2.5 Chapter 7 Thermochemistry (inked 3)

# 2.5 Chapter 7 Thermochemistry (inked 3) - 7-6 Heats of...

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1 7-6 Heats of Reaction at Constant Volume: Δ U Reactants Products U i U f Δ U = U f -U i Δ U = q rxn + w In a system at constant volume : Δ U = q rxn + 0 = q rxn = q v But we live in a constant pressure world! How does q p relate to q v ? Heats of Reaction, Δ U Heats of Reaction at Constant Pressure: Δ H q V = q P + w We know that w = - P Δ V and q V = Δ U, therefore: Δ U = q P -P Δ V q P = Δ U + P Δ V U, P and V are all state functions, so define a new function: H = U + PV (H = Enthalpy) Δ H = H f –H i = Δ U + Δ (PV) = Δ U + Δ P•V + P• Δ V At constant pressure ( Δ P•V = 0) and temperature: Δ H = Δ U + P Δ V = q P Changes of State of Matter H 2 O (l) H 2 O(g) Δ H = 44.0 kJ at 298 K Molar enthalpy of vaporization: Molar enthalpy of fusion: H 2 O (s) H 2 O(l) Δ H = 6.01 kJ at 273.15 K Changes of State of Matter H 2 O (l) H 2 O(g) Δ H = 44.0 kJ at 298 K Molar enthalpy of vaporization: Molar enthalpy of fusion: H 2 O (s) H 2 O(l) Δ H = 6.01 kJ at 273.15

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2.5 Chapter 7 Thermochemistry (inked 3) - 7-6 Heats of...

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