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q and delta H interchangeable in calculations?

Posted: Fri Jan 19, 2018 12:51 pm
by Jade Fosburgh Discussion 2C
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
by Ethan-Van To Dis2L
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
by Katie Lam 1B
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
by Mika Sonnleitner 1A
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
by Brigitte Phung 1F
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.