Q10.84CP - Chemistry: Molecular Nature Of Matter And Change

Question

A major short-lived, neutral species in flames is OH.

1. What is unusual about the electronic structure of OH?
2. Use the standard heat of formation of OH(g) (39.0 kJ/mol) and bond energies to calculate the O-H bond energy in OH(g).
3. From the average value for the O-H bond energy in Table 9.2 (p. 353) and your value for the O-H bond energy in OH(g), find the energy needed to break the first O-H bond in water

Step-by-Step Solution

Answer

a) The O atom in the OH molecule has 7 electrons in its valence shell is quite unusual. 

b) The bond energy of O-H in OH(g) is  $\mathbf{426}\mathbf{kJ}\mathbf{/}\mathbf{mol}$.

c) energy needed to break the first O-H bond in water is  $\mathbf{508}\mathbf{kJ}\mathbf{/}\mathbf{mol}$.

Step 1: Subpart (a) The electronic structure of OH.

The O atom in the OH molecule has 7 electrons in its valence shell. While hydrogen only has one valence electron, oxygen has six. They form a single bond through sharing of two electrons. So, one unpaired electron remains on the oxygen atom. Thus, none of the atoms in O-H has a complete octet.

 

Step 2: Subpart (b) The calculation of the O-H bond energy in OH(g).

$\frac{1}{2}{\mathbf{O}}_2\mathbf{\left(}\mathbf{g}\mathbf{\right)}\mathbf{+}\frac{1}{2}{\mathbf{H}}_2\mathbf{\left(}\mathbf{g}\mathbf{\right)}\to \mathbf{OH}\mathbf{\left(}\mathbf{g}\mathbf{\right)}$

As

 $\begin{array}{c}\text{The heat of formation is = }\left(\begin{array}{l}\text{Heat required to break bonds}\\ -\text{heat released when bonds formed}\end{array}\right)\\ \text{39 kJ/mol =}\left(\begin{array}{c}\frac{\text{1}}{\text{2}}\text{ Bond Energy}\left(\text{O=O}\right)\text{ +}\frac{\text{1}}{\text{2}}\text{ Bond energy}\left(\text{H-H}\right)\\ \text{– Bond energy}\left(\text{O-H}\right)\end{array}\right)\end{array}$

 

Therefore,

$\begin{array}{c}\text{39 kJ/mol = }\frac{\text{498}}{\text{2}}\text{+}\frac{\text{432}}{\text{2}}-\text{ BE(O-H)}\\ \text{BE(O-H) = 249 + 216 }-\text{ 39}\\ \text{ = 426 kJ/mol}\end{array}$

Step 3: Subpart (c) The energy required for the first bond.

In a water molecule, there are 2 OH bonds.

The average bond energy for one OH bond is $\text{= 467kJ/mol}$  .

Therefore, for two OH bonds, the energy is $\text{= 467×2 = 934kJ/mol}$ . 

Thus, the energy needed to break the first bond  $\text{= 934-426 = 508kJ/mol}$ .