Gas molar heat capacity [ENDORSED]

[Miguel Velasco 2J]

why does gas have two values for molar heat capacity?

[Katherine Jordak 1H]

All I know is that one molar hear capacity is for when there is a constant volume and the other one is for when there is a constant pressure.

[Chem_Mod]

In class I talked about heating a solid or liquid results in essentially no volume change.

Unlike a gas there is a change in volume if heating occurs under constant pressure conditions.
Volume has to increase if temperature increases and pressure is constant: PV = nRT

If volume increases the gas (system) is doing work and this takes additional energy.

However, if the gas is heated under constant volume conditions then no additional energy is needed as there is no work of expansion.

Therefore, as discussed in class gases have two different values for their molar heat capacity: C_p and C_v

[Chem_Mod]

In my answers I sometimes refer to class discussion or lecture notes ("as discussed in class").
I do this as a way of suggesting to compare and discuss class notes which students can do in the 60 hours of group peer learning sessions and office hours per week.

In addition these lecture topics can also be discussed in discussion sections. Just ask/email your TA before discussion section a list of topics or homework problems that would be helpful to cover in discussion section and/or office hours.

Hope this helps. :-)

[manasa933]

One is under constant volume, and the other under constant pressure.