Why is energy released when a bond is formed?

Why is energy released when a bond is formed?

Energy change during formation and breaking of bonds

• During a chemical reaction, the bonds between the atoms in the reactans need to be broken first before new bonds between the atoms in the products can be formed.
• Chemical reactions involve bond breaking and bond formation:
  (a) Bond breaking requires energy. Thus, bond breaking is an endothermic change.
  (b) Bond formation releases energy. Thus, bond formation is an exothermic process.
• The quantity of heat energy absorbed or released during the breaking and formation of bonds in a chemical reaction depends on the strength of the bond.
  (a) More energy is required to break a strong bond compared to a weak bond.
  (b) More energy is released when a strong bond is formed compared to a weak bond.
• Table shows the average bond energies of some common covalent bonds.
  
• The heat absorbed or given out in a reaction comes from the bonds being broken or made in the reaction.
  ΔH = E_b– E_f
  where,
  E_b is the total energy absorbed during the breaking of bonds, and
  E_f is the total energy given out during the formation of bonds.
  (a) If bond formation releases more energy than is required in the bond breaking (E_f > E_b), ΔH = E_b – E_f = negative. The reaction is exothermic.
  (b) If bond breaking requires more energy than the energy that is released during bond formation (E_f > E_b), ΔH = E_b – E_f = positive. The reaction is endothermic.

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Formation of hydrogen bromide, HBr :

H₂(g) + Br₂(g) → 2HBr(g)
H – H + Br – Br → 2 H – Br

• Total energy absorbed to break the bonds in the reactants,
  E_b = energy to break 1 mole of H – H bonds + energy to break 1 mole of Br – Br bonds
  = 436 kJ + 192 kj = 628 kJ
• Total energy given out to form the bonds in the products,
  E_f = energy to form 2 moles of H – Br
  = 2 x 366 kJ = 732 kJ
• Overall heat change in the reaction,
  ΔH = E_b – E_f
  = 628 kJ – 732 kJ = -104 kJ
• Thus, formation of hydrogen bromide is an exothermic reaction.
  H₂(g) + Br₂(g) → 2HBr(g)    ΔH = -104 kJ
• The energy level diagram below illustrates the formation of hydrogen bromide.
  

Decomposition of ammonia:

• Total energy absorbed to break the bonds in the reactants,
  E_b = energy to break 6 moles of H – N bonds
  = 6 x 391 kJ = 2346 kJ
• Total energy given out to fo.rm the bonds in the products, E_f
  
  
• Overall heat change in the reaction,
  ΔH = E_b-E_f
  = 2346 kJ – 2254 kJ = +92 kJ
  The decomposition of ammonia to form nitrogen and hydrogen is an endothermic reaction.
  2NH₃(g) → N₂(g) + 3H₂(g)    ΔH = +92 kJ
• The energy level diagram below illustrates the decomposition of ammonia.
  

Example: The following is the chemical equation of the formation of hydrogen chloride.
H₂(g) + Cl₂(g) → 2HCl(g)
Calculate the overall heat change in the reaction.
Given that the bond energy of H – H = 436 kJ mol^-1, Cl – Cl = 242 kJ mol^-1, H – Cl = 432 kJ mol^-1.
Solution: