Biology Forum Community General Discussion How do i know if a reaction is endothermic or exothermic?

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    • #17613

      Consider the Reaction: 2Hg) (s) –> 2Hg (l) + O2 (g)

      Hg=O 340kJ/mol
      O=O 498 kJ/mol

      How do i solve this?

      My answer was exothermic but it was wrong. It should be endothermic.

      This is how i solved it:

      Enthalpy Before: 2(340) = 680
      Enthalpy After: 498

      Change in Enthalpy: 498 – 680= -182

      Therefore, the reaction is exothermic because energy is released from the bond.

      Can you please tell me what i did wrong and how to solve it.

      In additon, my textbook states that an endothermic reaction is a reaction which requires energy overall and absorbs heat. While exothermic reactions release energy overall and releases heat.

      It also states that Catabolic reactions are exothermic reactions while anabolic reactions are endothermic reactions because it requires a net input of energy.

      A reaction is exothermic when change in entalphy is less than 0 and endothermic when change in enthalpy is greater than 0.

      How come in the reaction H2 + Cl2 –> 2HCl, it requires energy to break the bonds into H H Cl Cl when it is catabolic and releases energy when reforming the bonds to H-Cl and H-Cl when it is anabolic?

      Thank You

    • #114685

      Change in enthalpy = sum of bond energies of reactants – sum of energies of products
      = Total input energy – Total output energy

      From: Enthalpy Changes in Chemical Reactions … tabid=1871

      This reverses the sign of the enthalpy.

      Enthalpy Before: 2(340) = 680
      Enthalpy After: 498

      Change in Enthalpy: 680 – 498= 182

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