Reaction Energy & Bond Energy | Interactive Chemistry model

Investigate reaction energy

Reaction Activation energy, Ea (kJ mol⁻¹)

Energy absorbed—

Breaking reactant bonds

−

Energy released—

Forming product bonds

Total energy to break bonds—

Total energy released forming bonds—

Reaction enthalpy, ΔH—

Reaction type—

Activation energy, Ea—

Products relative to reactants—

Side	Bond	Number	Bond energy (kJ mol⁻¹)	Contribution (kJ mol⁻¹)

Important distinction: Breaking a bond is always an energy-absorbing process. Forming a bond is always an energy-releasing process. An exothermic reaction does not mean that breaking its reactant bonds releases energy; it means that more energy is released by forming product bonds than is required to break the reactant bonds.

Calculate ΔH from Bond Energies

The Python model adds the energies needed to break all relevant reactant bonds and subtracts the energies released when the product bonds form.

# Bond-energy calculation
#
# Example:
# 2H2 + O2 -> 2H2O

# Approximate average bond enthalpies
# in kJ mol^-1

bond_energy = {
    "H-H": 436,
    "O=O": 498,
    "O-H": 463
}

# Bonds broken:
#
# 2 H-H bonds
# 1 O=O bond

bonds_broken = (
    2 * bond_energy["H-H"]
    + 1 * bond_energy["O=O"]
)

# Bonds formed:
#
# 2 H2O molecules contain
# 4 O-H bonds in total

bonds_formed = (
    4 * bond_energy["O-H"]
)

# Reaction enthalpy estimate
delta_H = (
    bonds_broken
    - bonds_formed
)

print(
    "Energy absorbed breaking bonds:",
    bonds_broken,
    "kJ mol^-1"
)

print(
    "Energy released forming bonds:",
    bonds_formed,
    "kJ mol^-1"
)

print(
    "Delta H:",
    delta_H,
    "kJ mol^-1"
)

if delta_H < 0:
    print("Reaction is EXOTHERMIC")

elif delta_H > 0:
    print("Reaction is ENDOTHERMIC")

else:
    print("No net energy change")


# Why?
#
# Breaking bonds requires energy.
# Forming bonds releases energy.
#
# If:
#
# energy released > energy absorbed
#
# then Delta H is negative and
# the reaction is exothermic.
#
# Bond enthalpies are average values,
# therefore this method gives an
# approximate reaction enthalpy.

Activation Energy

A reaction can be exothermic overall and still require an initial energy input. The activation energy is the energy barrier that must be overcome before reactant bonds can rearrange into products.

A catalyst lowers the activation-energy pathway but does not change the energy levels of the reactants and products, so it does not change ΔH.