Reversible Heating and Cooling

[12boone]

Homework Statement

I have two problems.

What is Delta U, Delta H, q, and w for the Reversible heating of an ideal gas at constant P.

Also, what are these values for the Reversible cooling of an ideal gas at constant V.

Homework Equations

The Attempt at a Solution

For the first equation I decided that all values would be negative except for heat which is positive. I consider this because w=-pex(deltaV) and I decided q= positive because when u are heating somethign you add heat.

for the second problem I know work=0 because the system is at constant volume. Also, q=Delta U because Delta U=q+w. q is negative because soemthing that is cooling is removing heat. I can not figure out delta H.

 

[Redbelly98]

Welcome back to PF

Are you being asked to provide the expressions for ΔU, etc., in terms of P and V? Or just to state whether they are positive, negative, or zero?

For the first equation I decided that all values would be negative except for heat which is positive. I consider this because w=-pex(deltaV) and I decided q= positive because when u are heating somethign you add heat.

That's good reasoning on w and q. How did you figure out that U and H are negative?

for the second problem I know work=0 because the system is at constant volume. Also, q=Delta U because Delta U=q+w. q is negative because soemthing that is cooling is removing heat.

Looks good so far.

I can not figure out delta H.

What is the definition of H?

Also, the fact that it is an ideal gas might or might not come into play.

 

[12boone]

H is enthalpy. I just have to say whether or not the values of these will increase decrease or if they are zero in each case. I am really only concerned about the last two.

 

[Redbelly98]

How is enthalpy H defined in terms of U, P, and V? You can use this defining equation to work out whether ΔH is +, -, or 0.