Question

As a metal such as lead melts, what happens to (a) the average kinetic energy of the atoms, (b) the average distance between the atoms?

Short answer

As lead melts: (a) The average kinetic energy of the atoms increases due to increased vibrations from the temperature increase. (b) The average distance between the atoms increases as they move more freely in the liquid state compared to the solid state.

Step-by-step solution

Understand the solid state of the metal

Before the metal melts, it is in a solid state. In this state, the atoms are tightly packed and organized in a crystalline structure. They vibrate in fixed positions and have limited freedom of movement.

Understand the process of melting

Melting is the process of transitioning from the solid state to the liquid state due to the increase of temperature. As temperature increases, the atoms in the solid absorb energy and start vibrating faster. At the melting point, the vibrations become strong enough to break the bonds holding the atoms together in the crystalline structure, allowing them to move more freely and transition into the liquid state.

Analyze the changes in average kinetic energy during melting

(a) In the melting process, the atoms gain energy due to the increase in temperature. This causes their vibrations to become faster, leading to an increase in their average kinetic energy. During this phase transition, it's important to notice that the temperature remains constant, meaning that the energy absorbed by the atoms goes into breaking the intermolecular bonds instead of further increasing the kinetic energy. Once the metal has completely melted, any further increase in temperature would certainly lead to an increase in average kinetic energy.

Analyze the changes in average distance between atoms during melting

(b) As the metal melts, the organized crystalline structure is broken, allowing the atoms to move more freely. As a result, the average distance between the atoms increases. In the liquid state, the atoms are still close to each other but have more freedom of movement, causing them to be less ordered than in the solid state. This means that the average distance between the atoms is larger in the liquid state compared to the solid state. In conclusion, as a metal such as lead melts: (a) The average kinetic energy of the atoms increases due to the increased vibrations caused by the temperature increase. (b) The average distance between the atoms increases as the tightly packed crystalline structure breaks and the atoms gain more freedom of movement in the liquid state.

Key concepts

Solid to Liquid Transition

Understanding the transition of a substance from a solid to a liquid state, also known as melting, can be fascinating. The process involves heating the solid until its constituent particles have enough energy to overcome their fixed positions within a solid structure. When examining the melting of metals like lead, it's crucial to recognize that melting occurs at a specific temperature, called the melting point. At this temperature, the solid begins to break down into a liquid form, and we notice two fundamental changes:

• The particles within the substance vibrate with greater intensity as they gain thermal energy.
• These enhanced vibrations eventually overcome the forces that hold the particles in a rigid structure, leading to a more fluid arrangement.

During the melting point, the temperature of the substance remains constant as energy is absorbed to initiate the phase change, a concept best understood through the lens of latent heat.

Average Kinetic Energy

Diving into the concept of average kinetic energy, it's all about the movement of particles. In the solid phase, particles are restricted to vibrational movements within their fixed positions. As a metal heats up and approaches its melting point, the particles receive more energy and move with increased speed. This movement is a direct measure of the kinetic energy. During melting, the average kinetic energy of the atoms indeed ramps up due to the heightened vibration brought about by the uptick in thermal energy. However, a curious phenomenon to note here is that the temperature doesn’t rise during the actual phase change. This steadfast temperature hints that all the absorbed energy is being utilized to break down the rigid structure of the solid rather than boosting the particle velocity further. In simpler terms, the average kinetic energy of the particles is more during the liquid state than in the solid state but does not increase during the phase change itself.

Intermolecular Bonds

Intermolecular bonds are the forces that hold substances together. In metals, these are typically metallic bonds where electrons are free to move around positively charged ions. These bonds are strong, which is why metals have high melting points. Upon heating, the energy provided isn't immediately evident in particle motion; first, it works to break the intermolecular bonds that define the metal’s crystalline structure. In the melting process, once the threshold energy called the heat of fusion is reached, these bonds start to break apart. This breaking does not mean the bonds disappear entirely in the liquid phase; rather, they simply allow for more freedom of movement. While these intermolecular forces are weaker in the liquid state, they are still present and significant. It's this delicate balance between energy input and intermolecular bond dissociation that governs the phase transition from solid to liquid.

Crystalline Structure

The crystalline structure in metals is a well-ordered, repetitive arrangement of atoms. This geometric pattern is the foundation for a metal’s solid state and contributes to its strength and durability. When a metal transitions from solid to liquid, this refined order is disrupted. The heat applied to melt the metal injects energy into the system, making the atoms vibrate with more vigor. Eventually, the crystalline structure loses integrity as these bolstered vibrations essentially jostle the atoms out of their snug, orderly arrangement. Once the crystalline bonds are broken, the atoms drift apart, and the material adopts a disorderly, amorphous form typical of liquids. In essence, while the atoms in a solid metal are neatly lined up in a fixed grid, their arrangement in the melted liquid state becomes random and unstructured. The average distance between atoms, hence, is larger in the liquid because of the collapse of the well-ordered crystalline framework.