temperature change and enthalpy change

[Megan Bundy 2K]

can someone explain what Dr. Lavelle mentioned in Wednesday's lecture about how there is an enthalpy change in a reaction but there is no net temperature change?

[Sabira Mohammed 3I]

Not sure if I'm remembering the right moment, but I think he was saying that we measure enthalpy change of a constant temperature, because in labs they measure it at a set temperature (25 degrees, for example), and then people get confused because enthalpy changes heat, but then they measure enthalpy again when the reaction is back to the original temperature (25 degrees) so then that means that even though there was a net release/absorption of heat, the temperature is constant?

[Julie Tran 3C]

Hello,

This was referring to what happens during a phase change (e.g. liquid to solid). As this process occurs, heat energy being added to the substance is contributed towards breaking bonds (which would lead to a phase change like melting or vaporization) rather than increasing the motion of the substance's molecules (which would increase temperature). Thus, even though there is a change in the amount of heat energy (enthalpy) the system has, the temperature still stays constant. This can also be seen in the plateaus of energy vs temperature graphs for phase changes.

Hope this helps!

[Sidharth Paparaju 3B]

During phase changes, energy is being used to break the hydrogen bonds/ IMF forces between molecules rather than increasing the average kinetic energy of molecules. As inter molecular forces are broken, the temperature does not increase, but the phases change from s-->l or l-->g , while when the average kinetic energy increases, so does temperature.

[Libby Stafford 1L]

Also, sometimes in a lab the reaction is kept constant in a temperature bath or something similar, e.g. the beaker is placed in a vat of water at 25 degrees Celsius. This means that even after the reaction has been carried out and released/taken in heat, it will return to the initial temperature, and there is no net temperature change (even though enthalpy has changed based on the reaction itself).