Question

Why is there no change in intramolecular forces when a solid is melted? Are intramolecular forces stronger or weaker than intermolecular forces?

Short answer

During the melting process, a solid changes to a liquid state due to increased temperature and particle vibration. This affects the intermolecular forces, causing them to weaken as particles move more freely. However, intramolecular forces within molecules remain unchanged, as the energy input is not enough to break chemical bonds like covalent or ionic bonds. Intramolecular forces are generally stronger than intermolecular forces, as they involve the sharing or transfer of electrons between atoms, while intermolecular forces result from interactions between molecules, like van der Waals forces, hydrogen bonding, and dipole-dipole interactions.

Step-by-step solution

Define Intramolecular and Intermolecular Forces

Intramolecular forces are the forces holding the atoms within a molecule together. These forces result from the chemical bonds, such as covalent or ionic bonds, formed between the atoms. Intermolecular forces, on the other hand, are forces that exist between different molecules. These forces include van der Waals forces, hydrogen bonding, and dipole-dipole interactions.

Describe the Melting Process

Melting is a process where a substance changes its phase from solid to liquid due to an increase in temperature. During melting, the energy input to the substance causes its particles to vibrate faster, overcoming the intermolecular forces that hold them in the solid state. This increase in vibration allows the particles to move more freely and transition to the liquid state.

Explain No Change in Intramolecular Forces during Melting

When a solid melts, the increase in energy and vibration of particles affects the intermolecular forces, causing them to weaken and allowing the particles to move more freely. However, there is no change in intramolecular forces, as these forces result from the chemical bonds within the molecule. The energy input during melting is not sufficient to break the chemical bonds holding the atoms together, so intramolecular forces are not affected during the melting process.

Compare Intramolecular and Intermolecular Forces

Intramolecular forces are generally stronger than intermolecular forces. This is because intramolecular forces involve the sharing or transfer of electrons between atoms, resulting in stable chemical bonds such as covalent or ionic bonds. Intermolecular forces, on the other hand, are weaker forces that arise from interactions between molecules, such as van der Waals forces, hydrogen bonding, and dipole-dipole interactions. The fact that intramolecular forces remain unchanged during the melting process while intermolecular forces weaken further supports the idea that intramolecular forces are stronger than intermolecular forces.

Key concepts

Melting Process

Understanding the melting process is fundamental in the study of physical states and phase transitions. Melting occurs when a substance transitions from a solid to a liquid state.

This is caused by an increase in temperature, resulting in a rise in the kinetic energy of the particles. When this energy exceeds the energy of the intermolecular forces holding the particles together in a solid arrangement, the substance begins to melt.

Throughout this process, the internal structure of the particles doesn't change, meaning that the atoms within the molecules remain bonded to each other. Instead, they gain enough energy to overcome the weaker forces between separate molecules, leading to the phase change.

Chemical Bonds

Chemical bonds are intramolecular forces responsible for holding atoms together within a molecule.

The two main types of chemical bonds are covalent bonds, where atoms share electrons, and ionic bonds, where atoms transfer electrons.

Covalent Bonds

They involve two atoms sharing a pair of electrons to achieve stability.

Ionic Bonds

They involve an electron transfer from one atom to another, creating charged ions that attract each other.

These forces are significantly robust, requiring substantial energy to alter, much more than the energy needed to simply melt a substance, which is why the internal structure of molecules remains unchanged during the melting process.

Phase Change

A phase change refers to the transition of matter from one state (solid, liquid, gas) to another.

Phase changes are physical changes that involve altering the energy of a system but not its chemical composition. When a solid melts into a liquid, it's undergoing a phase change.

• Solid to Liquid (Melting)
• Liquid to Gas (Evaporation or Boiling)
• Gas to Liquid (Condensation)
• Liquid to Solid (Freezing)

While the phase change involves a change in intermolecular forces, intramolecular forces, the bonds that make up a molecule's structure, remain untouched.

Van der Waals Forces

Van der Waals forces encompass several intermolecular attractions.

These forces are relatively weak compared to chemical bonds and include attractions between all molecules, regardless of polarity. Van der Waals forces are crucial in holding materials together in the solid or liquid state and must be overcome during a phase change like melting.

Types of Van der Waals forces:

• London Dispersion Forces, which occur in all molecular interactions.
• Dipole-Dipole interactions between polar molecules.

These forces are prominent in non-polar substances and dictate properties like boiling points and melting points for gases and liquids.

Hydrogen Bonding

Hydrogen bonding is a special type of intermolecular force that occurs when a hydrogen atom, covalently bonded to a highly electronegative atom such as nitrogen, oxygen, or fluorine, is attracted to another electronegative atom.

These bonds are stronger than other types of van der Waals interactions due to the high polarity created by the large difference in electronegativities. Hydrogen bonding plays a key role in determining the physical properties of water and other molecules where it occurs. It significantly affects the melting and boiling points of substances and is essential in biological molecules such as DNA and proteins.

Dipole-Dipole Interactions

Dipole-dipole interactions arise between polar molecules, where there is a clear separation of positive and negative charges within the molecule.

Molecules with permanent dipoles will align themselves so that the positive end of one molecule is near the negative end of another. This type of intermolecular force is important in determining the physical properties of compounds.

Impact on Melting and Boiling Points

Substances with stronger dipole-dipole interactions typically have higher melting and boiling points, as these intermolecular forces are stronger than dispersion forces but still weaker than hydrogen bonds or intramolecular forces.