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HESS CYCLES!

HESS CYCLES!. By Ravi Mistry. Blud. Hess’ Law. “If a reaction can take place by more than one route, and the initial and final conditions are the same, the total enthalpy change is the same each route”. “The enthalpy change is independent of the path taken”. Enthalpy Change of Combustion!.

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HESS CYCLES!

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  1. HESS CYCLES! By Ravi Mistry. Blud.

  2. Hess’ Law “If a reaction can take place by more than one route, and the initial and final conditions are the same, the total enthalpy change is the same each route” “The enthalpy change is independent of the path taken”

  3. Enthalpy Change of Combustion! “The enthalpy change when ONE MOLE of a substance undergoes complete combustion under standard conditions. All reactants and products are in their standard states”

  4. Enthalpy Change of Combustion! ΔHr = Enthalpy Δ of Reaction ΔH? A B Combustion Product Combustion arrows DOWN!

  5. For example. ΔHr 3C + 4H2 C3H8 ΔHc C = -394 x 3 ΔHc H = -289 x 4 -2219 B = (-394x3) + (-289x4) = 2338 CO2 + H20 C = -2219 If arrow C conflicts, it changes the sign. Therefore, A = B - C A = B + C A = B - C A = -2338 - (-2219) = -119kJ/mol-1

  6. Enthalpy Change of Formation! “The enthalpy change when ONE MOLE of a compound is formed in its standard state from its elements in their standard states”

  7. Enthalpy Change of Formation! ΔH? A B Combustion Product Formation arrows UP!

  8. Substance ΔHC/kJmol-1 C -394 H2 -286 C6H14 -4163 In an exam question…

  9. For example. ΔHr 6C + 7H2 C6H14 ΔHc C = -394 x 6 ΔHcH2 = -286 x 7 -4163 B = (-394x6) + (-286x7) = -4366 CO2 + H20 C = -4163 A = B + C A = C - B A = -4163 - (-4366) = -203kJ/mol-1

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