5.0 Unit 5 Overview: Kinetics

According to the College Board, "Unit 4 focused on chemical changes💥; in Unit 5 students will develop an understanding of the rates at which chemical changes occur and the factors that influence the rates📈. Those factors include the concentration of reactants, temperature, catalysts, and other environmental factors. Chemical changes are represented by chemical reactions, and the rates of chemical reactions are determined by the details of the molecular collisions.

Rates of change in chemical reactions are observable and measurable📏. When measuring rates of change, students are measuring the concentration of reactant or product species as a function of time. These chemical processes may be observed in a variety of ways and often involve changes in energy as well. In subsequent units, students will describe the role of energy⚡ in changes in matter."

Kinetics is the study of the rate of chemical reactions, essentially how fast a chemical reaction goes💨. While we can study whether a reaction will actually happen or not using many tools, such as solubility rules and activity series, one cannot use these same tools to study the specific rate at which a reaction will happen. This is analogous to knowing whether someone moved or not since the last 5 seconds have passed, versus knowing with which speed they moved in that time interval.

✍️About 7-9% of the exam tests your knowledge from this unit.

To study the rates at which reactions take place, physical chemists (who study chemistry from a mathematical and physics-motivated perspective) model reactions using the collision model, which we will discuss shortly. This is the same theory used to model ideal gases.

Certain reactions are then categorized depending on a parameter n, which essentially describes how the amount of atoms in a sample (concentration) affects the rate of collisions (as a higher concentration --> more atoms = more collisions). This parameter n is called the reaction order. For example n = 1 is equivalent to first order, n = 2 is equivalent to second order, and the same pattern holds for higher values of n. We will come to learn of how reaction rate can be defined using a "rate law", essentially an equation that describes how changes in concentration impact the rate of a reaction (Sneak peek: R = k[A]^n)

The order of a reaction must either be determined experimentally, or through analyzing a reaction mechanism, which deeper describes the chemical details of a reaction. Instead of analyzing the reactions macroscopically, this method allows chemists to see a relatively more accurate picture regarding how and why a chemical reaction happens, instead of simply analyzing what products come from what reactants.

These reaction mechanisms motivate finding overall rates, as the rates of each mechanistic step can be used to find the overall rate of the reaction by analyzing something called the rate-limiting step, which is conceptually similar to the concept of limiting reactants in stoichiometry.