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

Short answer

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

b) The bondenergy ofO-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-by-step solution

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 onevalence 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.

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

$\begin{array}{c}\text{Theheatofformationis=}\left(\begin{array}{l}\text{Heatrequiredtobreakbonds}\\ -\text{heatreleasedwhenbondsformed}\end{array}\right)\\ \text{39kJ/mol=}\left(\begin{array}{c}\frac{\text{1}}{\text{2}}\text{BondEnergy}\left(\text{O=O}\right)\text{+}\frac{\text{1}}{\text{2}}\text{Bondenergy}\left(\text{H-H}\right)\\ \text{–Bondenergy}\left(\text{O-H}\right)\end{array}\right)\end{array}$

Therefore,

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

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 $\text{=934-426=508kJ/mol}$.