Change in Enthalpy

[Eric Quach 1C]

Is the change in enthalpy from water as a liquid to gas positive and negative? Why?

[Tuong-Minh Tran 1C]

The enthalpy of water from liquid to gas is positive. Enthalpy is defined as the internal energy of a substance. In order for water to turn from a liquid to a gas, energy must be added into the system in order to excite the molecules enough to change from a liquid state to a gas state.

[LedaKnowles2E]

The change in enthalpy of liquid water to water vapor is positive because the phase change from liquid to vapor requires heat to be added to the water. Since heat was added to the water for it to become vapor, the vapor will have higher enthalpy than the water. The change in enthalpy is calculated as the enthalpy of the vapor minus the enthalpy of the water, so it will be a positive value.

[Lorena Zhang 4E]

The change in enthalpy would be positive in this case because in order to have the liquid water change phase into vapor, the process requires heat. The final enthalpy minus initial would generate a positive change of enthalpy.