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c161l17_10_12b

# c161l17_10_12b - Energy calculations Energy Thermochemical...

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Energy calculations Energy calculations W Thermochemical equations relate the number of moles used or produced in a reaction to the energy produced or consumed. W We can treat the enthalpy the same way we treat reactants or products.

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Enthalpy Enthalpy W Δ U = q + w=q - P Δ V W At constant volume Δ V = 0 Δ U = q v = C v Δ T W If we define a new function enthalpy H=U + PV Δ H = Δ U + P Δ V + V Δ P At constant pressure Δ P=0, so Δ H = Δ U + P Δ V =q - P Δ V + P Δ V = q p = C p Δ T
Properties of Enthalpy Properties of Enthalpy W Enthalpy is a state function Δ H = H final - H initial Cannot measure H, only Δ H W For an exothermic process Δ H<0, for an endothermic process Δ H>0 W Enthalpy is an extensive property Double the amount, double the enthalpy

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Changes of State Changes of State W Vaporization - “Boiling” - changing from liquid to gaseous Always endothermic process Evaporation for cooling Reverse process called condensation W Enthalpy of vaporization amount of energy (at constant pressure) to vaporize a substance Usually expressed as a molar quantity

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Heating curves Heating curves -350 -250 -150 -50 50 150 Heat (q) Temperature ms Δ T ms Δ T n Δ H

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c161l17_10_12b - Energy calculations Energy Thermochemical...

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