Enthalpy Changes 2

• What does Hess Law state?

  that the enthalpy change for a chemical reaction is independent of

  the route taken, (provided the initial and final conditions are the same)
• What is the equation for enthalpy change of reaction using enthalpy change of formation?

  ∆Hө(reaction) = ∑∆fHө(products) - ∑∆fHө(reactants)

  ө= standard conditions

  ∑= sum of
• What is the equation for enthalpy change of formation using enthalpy change of combustion ?


  ∆fHө = ∆cHө[reactants] – ∆cHө[products]
• What do bond enthalpies measure ?

  The strengths of covalent bonds
• Are bond enthalpies -ive or +ive ?

  +ive
• Why are bond enthalpies endothermic ?

  Bond breaking always requires energy = endothermic

  Bond making always releases energy = exothermic

  The more positive the bond enthalpy, the higher the amount of energy needed to break the bond, and so the stronger the bond.
• • In any chemical reaction, bond breaking is followed by bond formation

  • Bond breaking = endothermic = energy is required

  • Bond forming = exothermic = energy is released

  • Therefore, in an exothermic reaction (∆H –ve), the bonds that are formed must be stronger than the bonds broken in the reactants.

  • And, in an endothermic reaction (∆H +ve), the bonds that are formed must be weaker than the bonds broken in the reactants.
• Definition of average bond enthalpy.

  The average enthalpy change when one mole of covalent bonds are broken within gaseous molecules.
• What is the equation for enthalpy change of reaction using average bond enthalpies ?

  ∆reactionH = ∑∆H (bonds broken) - ∑∆H (bonds formed)
• Suggest the reason for the actual value for the enthalpy change of reaction would be different from the one calculated using average bond enthalpy ?

  The calculated value uses average bond enthalpy not the actual bond

  enthalpy. The average does not take into account the environment of the bond so there will be a small difference.