Question

One reason suggested to account 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 \(\mathrm{Si}-\mathrm{H}\) energy. Why would a similar mechanism not be expected to be very important in the decomposition of long carbon chains?

Short answer

The decomposition of long silicon chains has a transition state with an activation energy of 210 kJ/mol, which is less than the energy required to break Si-Si or Si-H bonds. This makes it easier for the chain to decompose, leading to instability. However, in long carbon chains, the C-C and C-H bonds are stronger than their silicon counterparts, requiring a higher activation energy to break these bonds. This higher energy makes the transition state harder to achieve, resulting in increased stability of long carbon chains and making a similar mechanism less important for their decomposition.

Step-by-step solution

Understand the transition state of long chains of silicon atoms

The given transition state for the decomposition of long silicon chains has an activation energy of 210 kJ/mol. This energy is required to break the bonds and overcome various energy barriers during the reaction, causing instability in the long chains of silicon atoms.

Discuss the activation energy and its influence on the stability of long chains

The activation energy of 210 kJ/mol is less than the energy required to break either the Si-Si or Si-H bonds in the silicon atoms' long chains. This means that it's easier for the transition state to occur and the long chains of silicon atoms to decompose, leading to their instability.

Compare the properties of silicon and carbon atoms

Silicon and carbon atoms belong to the same group in the periodic table (group 14), and they both form covalent bonds. However, their properties differ due to the significant difference in size and electronegativity. While silicon is a metalloid with relatively weaker Si-Si bonds compared to the stronger C-C bonds in carbon, carbon is a non-metal with relatively stronger C-C and C-H bonds.

Explain why the mechanism would not be expected for long carbon chains

Since the C-C and C-H bonds in carbon atoms are stronger than the Si-Si and Si-H bonds in silicon atoms, the activation energy required to break these bonds in long carbon chains would be higher than that found in long silicon chains. This higher activation energy makes it harder for the transition state to occur, leading to increased stability in long carbon chains. Therefore, a similar mechanism would not be expected to be very important in the decomposition of long carbon chains.

Key concepts

Transition State

A transition state is a fleeting, high-energy state during a chemical reaction, marking the point where bonds are breaking and forming simultaneously. Imagine a hill; the transition state is the peak you must climb over. For silicon chains, the transition state represents the arrangement of atoms right before bond reorganization. The transition state is essential in chemical reactions because it determines the path and rate of the reaction. The characteristic energy required to form this transition state significantly influences how readily a silicon chain decomposes.
Silicon chains are unstable because reaching their transition state requires less energy than breaking the actual bonds present. This means that decomposing silicon chains can be quick since there's not much of a hill to climb over, energetically speaking. In understanding silicon atom chains, knowing how the transition state operates clarifies why these chains lack stability over, say, long carbon chains.

Activation Energy

Activation energy is the minimum energy required for a chemical reaction to occur. Think of it as the push needed to get a boulder rolling down a hill. Without this energy, the reaction cannot proceed. For silicon atoms, the activation energy determined for their transition state is 210 kJ/mol, which is relatively low.
This lower threshold suggests that the decomposition of silicon chains can readily happen, as less energy is needed to reach the transition state. Although silicon possesses bonds like Si-Si and Si-H, the activation energy needed to overcome these barriers is less than the energy of the individual bond strengths themselves. This ease of breaking into the transition state is why long chains of silicon are thought to be inherently unstable, as opposed to carbon chains that require much higher activation energy due to stronger bond formations.

Bond Stability

Bond stability refers to how strongly atoms are held together within a molecule. Stable bonds resist being broken apart, while unstable bonds can easily transition into other configurations. In silicon atom chains, stability is compromised due to lower activation energy requirements. The Si-Si bonds in these chains are weaker than the carbon-carbon (C-C) bonds. Thus, less energy is required to disrupt these bonds.
Key factors affecting bond stability include:

• Bond Strength: Stronger bonds need more energy to break.
• Molecular Structure: How atoms are arranged affects stability.
• Energy Barriers: Higher barriers mean more stability.

Silicon’s weaker bonds lead to less stability when compared with carbon, which forms stronger, more stable bonds. This fundamental difference explains the structural frailty of silicon chains under conditions that would leave carbon chains largely intact.

Covalent Bonds

Covalent bonds involve the sharing of electron pairs between atoms, a fundamental concept in chemistry. Both silicon and carbon form covalent bonds, yet their characteristics differ substantially due to variations in atomic size and electronegativity. Silicon, being larger and less electronegative than carbon, forms longer and relatively weaker covalent bonds.
Key properties of covalent bonds:

• Sharing of Electrons: Core mechanism of bond formation.
• Bond Length: Affected by atomic size; longer in silicon.
• Bond Strength: Tends to be stronger in carbon.

In long silicon chains, the longer bond lengths and reduced strength relative to carbon bonds lead to easier decomposition. Contrastingly, carbon’s shorter, more robust covalent bonds contribute to the stability and resilience of carbon-based structures, minimizing decomposition under similar energetic conditions.