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How is enthalpy different than heat?

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  1. Dec 26, 2015 #1
    Background
    I'm taking grade 12 chemistry. During a lesson, my teacher briefly mentioned that enthalpy is equal to the internal energy of a system plus the product of the pressure and volume of that system. However, she remarked that the concept wasn't an important part of the curriculum and moved on after she wrote its mathematical definition on the board. (She's a good teacher; I don't mean to suggest that she short-changed us.)

    My Attempt to Answer my Question
    I spent several hours yesterday looking for examples that might give me an intuitive understanding of what that formula is describing: I read the Wikibooks chapter, spoke with an aerospace engineer, and watched the Khan Academy video.
    • The Wikibooks chapter quickly employs more calculus than I know.
    • The engineer provided some help, but noted that he couldn't distinguish change in enthalpy from heat.
    • The Khan Academy video1 defines it as "heat transfer for chemical reactions".
    I suspect that transfer in heat transfer (a phrase that the video presenter used) is redundant, as she defines heat as "energy transfer due to change in temperature". If it is redundant, then the presenter defined enthalpy as 'heat for chemical reactions'. Additionally, I know that heat is not a state function and that enthalpy is a state function. As such, it seems to me that the video presenter's definition would make sense only if she used enthalpy to denote 'change in enthalpy' as I assume she did.

    My Question
    The two sources that I could understand both equated change in enthalpy with heat. However, that still leaves me with my original question: What does the formula for enthalpy (not change in enthalpy) describe? Some everyday examples of things that exhibit much, or little, enthalpy might help me understand.

    Thank you,



    1 https://www.khanacademy.org/test-prep/mcat/biomolecules/principles-of-bioenergetics/v/enthalpy-1 (see 1:22).
     
    Last edited: Dec 26, 2015
  2. jcsd
  3. Dec 26, 2015 #2
    Just to start this off the combination u + pv occurs frequently in thermodynamic and because both u and pv are state variables the combination is a state variable and thus given its own name. U cannot be measured directly and so h also cannot be measured directly. However like internal energy change in enthalpy is directly measurable and physically relevant. The article https://en.wikipedia.org/wiki/Enthalpy gives a decent interpretation of enthaly and internal energy.
     
  4. Dec 27, 2015 #3

    Borek

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    Staff: Mentor

    Imagine a gas in a rigid tank. You heat the gas, and all heat goes into the internal energy of the gas.

    Imagine a gas in a cylinder with a movable piston. You heat the gas (with the same amount of heat as in the previous case) and it expands. When it expands it does some work moving the piston. This work equals PΔV (easy to show using a basic physics and the pressure definition). Now the increase in the internal gas energy is lower, as some of the energy (transferred in the form of heat) was used to do work.

    Imagine a gas in a well insulated cylinder with a movable piston. You don't heat it, but instead you squeeze the gas moving the cylinder. This time you did a PΔV work on the gas and its internal energy went up.

    Enthalpy change covers all these situations (and all their possible combinations, as in practice we never deal with ideal models), internal energy change doesn't.
     
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