Chapter 9, Problem 9.100QE

Interpretation Introduction

Interpretation:

The Lewis structure of ${\text{CH}}_{\text{3}}{\text{NH}}_{\text{2}}$ has to be determined. Also, the energy needed to break all of the bonds in 1 mol of ${\text{CH}}_{\text{3}}{\text{NH}}_{\text{2}}$ has to be identified.

Concept Introduction:

Lewis structures are representations of the covalent bond. In this, Lewis symbols show how the valence electrons are present in the molecule.

Steps to write Lewis structures are as follows:

1. The skeleton structure with single bonds between all bonded atoms has to be written

2. Sum the valence electrons of the atoms in the molecule.

(a) For cations, one electron is subtracted for each positive charge.

(b) For anions, one electron is added for each negative charge.

3. Subtract two electrons from total number of valence electrons for each bond in the skeleton structure.

4. Count the number of electrons required to satisfy the octet rule for each atom in the structure. If the number of electrons needed is less than the number remaining, add one bond for every two electrons needed between atoms to attain an octet.

5. The remaining electrons are placed as lone pairs on atoms that need them to satisfy the octet rule.

The bond dissociation energy or bond energy $\left(D\right)$ is defined as energy needed to break one mole of bonds in a gaseous species.

The equation that describes the bond dissociation for ${\text{H}}_{\text{2}}$ is as follows:

  ${\text{H}}_{\text{2}}\left(\text{g}\right)\to \text{H}\left(\text{g}\right)+\text{H}\left(\text{g}\right)$

Bond energies are always endothermic and have a positive sign. It takes energy to break a bond.

The enthalpy of reaction can be determined by the sum of the bond dissociation energies of all the reactants minus the sum of the bond dissociation energies of all products present in chemical reaction.

The equation to calculate enthalpy of reaction is as follows:

  $\Delta H_{\text{reaction}}=\left[\begin{array}{c}{\displaystyle \sum \left(\text{moles of bond broken}\times \text{bond energies of bond broken}\right)}\\ -{\displaystyle \sum \left(\text{moles of bond formed}\times \text{bond energies of bond formed}\right)}\end{array}\right]$

Negative sign in the equation depicts that bonds will form in the products. It is an exothermic process, so the energy charge is the negative of bond energy.