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Topic: Enthalpy = heat  (Read 2798 times)

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Offline zeoblade

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Enthalpy = heat
« on: August 16, 2009, 04:41:47 AM »
Energy total = energy potential + energy kinetic + energy internal

In the lab, we're not displacing so both energy potential is constant and energy kinetic = 0 leaving energy internal the component of interest in this situation. Energy internal (U) combined with pressure-volume work (PV) = enthalpy (H). Rearranging this equation U = H - PV but now I don't know how to define U because H is dependent on U and PV. I can see that -PV is from rearrangement of the equation but the negative value means the system is using energy to do work so that energy leaves the system to increase volume assuming constant pressure. Is this interpretation, correct or incorrect?

U component of energy total also has energy potential + energy kinetic within it that adds onto the greater energy potential + energy kinetic to create energy total. Now U can also be expressed as heat (q) + work (w). Is this the same work as H = U + PV? When U = q + w, w can also be defined as PV. Is the sign also -PV because q is being converted to w(PV) and leaves to system to give -PV?

This scenario is for exothermic reactions, so for endothermic reactions do we need to change the sign of H = U + PV --> H = U - PV and U = q + w --> U = -q - w?

I'm ashamed to admit I lost myself in all the steps and need to grasp the big picture again because the terminology of the components have such similar meaning. Can I differentiate H = U + PV as greater and U = q + w(-PV or -pv) as lesser? Is the U in H = U + PV the same U in U = q + w? Or is it different like U and u?

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