Kinetic study of a chemical reaction

Abstract

There are reactions so fast that they seem instantaneous, while others are so slow that they can last for hours, days, months and even years, as is the case with many radionuclide decays. An interesting example is the chemical reaction between the gases H₂ and O₂. When mixed under normal conditions in an isolated system, these gases react very slowly. However, if the mixture is activated by a spark or flame, an extremely fast uncontrolled exothermic reaction will occur, with a large release of energy, called a thermal explosion. This is what happened in the historic Hindenburg airship accident in 1937, in New Jersey, in the United States. On the other hand, when conducted under controlled burning conditions, this reaction can be used as a propellant. In fact, this is one of the most energetic chemical reactions known, capable of providing enough energy to launch large rockets, such as the Saturn V. The study of chemical kinetics (and, by implication, reaction speeds), as it has many scientific applications and practices, deserves to be highlighted at the beginning of Chemistry courses. The introduction of experiments involving kinetic studies leads students to recognize the importance of the effects of temperature on the speed of a reaction, as well as the influences that variations in concentration can cause. One of the many didactic experiments used for this purpose involves the decomposition reaction of thiosulfate ions in an acidic medium, with the formation of colloidal sulphur. Through this laboratory practice, it is possible to verify the influence of concentration and temperature on the reaction speed, measuring the time elapsed between the mixing of the reactants (t≅0) and the appearance of the first traces of turbidity in the reaction medium. This practice is interesting because it is quite simple to carry out and the results are very visible.