Lacey_Che131_S2012_Lect-15s

# Lacey_Che131_S2012_Lect-15s - Methods of Determining Hrxn 1...

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Lec-15: Hess’s Law; Food & Fuels Roy A. Lacey, Stony Brook University; Che 131, Spring 2012 1 Methods of Determining Δ H rxn 1. from calorimetry experiments 2. from enthalpies of formation 3. using Hess’s Law ` A calorimeter is the device used to measure the absorption or release of heat y a physical or chemical process by a physical or chemical process. Roy A. Lacey, Stony Brook University; Che 131, Spring 2012 2 State Functions The enthalpy (H) is a state function ` Enthalpy (H) = E + PV ` Change in Enthalpy ( Δ H) = Δ E + P Δ V ` At constant pressure q P = Δ E + P Δ V, therefore q P = Δ H ` Δ H = change in enthalpy: an energy flow as heat (at constant pressure) ` Δ H > 0, Endothermic ; Δ H < 0, Exothermic Roy A. Lacey, Stony Brook University; Che 131, Spring 2012 3 Fuels CH 4 ( g ) + 2 O 2 ( ) CO 2 ( ) + 2 H 2 O( ); H 0 rxn = 802.2 kJ. Δ H = -q cal = -C cal Δ T A bomb calorimeter t t l is a constant-volume device used to easure the heat of a measure the heat of a combustion reaction. Roy A. Lacey, Stony Brook University; Che 131, Spring 2012 4

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Calculating Δ H rxn ° ` Δ rxn °= Σ n p Δ f ° (products) −Σ r Δ f ° (reactants) The enthalpy (H) is a state function Roy A. Lacey, Stony Brook University; Che 131, Spring 2012 5 Hess’s Law ` Hess’s Hess’s law law: if a reaction is carried out in a number of steps, Δ for the overall reaction is the sum of Δ for ach dividual tep each individual step.
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Lacey_Che131_S2012_Lect-15s - Methods of Determining Hrxn 1...

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