CHEMISTRY COMMUNITY

When do you use PdeltaV=deltanRT?

You would use this as another way to calculate work when the external pressure is constant.

When Pex is constant, w = -P(delta)V. If the change in volume is not given and say they give you a reaction, then you would look to see if there is a change in the number of gas molecules (delta)n. If so, you can calculate work as: - (delta)n * R * T since as you stated, P(delta)V = (delta)n * R * T. However if there is no change in moles, then you can assume there is no change in volume (as long as temperature is constant as well) and thus there is no work done.

Like the reply above stated, it's just a different way to calculate work. We can use - delta n R T if we know the net change in moles of the equation in order to find the work.

You can only use this equation when you are dealing with an ideal gas. The deltas imply that a change is occurring. You can use this in these 2 formulas:

delta H = delta U + nR (delta T)
delta H = delta U + (delta n) RT

Because w=PdeltaV, work could also be found using w=deltanRT when a value in the other equation is not known. This is because of the equation pV=nRT