Problem 94 - Chemistry Matter and Change

Question

Apply A solid being heated stays at a constant temperature until it is completely melted. What happens to the heat energy put into the system during that time?

Step-by-Step Solution

Answer

The heat energy is used to break intermolecular bonds during the melting process, known as the latent heat of fusion.

Step 1: Understand the Latent Heat Concept

When heat is added to a solid at its melting point, it transitions from solid to liquid without increasing in temperature. This is because the heat energy is used to change the state of the substance.

Step 2: Recognize the Phase Change

During the phase change from solid to liquid, the heat energy goes into breaking the intermolecular forces rather than increasing kinetic energy, hence no temperature rise is observed.

Step 3: Relate to Latent Heat of Fusion

The heat input at constant temperature during melting is called the latent heat of fusion. It quantifies the energy required to change a given amount of solid into liquid without changing its temperature.

Step 4: Calculate Heat Energy if Needed

To find the heat energy used during the process, you can use the formula: \[ Q = m \cdot L_f \]where \( Q \) is the heat energy, \( m \) is the mass of the substance, and \( L_f \) is the latent heat of fusion.

Key Concepts

Phase Change

A phase change occurs when a substance transitions from one state of matter to another—such as solid to liquid, liquid to gas, or vice versa. These changes happen because of energy variations within the system. During a transition from solid to liquid, such as melting, the material absorbs heat. However, instead of this energy raising the substance's temperature, it goes into changing its phase. This process is why a solid being heated remains at a constant temperature until it has fully turned into liquid. The absorbed heat breaks down the structured arrangement of particles in a solid, leading to a more disordered, fluid form—without a temperature rise. The absence of temperature change during such transformations is crucial, as it underscores the energy's role in altering the material's state rather than its thermal motion. This specific period where only the phase changes is pivotal in thermodynamics and can be observed in substances like ice melting into water.

Latent Heat of Fusion

Latent heat of fusion is the specific amount of energy required to change a unit mass of a solid into a liquid at constant temperature. This energy is essential as it helps overcome the intermolecular forces holding the particles together in a solid. When you add heat to ice at 0 degrees Celsius, for example, it begins to absorb the latent heat of fusion. The ice doesn't get warmer; instead, it uses this energy to change into water. This systematic absorption of energy without a change in temperature is a classic demonstration of latent heat. The formula used for this is: \[ Q = m \cdot L_f \]where \( Q \) is the heat energy, \( m \) is the mass of the substance, and \( L_f \) represents the latent heat of fusion. Each substance has its unique latent heat of fusion value, indicating it's a specific physical property tied to the material's chemical nature.

Intermolecular Forces

Intermolecular forces are the attractions that exist between particles in a substance. These forces play a pivotal role in determining the physical state of a material at a given temperature and pressure. In a solid, particles are held closely by strong intermolecular forces, leading to a fixed, defined shape. As heat is introduced into the system, like during melting, these forces begin to weaken as they absorb energy. This energy undermines the forces, allowing particles to move more freely, transitioning the matter into a liquid state. Different substances exhibit varying strengths of intermolecular forces. Those with stronger forces typically have higher melting points, requiring more energy for a phase transition. Understanding these forces gives insight into why different materials behave distinctively under thermal conditions, influencing their melting and boiling processes.