Problem 1 - Chemistry A Guided Inquiry

Question

Explain why \(\Delta H\) is positive for the melting of ice.

Step-by-Step Solution

Answer

The \(\Delta H\) for the melting of ice is positive because it's an endothermic process. During the transition from solid (ice) to liquid (water), heat energy is absorbed from the surroundings to overcome the intermolecular forces of attraction, leading to a positive change in enthalpy.

Step 1: Understanding Phase Transition

Melting is a type of phase transition where a substance changes from a solid phase to a liquid phase. It is necessary to understand that during phase transitions, the arrangement and movement of particles change.

Step 2: Ice to Water Transformation

In the specific case of ice melting, the ice in the solid state has its water molecules arranged in a highly ordered, rigid structure with hydrogen bonds holding them together. For it to melt into liquid water, enough heat energy must be provided to break these bonds.

Step 3: Enthalpy Change Explanation

This heat absorption causes a positive change in the system's enthalpy (\(\Delta H\)). The energy taken in is used to overcome the forces of attraction between the molecules, enabling them to move more freely in the liquid state. Thus, the \(\Delta H\) for the melting of ice is positive because heat energy needs to be absorbed from the surroundings to transform the solid into a liquid.

Key Concepts

Phase Transition

Phase transitions involve the transformation of a substance from one state of matter to another. In the case of melting, a solid becomes a liquid. This change requires an understanding of particle movement and arrangement.
When a substance undergoes a phase transition, the particles experience a change in their energy levels. In a solid, particles are tightly packed with limited movement. However, during a phase transition such as melting, these particles need energy to loosen up and move more freely.
This transition does not change the chemical composition of the substance. It only alters how particles are organized and how they interact with each other.

Melting of Ice

The melting of ice is a classic example of phase transition, where solid ice becomes liquid water. In its solid form, water molecules are structured in an orderly way due to hydrogen bonds.
These hydrogen bonds are relatively strong, holding the molecules in place. To convert into liquid water, these bonds must be partially broken.

• As heat is introduced, molecules gain enough energy to overcome these interactions.
• This allows the molecules to slide past each other, creating the liquid form of water.

This process is essential for various natural and industrial processes. Understanding how ice melts helps grasp fundamental principles in thermodynamics and material science.

Heat Absorption

Heat absorption plays a vital role in melting. When ice absorbs heat, it takes in energy from its environment. This energy is crucial as it breaks the bonds holding the molecules in their solid structure.
The process of absorbing heat leads to a positive change in enthalpy, represented by \(\Delta H\\). For the melting of ice, \(\Delta H\) is positive because energy is required for the transition from solid to liquid.

• This energy absorption makes the enthalpy change a key marker of endothermic reactions like melting.
• Without sufficient heat absorption, ice cannot transition to liquid.

Recognizing the importance of this absorption helps in understanding broader concepts like energy transfer in physical and chemical processes.