Question

One reason suggested for the instability of long chains of silicon atoms is that the decomposition involves the transition state shown below: The activation energy for such a process is $210\mathrm{kJ}/\mathrm{mol}$, which is less than either the $\mathrm{Si}-\mathrm{Si}$ or the $\mathrm{Si}-\mathrm{H}$ bond energy. Why would a similar mechanism not be expected to play a very important role in the decomposition of long chains of carbon atoms as seen in organic compounds?

Short answer

The main reason why a similar decomposition mechanism to that of silicon chains is not expected to play a significant role in the decomposition of long chains of carbon atoms in organic compounds is because carbon chains have higher bond energies and a higher activation energy for decomposition. This results in carbon chains being more stable and resistant to decomposition compared to silicon chains.

Step-by-step solution

Identify the decomposition process

The decomposition process of silicon chains involves a transition state with an activation energy of 210 kJ/mol. This activation energy is less than the Si-Si or Si-H bond energies. This fact implies that the decomposition process is easier/more feasible due to having less energy needed to break the bonds.

Discuss the stability of carbon chains

When comparing silicon chains to carbon chains in organic compounds, carbon chains are more stable due to having stronger C-C and C-H bonds and a lower activation energy for decomposition. Carbon chains have higher bond energies, which means that the energy required to break the interactions between the atoms is higher, leading to more stable structures.

Examine the decomposition mechanism of carbon chains

Similar to silicon chains, carbon chains can also undergo chemical reactions and decompositions. However, these processes usually involve different mechanisms than those in silicon chains, and often they require a higher activation energy. This higher activation energy makes the carbon chains more stable and resistant to decomposition.

Conclusion

A similar decomposition mechanism to that of silicon chains is not expected to play a very important role in the decomposition of long chains of carbon atoms in organic compounds. The reason is that carbon chains have higher bond energies and a higher activation energy for decomposition, making them more stable and resistant to decomposition compared to silicon chains.

Key concepts

Silicon Bonds

Silicon bonds, specifically the Si-Si and Si-H bonds, play a critical role in understanding the decomposition of silicon-based compounds. Silicon, being located in group 14 of the periodic table, shares some similarities with carbon. However, its bonds are generally weaker than those of carbon. This means that the energy required to break silicon bonds in a chemical reaction is relatively lower.
The Si-Si bond energy is usually around 226 kJ/mol, while the Si-H bond energy is roughly 297 kJ/mol. Since the activation energy for the decomposition of silicon chains is 210 kJ/mol, which is lower than both the Si-Si and Si-H bond energies, this suggests a predisposition for easier decomposition of silicon-based structures.
This lower bond energy implies a greater reactivity and a tendency towards instability in larger silicon structures, a key factor in their decomposition process.

Carbon Chains

Carbon chains are characteristic of organic compounds and have a foundational role in organic chemistry. Unlike silicon, carbon atoms form robust C-C and C-H bonds, making carbon chains far more stable. The bond energy of C-C is around 348 kJ/mol and for C-H, it is approximately 413 kJ/mol.
This difference in bond energies when compared to silicon means that carbon forms significantly more stable and durable chains. The higher energy required to break these bonds translates to fewer spontaneous decompositions under normal conditions.
The robust nature of carbon chains is due to the small size of the carbon atom and the strong overlap of its orbitals, which results in a stable and durable lattice that resists breakdown. As a consequence, carbon chains are prevalent in countless organic compounds, including vital biological macromolecules.

Activation Energy

Activation energy is an essential concept in the study of chemical reactions, representing the minimum energy needed to initiate a chemical process. In the context of silicon decomposition, the activation energy is 210 kJ/mol. This value is crucial as it is lower than the Si-Si and Si-H bond energies, facilitating easier decomposition of silicon chains.
Activation energy acts as a threshold that the reacting species must overcome to proceed to the transition state and further to the products. The lower the activation energy, the more readily the reaction occurs. This explains why silicon structures are less stable and decompose more readily than their carbon counterparts.
In contrast, carbon compounds exhibit higher activation energies for decomposition, which correlates with their higher bond energies. As a result, carbon structures demonstrate greater resistance to decomposition, showcasing how activation energy impacts chemical stability and reaction feasibility.

Bond Energy Stability

Bond energy stability refers to how strongly atoms are held together within a molecule, which significantly influences the molecule's overall stability. For silicon compounds, the bond energies like Si-Si and Si-H are relatively low, promoting instability in long silicon chains. This leads to easier decomposition since less energy is needed to disrupt the bonds.
In contrast, carbon bonds are much stronger. These include the aforementioned C-C and C-H bonds, offering considerable stability and resistance to decomposition, thereby supporting the structural integrity of carbon-based molecules.
Bond energy stability not only affects how readily a compound may decompose but also influences its overall behavior in chemical reactions. Understanding these concepts helps explain why certain molecules decompose more readily than others and why some are used over others in various industrial applications.