Silylene \(\left(\mathrm{SiH}_{2}\right)\) is a key intermediate in the thermal decomposition of silicon hydrides such as silane \(\left(\mathrm{SiH}_{4}\right)\) and disilane \(\left(\mathrm{Si}_{2} \mathrm{H}_{6}\right)\). Moffat et al. (H.K. Moffat, K.F. Jensen, and R.W. Carr, J. Phys. Chem. 95,145 (1991)) report \(\Delta_{1} H\) \(\left.\Theta_{\mathrm{SiH}_{2}}\right)=+274 \mathrm{kl} \mathrm{mol}^{-1} .\) If \(\Delta_{1} H^{\Theta}\left(\mathrm{SiH}_{4}\right)=+34.3 \mathrm{kj} \mathrm{mol}^{-1}\) and \(\Delta_{1} H^{\mathrm{O}}\left(\mathrm{Si}_{2}, \mathrm{H}_{6}\right)\) \(=+80.3 \mathrm{kj} \mathrm{mol}^{-1}\) (CRC Handbook (2004)), compute the standard enthalpies of the following reactions: a. \(\mathrm{SiH}_{4}(\mathrm{~g}) \rightarrow \mathrm{SiH}_{2}(\mathrm{~g})+\mathrm{H}_{2}(\mathrm{~g})\) b. \(\mathrm{Si}_{2} \mathrm{H}_{6}(\mathrm{~g}) \rightarrow \mathrm{SiH}_{2}(\mathrm{~g})+\mathrm{SiH}_{4}(\mathrm{~g})\)

When we're looking at chemical reactions, one particularly useful concept is Hess's Law. This principle states that the total enthalpy change in a chemical reaction is the same regardless of the number of steps the reaction is carried out in. Put differently, the total enthalpy change is path-independent.

Imagine you're climbing a hill, and whether you take a direct route or a winding path, you end up at the same height above sea level. Similarly, in chemistry, whether a reaction occurs in one step or multiple steps, the overall change in heat will be the same. This is incredibly useful when we can't measure enthalpy changes directly; we can simply add or subtract the enthalpies of known steps to find the enthalpy change of the overall process.

To apply Hess's law, you often rearrange, multiply, or divide the standard enthalpies of known reactions to reflect the process at hand. For instance, in the exercise you're working on involving silanes, Hess's Law allows us to find the enthalpy change for the decomposition of silane into silylene and hydrogen by subtracting the enthalpy of formation of silylene from the enthalpy of formation of silane.

The term enthalpy change refers to the heat exchange that occurs at constant pressure during any chemical reaction. It's denoted as \( \Delta H \) and represents the difference in heat content between the reactants and the products. In the context of chemical thermodynamics, it's a crucial concept since it helps us quantify the energy involved in chemical processes.

There are various types of enthalpy changes, like formation, combustion, vaporization, but we're focusing on reaction enthalpy here. When you read that the enthalpy change for the decomposition of \( \mathrm{SiH}_{4} \) is \( +34.3 \, \mathrm{kJ/mol} \) it means that \( 34.3 \, \mathrm{kJ} \) of heat is absorbed when one mole of silane is formed under standard conditions. A positive enthalpy change, as seen here, indicates an endothermic reaction, where heat is taken in from the surroundings. Conversely, a negative value signifies an exothermic process where heat is released.

The field of chemical thermodynamics deals with the relationships between heat and other forms of energy in chemical processes. It helps predict whether a reaction will occur spontaneously, how much energy is needed to drive the reaction, and what changes in energy the reactants and products will experience.

Key principles like the First Law of Thermodynamics, which states energy conservation, play a huge role in understanding heat changes during chemical reactions. Another important concept is Gibbs free energy, which combines enthalpy, temperature and entropy to predict whether a process will occur spontaneously. It's important to understand that even when a reaction is exothermic, it might not be spontaneous; other factors like entropy change also affect spontaneity.

As it relates to our example, chemical thermodynamics will involve looking at the broader scope of the reaction, considering also the bonds breaking and forming in the silicon hydrides and how the system exchanges energy with its environment during the decomposition process.

Focusing on the thermal decomposition of silanes, it's a process where compounds containing silicon and hydrogen—like silane (\( \mathrm{SiH}_{4} \) and disilane (\( \mathrm{Si}_{2}\mathrm{H}_{6} \) break down into simpler substances when heated.

During the thermal decomposition, the bonds within the molecules are broken, often requiring an input of energy. Decomposition reactions are fundamentally important in understanding the stability of compounds and how they might behave under high-temperature conditions. Silylene (\( \mathrm{SiH}_{2} \) is often a key intermediate—a transient species that forms during the transformation from reactants to products. Knowing the enthalpy changes of these reactions helps researchers and engineers design processes for material synthesis, specifically in the field of silicon-based materials, which have a multitude of applications in electronics and materials science.

For instance, in the provided exercise, understanding the enthalpy changes allows for the determination of whether energy must be supplied or if it will be released when silane or disilane decompose into silylene, and this information is crucial for safely handling these materials in industrial settings.