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During a tranformation or process, the variation in Internal Energy of a body(system) is essentially defined by the first law of thermodynamics which states that energy is conserved:

ΔU = Q + W + W'

where :
ΔU is the change in internal energy of a system during a process.
Q is the heat added to a system.
W is the mechanical work done on a system and W' is energy added by all other processes.

If the mechanical work happens to be at constant pressure P, W is equal to (-P*ΔV). The minus sign follows the conventions of positive work when done on the system, when (ΔV<0).

We can rewrite the above equation as

ΔU + PΔV = Q + W'

If we introduce a new thermodynamic potential called H, Entalpy, for which H = E + PV, the left side of the equation is equal to ΔH, or enthalpy variation in the process. For a process in which the only work done is mechanical (PV), W' is equal to zero and ΔH represents the heat transferred at constant pressure.

ΔH values are listed for a number of important reactions.These values refer to the same number of moles of the stoichiometric coefficients of reacting compounds and of products, as shown in examples.

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