Bond Dissociation

Use ΔHrxn = BDE(bonds broken, reactants) - BDE(bonds formed, products) The bonds broken are: 3 C-H and 3 Cl-Cl. The bonds formed are: 3 C-Cl and 3 H-Cl ΔHrxn = (3 x 413 kJ/mol + 3 x 243 kJ/mol) - (3 x 339 kJ/mol + 3 x 427 kJ/mol) = (1968 kJ/mol) - (2298 kJ/mol) = -330 kJ/mol

Calculate ΔHrxn for: CH₄ + 3Cl₂ → HCCl₃ + 3HCl Use: BDE(C-H) = 413 kJ/ml, BDE(Cl-Cl) = 243 kJ/mol, BDE(C-Cl) = 339 kJ/mol, BDE(H-Cl) = 427 kJ/mol A) +330 kJ/mol B) -110 kJ/mol C) +110 kJ/mol D) -330 kJ/mol

The formation reaction is: H₂(g) + Cl₂ → 2HCl Bonds formed: 2 H-Cl Bonds broken: H-H and Cl-Cl Therefore: ΔHrxn = BDE(bonds broken, reactants) - BDE(bonds formed, products) = (436 kJ/mol + 242 kJ/mol) - (2 x 431 kJ/mol) = -184 kJ/ml

Calculate the heat of formation for hydrochloric acid using: BDE(H-H) = 436 kJ/mol, BDE(Cl-Cl) = 242 kJ/mol, BDE(H-Cl) = 431 kJ/mol A) +184 kJ/mol B) -247 kJ/mol C) +247 kJ/mol D) -184 kJ/mol

The bond dissociation energy (BDE) is the energy required to break a bond. The BDE is always a positive value because breaking a bond implies the input of energy (so ΔH > 0), versus the formation of a bond gives off energy (ΔH < 0).

How is the bond dissociation energy defined?

(1) Determine which bonds are broken. We break a N-H bond and a Cl-Cl bond. BDE(N-H) = 389 kJ/mol BDE(Cl-Cl) = 243 kJ/mol Determine which bonds are formed. We have a new N-Cl bond and a new H-Cl bond. BDE(N-Cl) = 201 kJ/mol BDE(H-Cl) = 431 kJ/mol The heat of reaction is:ΔHrxn = BDE(bonds broken, reactants) - BDE(bonds formed, products) = (389 kJ/mol + 243 kJ/mol) - (201 kJ/mol + 431 kJ/mol) = (+632 kJ/mol) - (632 kJ/mol) = 0 kJ/mol Note that the zero heat of formation is an unusual result.

How would you use the energy of broken bonds to calculate the heat of the total reaction?

The bond dissociation energy is another way to calculate the heat of a reaction. Breaking bonds requires a positive ΔH while forming bonds results in a negative ΔH.

What can be calculated using the bond dissociation energy?

Bonds can be broken homolytically or heterolytically. BDE usually refers to the homolytic case in which each of the two electrons in the bond go back onto their original atoms as the bond breaks. In heterolytic cleavage (seen in very polar solvents for example) both of the electrons in the bond go onto one of the atoms with other atom becoming electron deficient.

What type of bond cleavage does the bond dissociation energy typically refer to?

On a bond energy curve (or bond dissociation energy curve), the BDE is the minimum energy in the curve.

Where is the bond dissociation energy on a bond energy curve?