A reaction releases \(900 \mathrm{~kJ}\) of energy. (a) Is the reaction endothermic or exothermic? (b) Which are higher in the reaction-energy profile, reactants or products? Explain. (c) Does this reaction go uphill or downhill in energy? (d) Draw the reaction-energy profile.

Understanding how energy changes throughout a chemical reaction is crucial for predicting the behaviors of substances. A reaction-energy profile is a graphical representation that displays how energy levels vary during a chemical reaction from reactants to products. Imagine a reaction-energy profile as a snapshot of energy hills and valleys through which the substances travel during a reaction.

Starting from the reactants, they may be positioned on a metaphorical hill or valley depending on whether energy is required to initiate the reaction or if energy is released. As observed in the textbook solution, where 900 kJ of energy is released, the reactants began at a higher 'energy hill.' The products end up in a lower 'energy valley,' indicating that energy was given off, much like a ball rolling downhill. This visual helps students and scientists understand the energy flow and stability of compounds in a given reaction.

The terms endothermic and exothermic describe the two categories of chemical reactions based on their energy changes. Exothermic reactions release energy into the surroundings, typically in the form of heat, light, or sound — picture a campfire radiating warmth on a chilly night. The exercise provided shows an exothermic reaction, as it releases 900 kJ of energy. Furthermore, because the energy of the products is less than the energy of the reactants, the latter are situated higher on the reaction-energy profile.

In contrast, endothermic reactions absorb energy from their surroundings. This could be akin to the energy needed to melt an ice cube in your hand, leaving the area feeling cold. If the exercise described an endothermic reaction, the energy profile would illustrate the reactants sitting lower, and the reaction rolling 'uphill' to reach the higher-energy products.

During any chemical reaction, the energy content of the reactants and products often changes, and understanding these chemical reaction energy changes is pivotal. These changes are tied to bond breaking and making — breaking bonds requires energy (endothermic process) and making bonds releases energy (exothermic process). In the case of the exercise, the reaction is exothermic, indicating that the bond-forming process has released a substantial amount of energy, hence lowering the overall energy of the products compared to the reactants.

A reaction's direction, often referred to as 'going uphill' or 'downhill' in terms of energy, indicates the relative energy levels of reactants and products. The 'downhill' energy route in the given solution aligns with an exothermic reaction, where the system's energy decreases, which consequently increases the surrounding's temperature. Grasping these concepts helps students understand why certain reactions require heat to proceed, while others are self-sustaining once started.