Interpreting the Phase Change Graph for Water

[Karrin Evans 1C]

Hi! I had a quick question about understanding phase change diagrams. The example we went over in class compared the energy released by steam just after vaporization to the energy released by water just before this phase change. Vaporization requires about 46 kJ mol-1, but the heat of the sample remains consistent during this process. If heat stays the same, why should vapor release 46 kJ of energy while liquid only releases 5? Wouldn't that imply that the heat of the two samples just before and after boiling is different?


Phase Change Diagram for 1 mol Water

[Chem_Mod]

Hi!

The heat does not stay the same during a phase change; the temperature stays the same during a phase change. During the phase change, all the supplied energy is used to overcome attractive forces and/or break bonds between molecules.

Hope this helps!

[Vincent Palomo 3L]

First things first: heat and temperature are different. The two are related, but heat is essentially the net energy of a reaction, while temperature is the overall average kinetic energy of atoms in a substance. Knowing this, the reason why the temperature stays the same is because the energy coming into the system is being used to break the bonds and attractions between the atoms in substance, instead of increasing the temperature. Since the temperature stays the same but the amount of energy has increased this means that the vapor has more energy.

Hope this helps!!!

[Lizze Cardoso 2I]

Hi! After the lecture, I had the same question: why was there a difference in the energy released from vapor and liquid? However, the responses here helped clarify my understanding that temperature will remain constant during a phase change because heat breaks the molecules/bonds needed to change the state of matter. Thanks for clarifying and adding to my understanding!

[Karrin Evans 1C]

First things first: heat and temperature are different. The two are related, but heat is essentially the net energy of a reaction, while temperature is the overall average kinetic energy of atoms in a substance. Knowing this, the reason why the temperature stays the same is because the energy coming into the system is being used to break the bonds and attractions between the atoms in substance, instead of increasing the temperature. Since the temperature stays the same but the amount of energy has increased this means that the vapor has more energy.

Hope this helps!!!

That makes so much sense, thank you for clarifying!

[Audrey2B]

Are heat and temperature two different values?

[Nina Vu-Gia 1C]

Are heat and temperature two different values?

Heat and energy are different! Heat is a form of energy, and in this case the energy from heat is being used to break bonds resulting in a phase change. Temperature is a measure of average kinetic energy. Heat can increase the average kinetic energy of a system but during a phase change all the energy is going to bond breaking and therefore is being "used" and not contributing to the temperature, or kinetic energy, of the system.