q and delta H interchangeable in calculations?

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Jade Fosburgh Discussion 2C
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q and delta H interchangeable in calculations?

Postby Jade Fosburgh Discussion 2C » 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?

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

Postby Ethan-Van To Dis2L » 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.

Katie Lam 1B
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Re: q and delta H interchangeable in calculations?

Postby Katie Lam 1B » Fri Jan 19, 2018 1:39 pm

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?

Postby Mika Sonnleitner 1A » 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.

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

Postby Brigitte Phung 1F » 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.


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