Kinetics is the branch of physical chemistry that informs us on rates of processes as well as experimental methods to measure rates and reaction progress. In this section we introduce the basic concept of a rate law and associated rate constant. In this course we will focus exclusively on first and second order rate constants. However, we should be aware that kinetics accounts for processes that fall outside this scope. We discuss how to determine the reaction order experimentally based on the isolation method. Experimental measurements using spectroscopy, conductivity, pressure and so on are needed to quantitatively describe the rates and derive the rate laws.
The isolation method is a way to determine the reaction order. One changes only one reactant concnetration while keeping the others constant. Then one can compare the rates as a function of the change in concentration. The analysis of these ratios leads to a determination of the exponent in the rate law, i.e. the reaction order for that reactant. By carrying out this procedure separately for each reactant we can determine the rate law and overall reaction order.
Many first order processes involving a single species can be conveniently described in terms of the half-life formalism. Radioactive decay is one important example, but there are many others. In this section the significance of exponential kinetics is discussed in terms of the natural life time t = 1/k, where k is the rate constant and the half life t1/2 = ln(2)/k.
When a system is perturbed away from equilibrium it will approach equilibrium again with an observed rate constant that is the sum of the forward and reverse rate constants for the process. This result is proven in the video below and justified using the Principle of Microscopic Reversibility. This principle states that the pathway for the forward and reverse reaction must be the same. Therefore, there is an intrinsic connection between the forward and reverse rate constants. This leads to the equatoin K = kf/kr, which connects the equilibrium constant for the forward and reverse rate constants.
