2.2.3 Melting, boiling and evaporation (3)
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1.
Describe evaporation in terms of the escape of more-energetic particles from the surface of a liquid. In your answer, explain how temperature relates to the rate of evaporation.
Evaporation is a surface phenomenon where liquid molecules gain sufficient kinetic energy to overcome the intermolecular forces holding them in the liquid phase and escape into the gaseous phase. This occurs when individual liquid molecules possess a kinetic energy equal to or greater than the vapor pressure of the liquid at a given temperature.
The kinetic energy of molecules is directly related to temperature. As temperature increases, the average kinetic energy of the liquid molecules increases. This means a greater proportion of molecules will have enough energy to overcome the intermolecular forces and escape. Therefore, the rate of evaporation increases with increasing temperature. A higher temperature leads to a higher vapor pressure, and a greater number of molecules possessing the necessary energy to transition to the gaseous state.
2.
Water is a crucial substance in many chemical and biological processes. State the melting point and boiling point of water at standard atmospheric pressure (1 atm). Explain, in terms of intermolecular forces, why water has relatively high melting and boiling points compared to other molecules of similar size.
The melting point of water is 0°C (273.15 K) and the boiling point is 100°C (373.15 K) at standard atmospheric pressure (1 atm).
Water has relatively high melting and boiling points due to the presence of strong hydrogen bonds between water molecules. Hydrogen bonds are a type of dipole-dipole interaction that occurs when hydrogen is bonded to a highly electronegative atom like oxygen. These bonds are stronger than the weaker London dispersion forces found in nonpolar molecules of similar size. More energy is required to overcome these strong intermolecular forces, hence the higher melting and boiling points.
- Hydrogen bonds are responsible for the high melting and boiling points.
- Hydrogen bonds are stronger than London dispersion forces.
- More energy is required to break the intermolecular forces.
3.
Describe, in terms of energy input, the processes of melting and boiling. Explain how these processes occur without a change in temperature.
Melting is the process where a solid changes into a liquid. This occurs when heat energy is added to the solid. The added energy increases the kinetic energy of the particles within the solid. In a solid, particles are held in fixed positions and vibrate. As the temperature rises with energy input, the particles vibrate more vigorously. Eventually, the vibrations become strong enough to overcome the intermolecular forces holding the particles in their fixed positions. This allows the particles to move more freely, transitioning into a liquid state. Crucially, during the melting process, the temperature remains constant at the melting point. All the added energy is used to break the bonds between the particles, rather than increasing their kinetic energy (which would raise the temperature).
Boiling is the process where a liquid changes into a gas (steam). Similar to melting, boiling requires a significant amount of heat energy input. As heat is added, the kinetic energy of the liquid particles increases. In a liquid, particles are close together but can move past each other. With increasing energy input, the particles gain enough kinetic energy to overcome the intermolecular forces holding them together in the liquid state. They escape into the gaseous phase. Again, the temperature remains constant at the boiling point during the boiling process. All the added energy is used to overcome the intermolecular forces and allow the particles to escape as a gas, rather than increasing the average kinetic energy of the particles (which would raise the temperature).