What is the energy change when the temperature of 11.1 grams of gaseous oxygen is decreased from 38.1 C to 20.6 C?

Answer 1

The answer depends on whether the change in temperature is done at constant volume or at constant pressure and on just what you mean by change in energy.

11.1 g of O₂ is 11.1 / 32 = .347 moles.

For a change made a constant volume ( say the oxygen is in a steel tank) we use C_v - the specific heat at constant volume. For a diatomic gas like O₂ C_V = (5/2) R = 20.8 J /mole/degree

So at constant volume the decrease in energy is .347 ( 38.1 - 20.6) 20.8 = 126.3 J

For a change made a constant pressure (say the oxygen is in a toy balloon inflated just enough to keep its shape ) we use C_P - the specific heat at constant pressure. { In this case the pressure stays equal to 1 atmosphere.} For a diatomic gas like O₂ , C_P = (7/2) R = 29.1

So the decrease in energy is 176.8 J

A further note is that these decreases in energy are the amount of heat that flowed out of the system into the surrounding universe. In the constant volume case, that change is the same as the change in internal energy of the oxygen because no work was done on or to the system.