Bond enthalpies

Question 1

When is heat released and when is heat absorbed in chemical reactions?

Answer 1

• In chemical reactions, the chemical bonds of the reactants are broken and new chemical bonds are make when the products are formed
• Breaking bonds: endothermic (bendo) energy absorbed
• Making bonds: exothermic (mexo) energy released

Question 2

When is a reaction exothermic?

Answer 2

• When less energy is absorbed while breaking bonds and more energy is released when making bonds, the reaction is exothermic (negative change)
• When more energy is absorbed while breaking bonds and less energy is released when making bonds, the reaction is endothermic (positive change)
• The difference is: bendo (reactants) - mexo (products)

Question 3

What is bond enthalpy?

Answer 3

• The energy required to break one mole of chemical bonds in a gaseous state
• It is also called bond dissociation energy (E)
• Bond enthalpy’s are always positive (bonds broken)
• The substances are always in the gaseous state
• When new bonds are formed, the energy released is equal to the energy that was absorbed to break the bond (exothermic, negative)

Question 4

What is the average bond enthalpy?

Answer 4

Definition: The enthalpy change when one mole of bonds are broken in the gaseous state averaged for the same bond in similar compounds

• The same type of chemical bond in different compounds has different bond enthalpy values and the average of them are calculated to obtain the average bond enthalpy
• Also identical bonds in molecules with two or more types of bond also have different bond enthalpy values
• The bond enthalpy for the two bonds are calculated, and the average of them is used.
• They don’t take the intermolecular forces into account, hence the values may differ from the data booklet.

Question 5

How to calculate enthalpy changes using bond enthalpy values.

Answer 5

• The enthalpy change is equal to the sum of bond enthalpy values of the bonds broken - sum of bond enthalpy values of bond formed
  ΔH = bendo (reactants) -mexo (products)
• Add the values in the data booklet together
• These values differ from experimental values because they are average values taken from the same bonds in a range of similar compounds
• In exothermic reactions, the products are more stable than the reactants as they contain less energy and vice versa

Question 6

What is the purpose of ozone and where is it found?

Answer 6

• Ozone absorbs harmful ultraviolet (UV) radiation from the sun
• The stratosphere is the layer of atmosphere contains 90% ozone at a concentration of less than 10 ppm (ozone layer)
• Ozone levels are maintained by a continuous cycle of synthesis and breakdown reactions by the energy from UV radiation

Question 7

Discuss the bond strength of ozone layer compared to oxygen.

Answer 7

• Oxygen has bond energy 498 kJ mol-1 (has higher energy radiation of shorter wavelength, shorter than 242nm, λ < 242 nm)
• Ozone has bond energy 364 kJ mol-1 (has lower energy radiation of longer wavelength, λ < 330 nm)
• This difference is significant in the chemistry taking place in the lower stratosphere.
• The double bond in oxygen can be broken by ultraviolet radiation producing two free oxygen atoms (radicals)
• The higher the energy the shorter the wavelength ( λ ) and vice versa, O2 requires more energy than O3 which means the wavelength of O2 is shorter

Question 8

What are radicals and how are they related to the oxygen and ozone reactions?

Answer 8

• A highly reactive species due to the presence of an unpaired election. The unpaired electrons are represented by a dot
  For example:
  O2 (g) → O• (g) + O• (g) UV light, λ < 242 nm
  OR
  O2 (g) + O• (g) → O3 (g), λ < 330 nm
• These processes depend on the wavelength
• Oxygen = UV-C radiation
• Ozone = UV-B radiation
• The majoirty of the ultraviolet radiation reaching Earth is in the least harmful UV-A form
• Ozone protects life from the harmful radiation