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Hess’s LAw

Hess’s LAw. Hess’s Law . states that the enthalpy change for any chemical reaction is independent of the route provided the starting conditions and final conditions (because enthalpy is a state function)

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Hess’s LAw

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  1. Hess’s LAw

  2. Hess’s Law states that the enthalpy change for any chemical reaction is independent of the route provided the starting conditions and final conditions (because enthalpy is a state function) Allows us to calculate the enthalpy changes of reactions that we cannot measure directly in the laboratory Can use a direct or indirect method
  3. Standard enthalpy of formation ΔHf° Enthalpy change that occurs when one mole of the substance is formed from its elements in their standard states under standard conditions
  4. Standard enthalpy of reaction Direct method to find enthalpy change ΔHrxn° = ΣnΔHf(products) – ΣmΔHf(reactants) Enthalpy of a reaction carried out at 1atm Multiply coefficient of each substance by the ΔH Add product side and subtract the sum of the reactant side
  5. Example 1 2Al (s) + Fe2O3(s)  Al2O3(s) + 2Fe(l) Find the enthalpy change for the reaction if the ΔHf for Fe(l) = 12.40kJ/mol, the ΔHf for Fe2O3 is -822.2kJ/mol, and the ΔHf for Al2O3 is -1669.8kJ/mol
  6. Example 2 (indirect method) Find the enthalpy change for the conversion of graphite to diamond: Cgraphite(s)  C diamond (s) Cgraphite (s) + O2(g)  CO2(g) ΔH = -394kJ Cdiamond (s) + O2(g)  CO2(g) ΔH = -396kJ
  7. Indirect method tips Addition of multiple reactions to find the one you want Each individual reaction must be balanced Reversing the direction of a reaction reverses the sign Single elements do not have an enthalpy of formation value in their room temperature state Add the individual reactions together and make sure things cancel out to give you the desired reaction – then change enthalpy signs as needed and add together
  8. More tips for indirect ΔH is proportional to the coefficients- so if you multiply all coefficients by something, do the same for the enthalpy Work backward from the required equation to know how to manipulate the other reactions Reverse reactions as needed Multiply reactions to give the correct numbers of reactants and products
  9. Example 3 diboronhexahydride (old rocket fuel) is synthesized from elemental boron and hydrogen. Calculate the enthalpy change of formation of diboronhexahydride. 4B(s) + 3O2(g)  2B2O3(s) ΔH = -2546kJ B2H6(g) + 3O2(g)  B2O3(s) + 3 H2O(g) ΔH = -2035kJ 2H2(g) + O2(g)  2H2O(l) ΔH = -572kJ H2O(l)  H2O(g) ΔH = 44kJ
  10. Ex 3 solution 2B2O3 (s) + 6H2O  2B2H6 (g) + 6O2 2(2035kJ) 4B(s) + 3O2 (g)  2B2O3 (s) -2546 4B(s) + 6H2O (g)  2B2H6(g) +3O2 1524 6H2O (l)  6H2O(g) 6(44) 4B(s) +6H2O(l)  2B2H6(g) + 3O2(g) 1788 6H2(g) + 3O2(g)  6H2O(l) 3(-572) 4B(s) + 6H2(g)  2B2H6 (g) 72 Divide by 2 2B(s) + 3H2(g)  B2H6 (g) 36kJ
  11. HW1 2C(s) + O2(g)  2CO(g)
  12. HW 2 2N2(g) +6H2O(g)  3O2(g)g + 4NH3(g)
  13. HW 3 NO(g) + O (g)  NO2(g)
  14. HW 4 CaC2(s) + 2H2O(l)  Ca(OH)2(aq) + C2H2(g)
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