15: Chemical Kinetics
From thermodynamics, we can determine the spontaneity of a reaction and its extent, using \(\Delta G\) and \(K_{\text{eq}}\), respectively. However, thermodynamics does not provide any information on how fast the reaction is going to happen. For example, while the reaction that converts solid carbon from its diamond allotropic form into hexagonal graphite is thermodynamically spontaneous, it is so slow as to be virtually non-existent. Diamond is effectively a meta-stable phase. The speed of a chemical reaction is the subject of a branch of physical chemistry called chemical kinetics.
A chemical kinetics study aims to find the rate of a reaction and to find the microscopic steps that compose it, determining its mechanism .
-
- 15.1: Differential and integrated rate laws
- The rate law of a chemical reaction is an equation that links the initial rate with the concentrations (or pressures) of the reactants. Rate laws usually include a constant parameter, k , called the rate coefficient, and several parameters found at the exponent of the concentrations of the reactants, and are called reaction orders. The rate coefficient depends on several conditions, including the reaction type, the temperature, the surface area of an adsorbent, light irradiation, and others.
-
- 15.2: Complex Rate Laws
- It is essential to specify that the order of a reaction and its molecularity are equal only for elementary reactions. Reactions that follow complex laws are composed of several elementary steps, and they usually have non-integer reaction orders, for at least one of the reactants.
-
- 15.3: Experimental Methods for Determination of Reaction Orders
- To experimentally measure the reaction rate, we need a method to measure concentration changes with respect to time. The simplest way to determine the reaction rate is to monitor the entire reaction as it proceeds and then plot the resulting data differently until a linear plot is found.
-
- 15.4: Temperature Dependence of the Rate Coefficients
- The dependence of the rate coefficient, k , on the temperature is given by the Arrhenius equation. This formula was derived by Svante August Arrhenius (1859–1927) in 1889 and is based on the simple experimental observation that every chemical process gets faster when the temperature is increased. Working on data from equilibrium reactions previously reported by van ’t Hoff, Arrhenius proposed the following simple exponential formula to explain the increase of k when T is increased.