# PV work

#### inaccensa

10+ Year Member
7+ Year Member
is there any PV work done during phase changes? I understand for ex- water from -10c to 0c, as heat is supplied, the molecules move faster and the energy is used to raise the temp.
Q =mct
but when water reaches 0c, the temp stops rising and doesn't the volume increase? So there should be some W done by the substance, correct? If so, then why do we call that as the heat of fusion, change in enthalpy = q

#### crazybob

"When thermal energy is withdrawn from a liquid or solid, the temperature falls. When heat energy is added the temperature rises. However, at the transition point between solid and liquid (the melting point), extra energy is required (the heat of fusion). To go from liquid to solid, the molecules of a substance must become more ordered. For them to maintain the order of a solid, extra heat must be withdrawn. In the other direction, to create the disorder from the solid crystal to liquid, extra heat must be added."

http://en.wikipedia.org/wiki/Enthalpy_of_fusion

#### inaccensa

10+ Year Member
7+ Year Member
"When thermal energy is withdrawn from a liquid or solid, the temperature falls. When heat energy is added the temperature rises. However, at the transition point between solid and liquid (the melting point), extra energy is required (the heat of fusion). To go from liquid to solid, the molecules of a substance must become more ordered. For them to maintain the order of a solid, extra heat must be withdrawn. In the other direction, to create the disorder from the solid crystal to liquid, extra heat must be added."

http://en.wikipedia.org/wiki/Enthalpy_of_fusion

Let me rephrase that
Isn't there work done during phase change? What I'm asking is when we say enthalpy change, I feel like they are saying, at constant pressure, H=Q, but since the volume changes during phase change, isn't there work done? So, shouldn't this be H =Q+ PV

#### crazybob

Let me rephrase that
Isn't there work done during phase change? What I'm asking is when we say enthalpy change, I feel like they are saying, at constant pressure, H=Q, but since the volume changes during phase change, isn't there work done? So, shouldn't this be H =Q+ PV
The process takes place at constant pressure, so
w = -Pexternal(V2-V1)
and H does equal q. but H = U + PV. U = q + w. so H = q = U - w. and w = - PV

the only way that U = q is when w = 0. so if w = 0, H = Q + PV

#### inaccensa

10+ Year Member
7+ Year Member
The process takes place at constant pressure, so
w = -Pexternal(V2-V1)
and H does equal q. but H = U + PV. U = q + w. so H = q = U - w. and w = - PV

the only way that U = q is when w = 0. so if w = 0, H = Q + PV
If W = 0, how can H = Q+ PV? Anyhow, so there is work done during the phase change. EK said that the heat supplied is used the increase the PE of the substance. What exactly are they increasing?

#### crazybob

If W = 0, how can H = Q+ PV? Anyhow, so there is work done during the phase change. EK said that the heat supplied is used the increase the PE of the substance. What exactly are they increasing?
when W = 0, PV = 0, and H = Q + PV = Q + 0 = Q.

when you increase the potential energy, you increase the value of U.

#### inaccensa

10+ Year Member
7+ Year Member
when W = 0, PV = 0, and H = Q + PV = Q + 0 = Q.

when you increase the potential energy, you increase the value of U.

Let me get this straight,
When the phase change takes place, For Example at 0c ice to water, the enthalpy change is positive. Q =mH. There is an increase in the internal energy and the volume changes, So it technically boils down to

U =Q-W

There is internal energy change which is the Q-W, but the enthalpy change is not H = U +PV, its simply U=Q. I'm still confused!!!