6 Chemical Kinetics#
Introduction#
“Diamonds are forever,” claims a well-known advertising campaign. But are they? A review of enthalpy and entropy values reveals that the thermodynamically stable form of carbon is graphite, not diamond. Thermodynamic calculations indicate that the conversion of diamond into graphite is a spontaneous process, as evidenced by the condition \(\Delta _{rxn}G^\circ < 0\). So why don’t we see diamonds spontaneously transforming into graphite?
Thus far, we have focused on processes that reach equilibrium. However, most chemical reactions occur at different rates before achieving this state. While the conversion of diamond to graphite is spontaneous, it happens so slowly that we do not see this transformation occurring within the human timescale.
In this chapter, we focus on how to quantify reaction rates. Chemical kinetics describes the rate at which chemical reactions occur, specifically the speed at which reactants are transformed into products. This transformation can happen through abiotic processes or biological systems, such as microbial metabolism. Since a rate represents a change in quantity over time, our main concern is the change in the concentration of reactants as they convert into new chemical compounds.
Below are some examples of the application of chemical kinetics in environmental geosciences.
Radioisotope Decay#
Radioisotopes, unstable isotopes of elements, are found everywhere on Earth. They play a crucial role in helping us understand the planet in several ways. One significant method is radioisotope dating, which lets us determine the age of Earth’s rocks. This technique works because certain isotopes of elements, such as carbon (C), potassium (K), and uranium (U), are radioactive and decay into other elements over time. Geologists can calculate the age of the rock or mineral by measuring the quantities of the parent and daughter elements. However, the decay rates of these radioisotopes vary by element and can range from just a few seconds to billions of years.
For example, the radioisotope \(\ce{^{14}C}\) has a half-life (denoted as \(t_{1/2}\), which is the time required for half of the original mass of the isotope to decay) of approximately \(\pu{5.7e3 y}\) (5,700 years) and is very useful for dating fossils. In contrast, the radioisotope \(\ce{^{238}U}\) has a much longer half-life of about \(\pu{4.5e9 y}\) (4.5 billion years), making it suitable for studying very old rocks. On the other hand, \(\ce{^{99}Tc}\) has a very short half-life of just \(\pu{6 h}\) (6 hours) and is commonly used as a tracer in medical diagnostics. Thus, decay rates can vary significantly among different isotopes.
Chemical Weathering#
Chemical weathering is a natural process that occurs at varying rates depending on the type of parent rocks and environmental conditions. Rocks that contain carbonate minerals tend to weather more quickly than those made up of silicate minerals. Also, humid environments can significantly accelerate the weathering process compared to arid climates. Furthermore, atmospheric pollutants, such as sulfur dioxide (\(\ce{SO2}\)) from the combustion of fossil fuels can also increase weathering rates.
Fig. 46 Statues made from carbonate compounds such as limestone and marble typically weather slowly over time due to the actions of water, as well as thermal expansion and contraction. However, pollutants like sulfur dioxide can accelerate weathering. As the concentration of air pollutants increases, deterioration of limestone occurs more rapidly. Image source: 17.2 Factors Affecting Reaction Rates - Chemistry: Atoms First | OpenStax#
Redox Processes#
Redox reactions involve the transfer of electrons from one element to another, as discussed in the previous chapter, as shown in the reaction below.
Chemical reactions are often limited by their rates and can be influenced by several factors, including (a) the nature of the reactants—highly reactive substances typically react more quickly than those that are less reactive; (b) the concentration of the reactants—generally, higher concentrations lead to faster reactions; (c) the presence of catalysts—substances that increase the rate of a chemical reaction without being consumed in the process. Catalysts provide an alternative pathway for the reaction with a lower activation energy; (d) the temperature—increasing the temperature usually results in a faster reaction rate; and (e) the pressure—for gaseous reactions, increasing the pressure can enhance the reaction rate.
Learning Goals#
Learning Goals
The main goals for this chapter are to:
explain why reactions happen at different rates,
describe how reaction rates are quantified, and
identify where reaction kinetics are helpful in environmental geosciences.