Topic 5.3: Bond enthalpies Flashcards
Definition of average bond enthalpy
Energy required to break one mole of the same type of bond in the gaseous state averaged over a variety of similar compounds
Limitations of average bond enthalpy
a) Since the bond enthalpies are average to different molecules, , this can introduce some inaccuracies
b) All substances must be in gaseous state to ignore intermolecular forces
Bond formation
a) Removes heat bringing atoms closer together
b) Exothermic
Bond breaking
a) Adds heat separating atoms
b) Endothermic
Formula to calculate an approximate enthalpy change of reaction using bond enthalpies
∆Hθ=ΣEbonds broken- ΣE(bonds formed)
Endothermic reactions based on bond enthalpies of products and reactants
a) Strong bonds in the reactants
b) Weak bonds in the products
Exothermic reactions based on bond enthalpies of products and reactants
a) Weak bonds in the reactants
b) Strong bonds in the products
Allotropes of oxygen in the atmosphere
a) O2
b) O3
Why are oxygen and ozone broken by UV light of different wavelengths?
Because the bonds in oxygen, O2, are stronger than those in ozone, O3.
Modified Planck’s equation to calculate the wavelength of light needed to break the bonds of oxygen / ozone
E = (h) (c) / (A)
Ozone-Oxygen cycle
a) Ozone formation
b) Ozone deplation
Ozone formation
a) Higher UV light breaks O2 into two oxygen radicals
b) Single atomic oxygen connects to an oxygen molecules, forming ozone
Ozone deplation
a) Lower UV light breaks ozone into oxygen atom and oxygen molecule
b) Ozone reacts with oxygen atom to form two oxygen molecules.
Significance of ozone cycle
Both processes are essential for the survival of life on Earth.
a) Dangerous UV light has been absorbed
b) The stratosphere has become warmer.
