Enthalpy - Enthalpy :any .Gases ,butthis

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Enthalpy Many chemical reactions occur at constant pressure rather than constant volume: any  reaction in an open beaker will be under the constant 1 atmosphere of pressure. Gases  expanding out of open reaction vessel are doing p  V Δ  work on the surroundings, but this  work is lost from a practical point of view, since it is dissipated into the surroundings.  When we do a calorimetry experiment at constant (atmospheric) pressure and measure  the heat given off by the system, we don't know the p  V Δ  work done by the system on  the surroundings. U = q + w = q - p  V Δ Δ We want to measure  U Δ  for the reaction because it is a state function, with is history  independent (unlike heat and work). To get around this problem we do a mathematical  trick. We define a new state function called  Enthalpy : H = U + p V
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