Question

Values of measured bond energies may vary greatly depending on the molecule studied. Consider the following reactions: $$ \begin{aligned} & \mathrm{NCl}_{3}(g) \longrightarrow \mathrm{NCl}_{2}(g)+\mathrm{Cl}(g) & \Delta E &=375 \mathrm{kJ} / \mathrm{mol} \\ \mathrm{ONCl}(g) & \longrightarrow \mathrm{NO}(g)+\mathrm{Cl}(g) & & \Delta E=158 \mathrm{kJ} / \mathrm{mol} \end{aligned} $$ Rationalize the difference in the values of \(\Delta E\) 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.)

Short answer

The difference in bond energy values for the two reactions involving nitrogen and chlorine compounds can be rationalized based on the difference in bond orders of the NO bonds. In the NCl3 reaction, the bond breaking primarily involves the N-Cl bond, with an energy of 375 kJ/mol. In the ONCl reaction, although it also involves breaking an N-Cl bond, the simultaneous formation of a double bond between nitrogen and oxygen (in NO) releases energy, leading to a lower overall bond energy of 158 kJ/mol.

Step-by-step solution

Understand the bond formation in the given molecules.

First, we need to recall how bond formation occurs in molecules. A bond order indicates the number of chemical bonds formed between a pair of atoms. In a single bond, the bond order is 1; in a double bond, the bond order is 2; and in a triple bond, the bond order is 3. In NCl3, there are three single N-Cl bonds, while in ONCl, there is one N-Cl bond, one O-N bond, and one O-Cl bond. In the NO molecule, the bond order is 2, so the nitrogen and oxygen atoms are double-bonded.

Analyze the reactions and bond breaking process.

Now let's analyze each reaction: 1. NCl3(g) -> NCl2(g) + Cl(g): In this reaction, one N-Cl bond from NCl3 is broken, resulting in NCl2 and a free Cl atom. 2. ONCl(g) -> NO(g) + Cl(g): In this reaction, the N-Cl bond in ONCl is broken, leaving behind NO with a double bond between the nitrogen and oxygen atoms, and a free Cl atom.

Compare the bond orders of the NO bond.

In the ONCl molecule, the nitrogen forms a single bond with the oxygen and a single bond with the chlorine. This means that the NO bond order in ONCl is 1. In the NO molecule, the nitrogen forms a double bond with the oxygen, so the NO bond order in NO is 2.

Rationalize the difference in bond energy values.

The difference in bond energy values for these two reactions can be rationalized based on the difference in bond orders of the NO bonds. In the first reaction (NCl3), the bond breaking primarily involves the N-Cl bond, and the energy required to break this single bond is 375 kJ/mol. In the second reaction (ONCl), although it also involves breaking an N-Cl bond, the simultaneous formation of a double bond between nitrogen and oxygen (in NO) releases energy. This is because bond formation is an exothermic process, which compensates for the energy required to break the N-Cl bond. As a result, the overall bond energy for the second reaction is lower, at 158 kJ/mol. In conclusion, the difference in the bond energy values for these two reactions stems from the difference in the bond orders of the NO bonds in ONCl and NO molecules. The simultaneous formation of a double bond between nitrogen and oxygen (in NO) during the second reaction compensates for the energy needed to break the N-Cl bond, leading to a lower overall bond energy value.

Key concepts

Chemical Bonds

'Chemical bonds' are the glue that holds atoms together in molecules. They are formed when atoms share or transfer electrons. Bonds can be broadly classified into three types: ionic, covalent, and metallic. In our textbook example, we're exploring covalent bonds, where pairs of electrons are shared between atoms to achieve stability.

Understanding bond energies—the energy required to break a bond—is crucial in studying chemical reactions. High bond energies typically mean a bond is strong and requires more energy to break. Conversely, a lower bond energy suggests a weaker bond. This knowledge allows us to predict the reactivity and stability of different molecules, significantly contributing to advancements in fields such as chemistry and materials science.

Bond Order

The concept of 'bond order' is a simple yet powerful indicator of the strength between two bonded atoms. It represents the number of chemical bonds between a pair of atoms: a higher bond order suggests a stronger bond and higher bond energy. For instance, a single bond has a bond order of 1, a double bond has a bond order of 2, and a triple bond has a bond order of 3.

This contributes to the understanding of why molecules with double or triple bonds are often less reactive; those bonds are simply harder to break due to their higher bond order, and thus, higher bond energy. In the exercise, analyzing the bond orders gives insight into the energy differences observed and helps explain the chemistry behind the reactions.

Exothermic Process

An 'exothermic process' is one that releases energy, usually in the form of heat, into the surrounding environment. The formation of chemical bonds is an exothermic process because when a bond forms, potential energy is lowered, and this excess energy is released. This contrasts with endothermic processes where energy is absorbed.

Understanding exothermic reactions is essential for grasping why certain reactions release heat and others absorb it. Moreover, it explains why in our exercise, the formation of the NO bond in the second reaction releases energy, which in turn reduces the overall energy required for the reaction, leading to a smaller \( \Delta E \).

Molecular Structure

The 'molecular structure' refers to the three-dimensional arrangement of atoms within a molecule. It defines many properties of a compound, such as reactivity, polarity, phase of matter, color, and magnetism. The molecular structure is determined by the chemical bonds that hold the atoms together and by the bond angles and bond lengths that are characteristic for different types of bonds and molecules.

In the context of the given reactions, the molecular structure paints a comprehensive picture of how the atoms reorganize during the reaction and provides insight into the stability and reactivity of the molecules involved.

Reaction Analysis

Performing a 'reaction analysis' involves studying the changes that occur at the atomic and molecular levels during a chemical reaction. This includes looking at which bonds break and form, the energy changes that accompany the reaction, and the kinetics or rate at which the reaction occurs. Reaction analysis is vital for predicting reaction outcomes and designing chemical processes that are safe, efficient, and environmentally friendly.

Applying reaction analysis to our textbook problem reveals not just the difference in bond energies due to bond order but also illustrates the importance of taking into account the complete changes within the molecular structure—highlighting the transformation from reactants to products and the associated energy profiles.