05 Notes - Hess's Law

05 Notes - Hess's Law - Aim: How are multi-step reactions...

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Aim: How are multi-step reactions evaluated? QQ: What is the enthalpy of a reaction that produces 5 moles of N 2 from ammonia gas? Do Now: If a synthesis reaction is exothermic, what would you expect about the decomposition of that compound? Reactions that are exothermic in one direction are endothermic in the other! 2H 2 (g) + O 2 (g) 2H 2 O (l) ΔH = -571.6 kJ 2H 2 O (l) 2H 2 (g) + O 2 (g) ΔH = +571.6 kJ What would a potential energy diagram look like for a reaction that happens in multiple steps? How do we find H for the entire process? Enthalpy is a state function Hess’s Law: If a series of reactions are added together, the enthalpy change for the net reaction will be the sum of the enthalpy change of the individual steps. Given this information: N 2 (g) + 2 O 2 (g) 2 NO 2 (g) Δ H= 135.6 kJ 2NO 2 (g) N 2 O 4 (g) Δ H= -116.4kJ How can I figure out the enthalpy change of: N 2 O 4 (g) N 2 (g) + 2O 2 (g) Δ H = ?
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05 Notes - Hess's Law - Aim: How are multi-step reactions...

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