Consider the generic reaction: $$ \mathrm{A}+2 \mathrm{~B} \longrightarrow \mathrm{C} \Delta H_{\mathrm{ran}}=-55 \mathrm{~kJ} $$ Determine the amount of heat emitted when each amount of reactant completely reacts (assume that there is more than enough of the other reactant). (a) \(1 \mathrm{~mol} \mathrm{~A}\) (b) 2 mol \(\mathrm{A}\) (c) \(1 \mathrm{~mol} \mathrm{~B}\) (d) \(2 \mathrm{~mol} \mathrm{~B}\)

Chemical reactions are the processes by which substances, known as reactants, transform into different substances called products. Understanding the nature of chemical reactions is essential for predicting how compounds will interact under various conditions.

In a reaction, the alignment of atoms and molecules changes, which can result in a release or absorption of energy. The reaction in the given exercise is an example of this principle, where reactant A combines with reactant B to form a new product C. In such reactions, the law of conservation of mass tells us that no atoms are lost or gained; they are simply rearranged to create new compounds.

Identifying Reactants and Products

When looking at a chemical equation, it's crucial to identify the reactants and products first. In our exercise, A and B are the reactants, whereas C is the product. The coefficients in front of the reactants, such as the '2' in front of B, indicate the stoichiometry, or the ratio of reactants that react together.

Stoichiometry is the branch of chemistry that quantitatively relates the amounts of substances consumed and produced in chemical reactions. It is based on the law of conservation of mass and the concept of the mole.

With stoichiometry, we can calculate the amounts of reactants needed to produce a desired quantity of product, or vice versa. For instance, the provided exercise showcases a balanced chemical reaction in which 1 mole of A reacts with 2 moles of B to produce C.

Importance of Balance

The equation must be balanced to apply stoichiometry correctly, meaning the number of atoms of each element must be the same on both the reactant and product sides. Our exercise features a balanced equation, which is critical for accurate calculations. Students should always ensure equations are balanced before applying stoichiometric principles.

The heat of reaction, or the enthalpy change (H), is the amount of heat absorbed or released during a chemical reaction at constant pressure. It's an essential aspect of thermochemistry, as it helps predict energy changes during reactions.

In the given problem, the H is indicated as -55 kJ, representing an exothermic reaction where heat is released into the surroundings when the reactants A and B are converted to product C. The negative sign signifies the release of heat.

Exothermic vs. Endothermic

Exothermic reactions release heat, as our example does, while endothermic reactions absorb heat. This distinction significantly affects how chemical processes are managed in various industrial applications and natural phenomena.

Thermochemistry studies the relationships between chemical reactions and energy changes involving heat. It's concerned with the measurement and understanding of the energy changes that accompany reactions.

The concepts of enthalpy, entropy, and Gibbs free energy are central to this field. In practical terms, knowing the thermochemical aspects of a reaction can inform us about the feasibility of a process and its potential impact on the environment.

Applications

Thermochemistry is used in designing energy-efficient processes in the chemical industry, understanding metabolic pathways in biology, and in environmental impact assessments. It plays a key role in the development of new materials and energy sources, emphasizing the importance of calculating and interpreting heat changes in reactions.