Energetics: Bond Enthalpies

Question 1

What is the definition of bond enthalpy?

Answer 1

• Bond enthalpy is the energy needed to break one mole of the stated bond in a gaseous state, under standard conditions
• Stated bond is usually covalent

Question 2

What is the definition of mean bond enthalpy?

Answer 2

• The average enthalpy change for breaking 1 mole of a stated bond in a range of different compounds in the gas phase
• Stated bond is usually covalent

Question 3

Describe how enthalpy changes when breaking and making bonds

Answer 3

• When reactions happen, reactant bonds are broken and product bonds are formed
• Energy is needed to break bonds
• So bond breaking is endothermic
• ∆H is positive

• Energy is released when bonds are formed

• So bond forming is exothermic
• ∆H is negative

Question 4

How do you calculate the enthalpy change of a reaction?

Answer 4

• Enthalpy change of reaction = Total energy absorbed - Total energy released

• 𝚫𝑯=𝚺𝑯(Bonds Broken)−𝚺𝑯(Bonds formed)
• 𝚫𝑯 = Break - Make

• Calculating total energy absorbed/released
- Multiply the given average bond enthalpies for each type of bond by the number of each bond present

Question 5

Why do values from mean bond enthalpy calculations differ from those determined using Hess’s law?

Answer 5

• This is because the mean bond enthalpies are not always the same as the actual bond enthalpies in the reactants and products
• Same bond but in a range of different compounds