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## Main Question or Discussion Point

Why Do I Use U For Constant Volume and H For Constant Pressure?

Also, which can be calculated H, Q, U, W?

Also, which can be calculated H, Q, U, W?

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Why Do I Use U For Constant Volume and H For Constant Pressure?

Also, which can be calculated H, Q, U, W?

Also, which can be calculated H, Q, U, W?

- #2

Andrew Mason

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For an ideal gas:Why Do I Use U For Constant Volume and H For Constant Pressure?

Also, which can be calculated H, Q, U, W?

[itex]\Delta U = nC_v\Delta T[/itex]

[itex]\Delta H = \Delta U + \Delta (PV) = nC_v\Delta T + nR\Delta T = nC_p\Delta T[/itex]

I am not sure what you mean by your last question. They can all be calculated if you have sufficient information.

AM

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I can't tell you why you do that but if there is no non-volumetric work the change of U or H is equal to the exchanged heat if you do so.Why Do I Use U For Constant Volume and H For Constant Pressure?

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I like Serena

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dU=dQ - PdV

dH=dQ + VdP

For constant volume dV=0, so we can write the first as:

ΔU=ΔQ

For constant pressure dP=0, so we can write the second as:

ΔH=ΔQ

These are the quantities that we can actually measure.

Depending on circumstances we can calculate each of H, Q, U, and W.

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