- #1

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## Main Question or Discussion Point

I understand why entropy (a state function) is very useful in chemistry. Since most chemistry systems are at constant pressure, ##\Delta H=Q_v## from the derivation below:

##H=U+PV##

##dH=dU+dPV+PdV##

##dU=\delta Q-\delta W##

## dU=\delta Q-PdV##

## dH=\delta Q+dPV##

##dPV=0##

## dH=\delta Q_v##

However, for constant volume work is not done, so that seems to leave this pesky ##dPV## term. That is:

##dH=\delta Q_p+dPV## What is the meaning of this term. ##PdV## is obviously microscopic work, but what the other term resulting from the product rule means physically escapes me.

Thanks,

Chris

##H=U+PV##

##dH=dU+dPV+PdV##

##dU=\delta Q-\delta W##

## dU=\delta Q-PdV##

## dH=\delta Q+dPV##

##dPV=0##

## dH=\delta Q_v##

However, for constant volume work is not done, so that seems to leave this pesky ##dPV## term. That is:

##dH=\delta Q_p+dPV## What is the meaning of this term. ##PdV## is obviously microscopic work, but what the other term resulting from the product rule means physically escapes me.

Thanks,

Chris