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AP Chem

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What energy change is associated with the reaction to obtain one mole of H2 from one mole of water vapor? The balanced equation is 2 H2O(g) -> 2 H2(g) + O2(g)and the relevant bond energies are
H-H : 436 kJ/mol; H-O : 467 kJ/mol;
O-O : 146 kJ/mol; O=O : 498 kJ/mol.
1. +249 kJ
2. −436 kJ
3. +425 kJ
4. −425 kJ

  • AP Chem -

    2 H2O(g) -> 2 H2(g) + O2(g)

    H2O(g) -> H2(g) + 1/2 O2(g)
    1mol 1 mol 1/2 mole
    each mole H2O has 2 moles of O-H bonds to break (energy requiring) = 2 x 467 = 934 kJ

    Each mole H2 has 1 mole H-H bonds to make (energy releasing) = 436 kJ
    Each mole O2 has 1 mole O=O bonds to make. So 1/2 mole = 1/2 x 498 = 249 kJ

    Energy required in bond breaking = 934 kJ
    Energy released in bond making = 436 + 249 = 685 kJ

    Balance needed = 934 - 685 = 249 kJ
    So Delta H = +249kJ

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