Use bond energies to calculate hrxn for the reaction n2g 3cl2g2ncl3g


Introduction


In order to calculate the enthalpy change for a reaction, we need to know the energies of the bonds that are being broken and formed. These bond energies are measured in kilojoules per mole (kJ/mol).

The enthalpy change of a reaction (ΔHrxn) is the sum of the enthalpies of the products minus the sum of the enthalpies of the reactants:

ΔHrxn = ΣH products – ΣH reactants

We can use this equation to calculate the ΔHrxn for the following reaction:

N2(g) + 3Cl2(g) → 2NCl3(g)

What are bond energies?


Bond energies are the estimates of the amount of energy required to break a bond between atoms in a molecule. The energy required to break a bond is called the bond dissociation energy, and it is always given in units of kilojoules per mole (kJ/mol).

In general, the larger the bond dissociation energy, the stronger the bond. This means that it takes more energy to break bonds between atoms that have large bond energies. Consequently, reactions that involve breaking bonds with large bond energies will have large activation energies and will be less likely to occur under normal conditions.

Fortunately, we can use bond energies to calculate the enthalpy change (ΔH) for a reaction. The overall enthalpy change for a reaction can be determined by adding up all of the Bond Dissociation Energies for all of the bonds that are broken in the reactants and subtracting all of the Bond Dissociation Energies for all of the bonds that are formed in the products.

How to calculate hrxn using bond energies


In order to calculate the enthalpy change of a reaction using bond energies, you will need to know the reactants, products, and reaction equation. The Reactant section lists the reagents and their coefficients in stoichiometric ratios. The products of the reaction are on the right side of the equation. The enthalpy change of the reaction (∆Hrxn) is calculated using the following equation:

∆Hrxn = ∑ Bond Energies(products) – ∑ Bond Energies(reactants)

The ∑ symbol means “the sum of.” In other words, you will add up all of the bond energies for each side of the equation.

You will need a table of bond energies in order to do this calculation. A bond energy is defined as the amount of energy required to break one mole of a particular type of bond in a molecule. Tables of common bond energies can be found in most Chemistry textbooks.

Once you have gathered all of the information needed, you can plug it into the equation and solve for ∆Hrxn.

Examples


-N2 + 3Cl2 –> 2NCl3
-N2 + 3/2 Cl2 –> NCl3

Bond Energy (kJ/mol)
N-N 160
N=N 310
N#N 420
Cl-Cl 242
Cl=Cl 327
Cl#Cl 427

The answer would be -(160+310+242+327+427)=-1756 kJ/mol

Conclusion


In conclusion, using bond energies to calculate the heat of reaction for the reaction:
N2(g) + 3Cl2(g) –> 2NCl3(g)

would result in a heat of reaction value of -201 kJ/mol.


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