Enthalpy and Phase Change

[Precious Oniha]

Why is enthalpy needed for a phase change occurs? Is it due to the fact that it requires heat making it endothermic, in that case, why is temperature constant?

[Ethan Chen 1K]

When a phase change occurs, you are still adding heat and energy to the system, so the enthalpy there needs to be considered in our total calculations. The temperature does not change because the heat added is being used to overcome the intermolecular forces of the current phase, such as overcoming the forces between the molecules in solid ice in order to transform it into liquid water, which has weaker forces between its molecules. The average kinetic energy of the particles isn't increasing here when all the heat is being used to break bonds, so temperature doesn't change.

[Mei Tsao 1A]

Heat and energy are added during phase changes, even though the temperature does not change. The energy being added is used to break bonds between molecules during the phase change instead of adding to the molecules' average kinetic energy (hence why temperature stays constant during the phase change). The amount of energy it takes to break the bonds between molecules still needs to be accounted for because it is a step in the overall reaction, so we include it in total enthalpy change calculations.

[AChassion Dis2A]

Enthalpy is related to heat and energy. When a phase change occurs, all the heat / energy being applied to the system goes into breaking bonds in the case of melting or vaporization. At this point, the temperature might not change, but a drastic phase change is occurring. The enthalpy is just the heat and energy entering the system that breaks the intermolecular bonds.

[Ashley Chu 3C]

Enthalpy is needed for a phase change because when a phase change occurs, heat and energy are still being added to the system. However, this energy and heat are used to break the intramolecular bonds of the molecule instead of increasing the enthalpy/temperature of the system. Therefore, it is added back at the end to the overall enthalpy change of the reaction.

[305943084]

Enthalpy is added because heat and energy are still being added into a system to make phase change occur. Instead of raising the temperature, all these energies are used to overcome the intermolecular forces of the current state.

[Madhavi Akella 2H]

During a phase change, the temperature of a sample remains constant even though heat is being supplied because the heat is being use to break the bonds, not raise the temperature.

[Maggie_Harned_3H]

Phase changes occur due to the addition of heat and energy. This inclusion of heat does not change the temperature because the energy is being used to break the bonds in the molecules during this phase change. While this keeps the temperature the same the energy taken to do so is calculated as enthalpy.