9.9

[Wenting Hu 2H]

Why does isothermal mean that deltaU=0?

[Sean Monji 2B]

For ideal gases, all the energy is in the kinetic energy of the particles. Temperature is basically a measure of the average kinetic energy of the molecules. Isothermal means there is no change in temperature, and if there is no change in temperature, there must be no change in total kinetic energy. Therefore, delta U (internal energy) cannot be changing.

[skalvakota2H]

Internal energy is simply the total store of energy in a system, which is associated with the random and disordered motion of molecules. On the other hand, temperature measures how much the molecules of a substance are moving. Thus, internal energy is a state function that is dependent on temperature.

In an isothermal process, if the temperature is constant, then there is no change in the kinetic energy in the system. As a result, the change in internal energy is also zero.

[Jingyi Li 2C]

For ideal gas, ΔU=3/2nRΔT
For isothermal reactions, ΔT=0, so ΔU also equals to 0.