Values of measured bond energies may vary greatly depending...

Values of measured bond energies may vary greatly depending on the molecule studied. Consider the following reactions: $$ \begin{aligned} \mathrm{NCl}_{3}(g) & \longrightarrow \mathrm{NCl}_{2}(g)+\mathrm{Cl}(g) & \Delta H &=375 \mathrm{~kJ} / \mathrm{mol} \\ \mathrm{ONCl}(g) & \longrightarrow \mathrm{NO}(g)+\mathrm{Cl}(g) & \Delta H &=158 \mathrm{~kJ} / \mathrm{mol} \end{aligned} $$ Rationalize the difference in the values of $\Delta H$ for these reactions, even though each reaction appears to involve only the breaking of one $\mathrm{N}-\mathrm{Cl}$ bond. (Hint: Consider the bond order of the NO bond in ONCl and in NO.)

Values of measured bond energies may vary greatly depending on the molecule studied. Consider the following reactions:
$$
\begin{aligned}
\mathrm{NCl}_{3}(g) & \longrightarrow \mathrm{NCl}_{2}(g)+\mathrm{Cl}(g) & \Delta H &=375 \mathrm{~kJ} / \mathrm{mol} \\
\mathrm{ONCl}(g) & \longrightarrow \mathrm{NO}(g)+\mathrm{Cl}(g) & \Delta H &=158 \mathrm{~kJ} / \mathrm{mol}
\end{aligned}
$$
Rationalize the difference in the values of $\Delta H$ for these reactions, even though each reaction appears to involve only the breaking of one $\mathrm{N}-\mathrm{Cl}$ bond. (Hint: Consider the bond order of the NO bond in ONCl and in NO.)

Key Concepts

Bond Dissociation Energy

This concept refers to the energy required to break a specific bond in a molecule. Bond dissociation energies can vary greatly depending on the molecular environment, meaning that the energy needed to break an N–Cl bond in one compound might differ from that in another due to differences in adjacent bonding, electron distribution, and overall molecular structure.

Bond Order

Bond order is a measure of the number of bonding electron pairs shared between atoms. A higher bond order indicates a stronger bond, as observed in double or triple bonds. When a reaction alters the effective bond order of adjacent atoms—such as altering the NO bond’s order—the overall energy landscape changes, which can significantly influence the bond dissociation energy of bonds that might seem similar at first glance.

Molecular Structure and Electronic Effects

Molecular structure and the distribution of electrons significantly affect bond strengths. In some molecules, resonance stabilization or delocalization of electrons can increase or decrease the energy required to break a bond. When comparing similar bond breakings in different molecules, variations in these electronic effects, such as changes in adjacent bond orders or stabilization of reaction products, provide the rationale for differing enthalpy values.

Transcript

00:01 Okay, so to understand why the energy of the second reaction is actually lower than the one for the first reaction, we have to understand how the energy of reactions is actually calculated.

00:12 So for any reaction, we're going to have first to break bonds and then to form new bonds.

00:20 So the total energy of any reaction is going to be the energy required going in to break any bonds that need to be broken, and then we're going to subtract the energy that's released when new forms are formed, and therefore that gives us the overall energy of the reaction.

00:39 Now this is true for any chemical reaction.

00:41 So now let's focus on the two reactions that we have here.

00:44 Let's look at the first reaction.

00:46 In the first reaction, what we have is, and i'm just focusing on the bonds that are formed, i'm not showing the free electrons for these reactions because i think it makes it easier to understand if we just focus on the bonds between the different elements.

00:59 So if we look at the first molecule, ncl3, in order to reach a new molecule, ncl2, and release the chlorine atom, we're going to have to basically break one of these bonds.

01:12 So if we break this bond, for example, any of the three bonds release the same, we're going to reach two chlorine bonds for nitrogen and then the extra chlorine that's free from the bond that was broken.

01:26 So basically in the first reaction, all that we are doing is cutting a bond or breaking a bond from the reactant and then the new, the two new products are formed.

01:37 In the first reaction, we are not forming any new bonds on the product side, okay? that's key here.

01:44 We break, we break a bond on the reactant side, the ncl bond, and we do not form any new bonds on the product side, okay? so that's what happens for the first reaction.

02:09 Let's look at the second reaction.

02:11 In the second reaction, we're also actually breaking a bond between the ncl so that we can release the chlorine.

02:19 So we also break, we also break a bond, in this case also an ncl bond...

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