Question

give an example to calculate the bond enthalpy and it should be clearly explained.




please guys help me fast

Answer 1

Bond enthalpy (also known as bond energy) is defined as the amount of energy required to break one mole of the stated bond. For example, the bond energy of a O-H single bond is 463 kJ/mol. This means that it requires 463 kJ of energy to break one mole of O-H bonds. Note: In order to break any bond, energy must be absorbed. Therefore, bond breaking is always an endothermic change (ΔH > 0). Conversely, energy is always released upon the formation of a bond. Therefore, bond making is always an exothermic change (ΔH < 0).

Change in reaction enthalpy (ΔHrxn) can be approximated from bond energy data. One can assume that during a chemical change, all bonds in the reactants are broken yielding free atoms as shown in step 1 in the diagram below. Then those atoms recombine to form the new bonds found in the products (step 2).


The overall reaction enthalpy change is easily obtained via Hess' Law:

ΔHrxn=ΔH1+ΔH2ΔHrxn=ΔH1+ΔH2

To summarize, the change in enthalpy for a given reaction will the combined total of the energy required to break any bonds that are broken and the energy released from any bonds that are formed.  

ΔHrxn=ΣΔHbreaking+ΣΔHmakingΔHrxn=ΣΔHbreaking+ΣΔHmaking

This is the sum of the bond enthalpies of the broken bonds minus the sum of the energies of the bonds that are formed (since forming bonds will release energy).  One advantage of this method is that you only need to look at the bonds that are changing in a given reaction.

Example: Consider the hydrogenation of ethene to give ethane.

C2H4(g)+H2(g)→C2H6(g)C2H4(g)+H2(g)→C2H6(g)

Here, we need to break the C=C bond in ethene, and the H-H bond in H2. Then we need to form two new C-H bonds and a C-C bond in ethane. (See Bond Energy table at the bottom of this page) A H-H bond enthalpy (BE) is 436 kJ/mol, a C=C bond is 602 kJ/mol, a C-C bond is 346 kJ/mol, and a C-H BE is 413 kJ/mol.