To produce silicon, used in semiconductors, from sand \(\left(\mathrm{SiO}_{2}\right)\), a reaction is used that can be broken down into three steps: $$ \begin{aligned} \mathrm{SiO}_{2}(s)+2 \mathrm{C}(s) \longrightarrow \mathrm{Si}(s)+2 \mathrm{CO}(g) & & \Delta H=689.9 \mathrm{~kJ} \\ \mathrm{Si}(s)+2 \mathrm{Cl}_{2}(g) \longrightarrow \mathrm{SiCl}_{4}(g) & & \Delta H=-657.0 \mathrm{~kJ} \\ \mathrm{SiCl}_{4}(g)+2 \mathrm{Mg}(s) \longrightarrow 2 \mathrm{MgCl}_{2}(s)+\mathrm{Si}(s) & & \Delta H=-625.6 \mathrm{~kJ} \end{aligned} $$ (a) Write the thermochemical equation for the overall reaction for the formation of silicon from silicon dioxide; \(\mathrm{CO}\) and \(\mathrm{MgCl}_{2}\) are byproducts. (b) What is \(\Delta H\) for the formation of one mole of silicon? (c) Is the overall reaction exothermic?

Enthalpy change, denoted as \( \Delta H \), is a measure of the heat energy released or absorbed during a chemical reaction under constant pressure. Calculating enthalpy change requires understanding the concept of thermochemical equations, which link the change in enthalpy to the quantities of reactants and products in a balanced chemical equation.

For a step-by-step process, we add the \( \Delta H \) values of all individual reactions that make up the overall chemical process. If the sum is negative, the reaction releases heat to its surroundings, making it exothermic. In contrast, a positive total indicates that the reaction absorbs heat, which is endothermic. Providing a cumulative energy change figure offers a straightforward way for students to assess the energy dynamics of complex chemical reactions, enhancing their grasp on thermal changes during such processes.

An exothermic reaction is a type of chemical reaction that releases energy in the form of heat to its surroundings, typically resulting in a temperature increase in the reaction medium. This kind of reaction is characterized by a negative \( \Delta H \) value. A common example is the combustion of fuels, like burning wood or gasoline.

Understanding exothermic reactions is crucial for students because it's not just about the energy release; it also involves how molecular bonds are broken and formed, and the factors that drive these reactions energetically favorable. This concept underlines everyday phenomena such as heating, lighting, and biological processes. By learning about exothermic reactions, students can make sense of energy conservation and better evaluate the implications of chemical reactions in both industrial and environmental contexts.

Breaking down complex reactions into a series of steps is a fundamental technique in chemistry called the reaction mechanism. Each step represents a distinct, simpler reaction that features its own reactants, products, and enthalpy change. When pieced together, these steps reveal the intricacies of the complete reaction from start to finish.

By dissecting reactions into manageable parts, students can delve deeper into the nuances of chemical transformations, analyzing aspects like reaction intermediates, rate-determining steps, and the overall energy profile of the reaction. This step-wise approach not only simplifies problem-solving but also equips learners with the analytical skills to understand how reactions progress at the molecular level, fostering a richer and more holistic appreciation of chemical processes.