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### q and delta H interchangeable in calculations?

Posted: Fri Jan 19, 2018 12:51 pm
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?

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

Posted: Fri Jan 19, 2018 1:01 pm
Delta H is only equivalent to the heat absorbed/released (aka q) when the system has a constant pressure and no nonexpansion work.

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

Posted: Fri Jan 19, 2018 1:39 pm
They are equivalent only under certain conditions, in this case at constant pressure.

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

Posted: Fri Jan 19, 2018 2:03 pm
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.

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

Posted: Fri Jan 19, 2018 3:05 pm
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.