 - Chemistry The Science in Context

Question

Which symbol, \(\Delta H_{\mathrm{f}}\) or \(\Delta H_{\text {fus }},\) refers to a chemical change?

Step-by-Step Solution

Answer

Answer: \(\Delta H_{\mathrm{f}}\)

Step 1: Understand the meaning of the symbols: \(\Delta H_{\mathrm{f}}\) and \(\Delta H_{\text {fus }}\)

The symbol \(\Delta H_{\mathrm{f}}\) represents the enthalpy change of formation, which is the change in enthalpy that occurs when one mole of a compound is formed from its constituent elements in their reference state (most stable form) at a specified temperature and pressure. The symbol \(\Delta H_{\text {fus }}\) represents the enthalpy change of fusion, which is the change in enthalpy when one mole of a solid substance melts (transforms from solid to liquid phase) at its melting point.

Step 2: Identify the symbol associated with a chemical change

A chemical change involves the formation or breaking of chemical bonds between atoms, leading to the formation of new substances with different physicochemical properties than the original substances. The enthalpy change of formation (\(\Delta H_{\mathrm{f}}\)) involves the formation of a new compound from its constituent elements, which is indeed associated with the formation of new chemical bonds. Therefore, the symbol \(\Delta H_{\mathrm{f}}\) refers to a chemical change. On the other hand, the enthalpy change of fusion (\(\Delta H_{\text {fus }}\)) involves a transition between different phases of the same substance (Solid to Liquid in this case), and thus, it's a physical change, not a chemical one.

Step 3: Conclusion

In conclusion, the symbol \(\Delta H_{\mathrm{f}}\) represents a chemical change, as it involves the formation or breaking of chemical bonds to create a new compound.

Key Concepts

Chemical Change

A chemical change occurs when substances combine or break apart to form new substances. This involves the making or breaking of chemical bonds. For instance, when hydrogen gas and oxygen gas react, they form water. Here, bonds between hydrogen and oxygen atoms are created, resulting in a new substance.

• New substances with different properties emerge.
• Chemical changes are often irreversible.
• Indicators include color change, gas production, or temperature change.

An example of a chemical change is the enthalpy change of formation (\( \Delta H_{\mathrm{f}} \)). When elements in their stable states form a compound, new bonds are made, signifying a chemical change.

Enthalpy Change of Fusion

The enthalpy change of fusion (\( \Delta H_{\text{fus }} \)) refers to the energy required to change a substance from solid to liquid at its melting point. This is a physical transformation involving phase change rather than chemical bonds breaking or forming.

• It requires heat absorption.
• No new substances are formed; the substance remains chemically the same.
• This process is reversible.

An example is melting ice. The solid water (ice) absorbs heat and becomes liquid water, but it remains H₂O throughout.

Phase Transition

A phase transition occurs when a substance changes from one state of matter to another. Common transitions include melting, freezing, and vaporization. These changes are physical, not chemical, meaning they don't alter the substance's molecular composition.

• Involves changes in physical properties.
• The substance keeps its chemical identity.
• Relies on temperature and pressure changes.

For example, during the melting phase, as described in enthalpy change of fusion, ice turns to water. Despite the change in state, the chemical structure (H₂O) remains identical.