## q and delta H interchangeable in calculations?

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### q and delta H interchangeable in calculations?

If heat and enthalpy are by definition different, then why in some problems (like 93), we use them interchangeably? For example, we calculate the work in part a, then the delta H in part b, then in part c it asks us for delta U (which is q+w). But the solutions manual simply added part a (w) and part b (delta H) together. So delta H and
q are interchangeable even though they are not the same by definition?

Ethan-Van To Dis2L
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### Re: q and delta H interchangeable in calculations?

Delta H is only equivalent to the heat absorbed/released (aka q) when the system has a constant pressure and no nonexpansion work.

Katie Lam 1B
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Joined: Fri Sep 29, 2017 7:06 am

### Re: q and delta H interchangeable in calculations?

They are equivalent only under certain conditions, in this case at constant pressure.

Mika Sonnleitner 1A
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### Re: q and delta H interchangeable in calculations?

q and ∆H are only equal to each other when pressure is constant. This gives rise to the equation for change in internal energy of a reaction that happens in a sealed container. If you substitute ∆H for q, the change in internal energy equation becomes ∆U = ∆H - w, or ∆U = ∆H -P∆V.

Brigitte Phung 1F
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### Re: q and delta H interchangeable in calculations?

In addition, because the amount of heat absorbed or released at a constant pressure is defined as the enthalpy, we can write that q = ∆H.

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