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Unformatted text preview: 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 Since internal energy, pressure, and volume are all state functions, then enthalpy, H,...
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