Chapter 20, Problem 9RAT

How much energy, in kilojoules, is released or absorbed from the reaction of 1 mole of nitrogen, ${\text{N}}_2$, with three moles of molecular hydrogen, ${\text{H}}_2$, to form 2 moles of ammonia, ${\text{NH}}_3$? Consult Table 20.1 for bond energies.

(a) $+899\text{ kJ/mol}$

(b) $-993\text{ kJ/mol}$

(c) $+80\text{ kJ/mol}$

(d) $-80\text{ kJ/mol}$

To determine

The correct option for the statement “How much energy, in kilojoules, is released or absorbed from the reaction of 1 mole of nitrogen, ${\text{N}}_2$, with three moles of molecular hydrogen, ${\text{H}}_2$, to form 2 moles of ammonia, ${\text{NH}}_3$? Consult Table 20.1 for bond energies”.

Answer to Problem 9RAT

The correct option for the statement “How much energy, in kilojoules, is released or absorbed from the reaction of 1 mole of nitrogen, ${\text{N}}_2$, with three moles of molecular hydrogen, ${\text{H}}_2$, to form 2 moles of ammonia, ${\text{NH}}_3$? Consult Table 20.

1 for bond energies” is option (c).

Explanation of Solution

The quantity of energy essential in any chemical reaction to dissociate a bond is termed as bond energy. The same amount of energy is released if the same bond is formed.

The reaction given can be written in the chemical equation as given below,

${\text{N}}_2{\text{+3H}}_2\to {\text{2NH}}_3$

The amount of energy released or absorbed can be calculated as follows,

According to the given reaction,

There is one $\text{N}\equiv \text{N}$ bond and the bond energy absorbed to break this bond from the table is $946\text{ kJ/mol}$.

There are three $\text{H}-\text{H}$ bonds and the bond energy absorbed to break this bond from the table is $3\times 436\text{ kJ/mol}$ that gives the value required to break the bonds is $1308\text{ kJ/mol}$.

Thus, the total energy absorbed to break the bonds in the reactant is $\left(1308+946\right)\text{ kJ/mol}$ this gives out to be $-2254\text{ kJ/mol}$.

There are three $\text{N}-\text{H}$ bonds in one ammonia molecule and 2 moles of ammonia contains total of six $\text{N}-\text{H}$ bonds. Therefore the total energy released for six $\text{N}-\text{H}$ bonds will have $6\times \left(-389\right)\text{ kJ/mol}$ this comes out to be $+2334\text{ kJ/mol}$.

Thus, the net energy of the reaction can be given as,

$\begin{array}{c}\text{Net energy of reaction}=\text{energy absorbed}+\text{energy released}\\ =-2254+2334\text{ kJ/mol}\\ =+80\text{ kJ/mol}\end{array}$.

Conclusion:

Therefore, the correct option is option (c).