delta H and delta Q

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delta H and delta Q

Postby Grace_Stevenson_1A » Tue Jan 10, 2017 6:10 pm

What is the difference between delta H and delta Q? Thank you!!

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Re: delta H and delta Q

Postby Tycho_Meimban_2B » Tue Jan 10, 2017 8:11 pm

Q is the amount of heat transfered to a system. It is one of two means of energy transfer during most processes studied in thermodynamics, the other being work. Since these are the only means by which we can transfer energy between the system and its surroundings, we write the change of internal energy of the thermodynamic system as:

ΔE= q + w

Enthalpy, however, measures the potential energy of a system. Enthalpy can be defined as:

H = E + PV

and thus:

ΔH = ΔE + Δ(PV)

ΔH = q is valid for constant pressure ONLY.

Hope this explanation helps!

Vincent Tse 2B
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Re: delta H and delta Q

Postby Vincent Tse 2B » Tue Jan 10, 2017 10:35 pm

Since heat is not a state property, it isn't continuous. Therefore, heat can't be differentiated or taken as a difference between an initial and final value of heat. (Refer to p. 14 in Course Reader).

On the other hand, enthalpy is a state property and is denoted by ΔH; this means that you can add and subtract. In short, ΔH refers to the amount of heat released/absorbed at a constant pressure. Here's a link to a video that further clarifies enthalpy: (

Heat, however, is really just a process of energy transfer between two objects due to temperature differences. To calculate heat, you might want to look into the concept of specific heat capacity (Section 8.5 in the textbook). The examples and explanations are clear in helping you understanding how heat is measured.

Hope this helps!

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