AS-Level Chemistry Chapter 6 Chemical Energetics Bond Energy

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Bond Energy

Bond breaking and bond forming

1) i. Breaking bonds needs energy, energy is absorbed from the surroundings. Therefore, bond breaking is an endothermic process.

ii. Forming bonds releases energy to the surroundings. Therefore, bond forming is an exothermic process.

2) i. If the energy needed to break bonds is less than the energy released when new bonds are formed, then the reaction will be exothermic

ii. If the energy needed to break bonds is more than the energy released when new bonds are formed, then the reaction will be endothermic.

3) In most chemical reactions, bonds have to be broken first and then reformed into a new compound.

Bond energy

1) Bond energy, E is the energy needed to break one mole of covalent bonds between two atoms in the gaseous state.

Br-Br(g) → 2Br(g) ; E(Br-Br) = +193 kJ mol⁻¹

2) Bond energy is often called bond enthalpy or bond dissociation energy.

Average bond energy

1) Bond energy is often affected by the presence of other atoms in the molecule. For example, the O-H bond in water has a different bond energy value than the O-H bond in ethanol. Even in the same molecule, for example, CH4, the energy needed to break all the C-H bonds are not identical.

2) Therefore, bond energies are taken from a number of bonds of the same type from different environment and average out to get the average bond energy. The values are not always accurate.

Hess' law involving average bond energy

1) Average bond energies can also be used to estimate enthalpy changes of reactions involving covalent molecules

2) If the given reactants or products are not in the gaseous state, enthalpy change of vaporization or enthalpy change of fusion must be considered because the non-gaseous compounds needed to be converted to the gaseous state. Changing from other states to the gaseous state needs to absorb extra energy.