Bond Energy Calculation and Reversible Reactions

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Bond Energy Calculations

Bond Energy:

  • Each type of chemecal bond has a different bond energy. This bond energy can very depending on what type of compound the bond is in.
  • We can use the bond energy to calculate the enthalpy change for a reaction. 
  • You add up the energy of the bonds that are broken and subtract the energy of the bonds that are made.

enthalpy change = total energy used to break bonds - total energy released on making bonds

The Formation of Hydrochloric Acid:

  • H-H = +436 kJ/mol
  • Cl-Cl = +242 kJ/mol
  • H-Cl = +431 kJ/mol
  • Breaking one mole of H-H and Cl-Cl requires -> 436 + 242 = 678 kJ
  • Forming two moles of H-Cl releases -> 2 x 431 = 862 kJ
  • /\H = 678 - 862 = -184 kJ 
  • This tell us that the reaction is exothermic as the /\H is negative.
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Reversible Reactions

A reversible reaction is a reaction where the products can react together and convert back to the original reactants. The reaction for ammonia is reversible.

Dynamic Equilibrium -> the relative quantities of reactants and productswill reach a certain balance and stay there.

There are different things that affect the way in which the dynamic equilibrium works.

Change in Temperature: 

  • All reactions are endothermic in one direction and exo in the other.
  • If you raise the temperature, the endothermic reaction will occur and if you lower the temperature, the exothermic reaction will occur to replenish the heat.

Change in Pressure:

  • If you increase the pressure, the reaction will occur where the products have less molecules than the reactants and vice versa.
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