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Internal energy vs enthalpy
Megan_5942
#1 Posted : Wednesday, June 16, 2021 10:17:55 PM
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What exactly is the difference between internal energy and enthalpy? In my text I see it says enthalpy is equal to heat in closed systems (which makes sense) but enthalpy equals internal energy in solids/liquids. But they act so similar I don't even know what this means (bc they have similar formulas). Any help in distinguishing would be appreciated!
INSTR_Calla_139
#2 Posted : Thursday, June 17, 2021 4:53:44 PM
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Hello,

I am going to start out with some quick definitions for you:

"Internal energy (E, sometimes U) is the sum of all the kinetic and potential energy in a system.
It is difficult to measure an absolute value for the internal energy of a system, so we are only
interested in determining the change in internal energy of a system"

"Enthalpy (H) is the total energy transferred during a chemical process, including both the change
in internal energy of the system (E) plus any energy (work) that is required to make room within
the surroundings for the products of the reaction. Enthalpy is energy and therefore has units of
Joules."

So knowing these definitions lets take a look at this equation:

ΔH = ΔE + Δ(PV)
ΔH - change in enthalpy
ΔE - change in internal energy
Δ(PV) - change in pressure x volume


your question asked how internal energy equals internal energy in solids and liquids. this is because in solids and liquids the pressure and volume stay constant so in this case ΔH = ΔE.

I recommend skimming the chem. cc lesson 4 (thermodynamics) ~pg.194
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