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Hess’s Law

Hess’s Law. section 5.4 p322 -. sometimes it is not possible to use a calorimeter for finding enthalpy eg : _______ Hess = _______________________________________________________________________________________________________________

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Hess’s Law

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  1. Hess’s Law

    section 5.4 p322 -
  2. sometimes it is not possible to use a calorimeter for finding enthalpy eg: _______ Hess= _______________________________________________________________________________________________________________ Analogy sum of vectors – doesn’t matter the path only initial and final destinations if you reverse the order – change direction!
  3. Potential Energy Diagrams Explanation Path doesn’t matter – only the beginning and end are important!
  4. Hess’ Law any reaction can be written in steps enthalpy for the reaction will be the sum of all enthalpies for all steps So: ΔHtarget= ___________________________ ΔHtarget = Σ ΔHsteps
  5. Target: H2O(l) → H2O(g) ΔH = ?? H2O(l) → H2(g) + ½ O2ΔH1 = +285.8kJ/mol H2(g) + ½ O2 → H2O(g)ΔH2 = -241.8kJ/mol Does this make sense??
  6. Target: 2C2H6(g) + 7O2(g) → 4CO2(g) + 6H2O(g) Given: (1) 2C(s) + 3H2(g) → C2H6(g) ∆H= -83.8kJ/mol (2) H2(g) + ½ O2(g) → H2O(g) ∆H= -241.8kJ/mol (3) C(s) + O2(g) → CO2(g) ∆H= -393.5kJ/mol What is the enthalpy for the target reaction?
  7. See next for more sample problemspp324-325 ** next step – combine this with a q=mcΔT !
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