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04 Notes - Enthalpy - Aim W hat is enthalpy QQ Draw a...

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Aim: What is enthalpy? QQ: Draw a potential energy diagram for an endothermic reaction and label key aspects. Do Now: Why do chemists use moles instead of grams when comparing two pure substances? Heat of Fusion: energy required to melt a substance Heat of Vaporization: energy required to vaporize a substance Heat of Fusion q = mH f Heat of Vaporization q = mH v Use the Reference Table B to obtain the values of H f and H v for water. How much energy is required to vaporize 100.0 g of water? Enthalpy: the heat that is absorbed or released in a chemical reaction, takes into account the molar quantity of the reactants ( Δ H). Δ H reaction = n Δ H products n Δ H reactants Δ H is looked at from the perspective of the system… Exothermic Reactions The system loses energy Δ H is negative 2H 2 (g) + O 2 (g) 2H 2 O (l) + 571.6 kJ 2H 2 (g) + O 2 (g) 2H 2 O (l) Δ H = -571.6 kJ Endothermic Reactions The system gains energy Δ H is positive N 2 (g) + O 2 (g) + 182.6 kJ NO (g) N 2 (g) + O 2 (g) NO (g) Δ H = 182.6 kJ Reactions that are exothermic in one direction are endothermic in the other!
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