6 Chemical Kinetics

Introduction

“Diamonds are forever,” says a famous advertising campaign. Are they? A look at the table of enthalpy and entropy values tells us the thermodynamically stable form of carbon is actually graphite, not diamond. Further, thermodynamics calculations shows us that the conversion of diamond into graphite is a spontaneous process (\(\Delta _{rxn}G^\circ <0\)). So why do we not see diamond spontaneously forming graphite? Thus far we have focused on processes that are at equilibrium. However, most chemical processes occur at different rates before achieving equilibrium. While the conversion of diamond into graphite is a spontaneous process, it occurs so slowly that we do not observe the conversion taking place in human life scale.

In this chapter we focus on how to quantify reaction rates. Chemical kinetics is the description of the rate of chemical reactions. This is the rate at which the reactants are transformed into products. This may take place by abiotic or by biological systems, such as microbial metabolism. Since a rate is a change in quantity that occurs with time, the change we are most concerned with is the change in the concentration of reactants into new chemical compounds.

Below are some examples of the application of chemical kinetics in environmental geosciences.

Radioisotope Decay

Radioisotopes (unstable isotopes of elements - covered in the next chapter) are ubiquitous on the Earth. Radioisotopes are important for understanding the Earth in a number of ways. One way is through the use of radioisotope dating, which allows us to determine the age of rocks that make up the Earth. This is possible because some (or all) isotopes of elements, such as C, K, and U, are radioactive and will decay into other elements over time. By measuring the amount of the parent element and the amount of the daughter element, geologists can calculate the age of the rock or mineral. However, the decay rates of these radioisotopes vary with element and range from seconds to billions of years.

For example, \(\ce{^{14}C}\) (radioisotope of C) has a half-life (\(t_{1/2}\) or the time required to decay half of the original mass of the isotope) of \(\pu{5.7e3 y}\) and is very useful for dating fossils. \(\ce{^{238}U}\) (radioisotope of U) has a half-life of \(\pu{4.5e9 y}\) (4.5 billion years!) and can be used to study very old rocks. In contrast, \(\ce{^{99}Tc}\) has a very short half-life (\(\pu{6 h}\)) and is commonly used as a tracer in medical diagnostic purposes. So in each case decay rates are variable.

Chemical Weathering

Chemical weathering is a spontaneous process, but proceeds at different rates depending on the parent rocks and the conditions. Rocks containing carbonate minerals are more likely to weather rapidly compared to rocks containing silicate minerals. Likewise, humid environments are more likely to accelerate weathering rates compared to arid climates. Atmospheric contaminants such as \(\ce{SO2}\) from fossil fuel combustion can also increase weathering rates.

Fig. 45 Statues made from carbonate compounds such as limestone and marble typically weather slowly over time due to the actions of water, and 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 transfer of \(\ce{e-}\) from one element to another (as covered in the previous chapter) as shown in the reaction below.

\[\ce{ FeS2 (s) + 14 Fe^3+ (aq) + 8 H2O (l) -> 16 H+ (aq) + 15 Fe^2+ (aq) + 2 SO4^2- (aq) }\]

Such reactions are rate-limited and can be influenced by many factors including: (a) the nature of the reactants - for example, highly reactive substances tend to react more quickly than less reactive substances, (b) the concentration of the reactants - generally, the higher the concentration of reactants, the faster the reactions, (c) the presence of catalysts (substances that can increase the rate of a chemical reaction without being consumed in the process) - they provide an alternative pathway for the reaction that has a lower activation energy, (d) the temperature - generally, an increase in temperature leads to an increase in the rate of the reaction, and (e) the pressure - for gases, an increase in pressure can lead to an increase in the rate of the reaction.

Learning Goals

Learning Goals

The main goals for this chapter are to:

1. explain why reactions happen at different rates,

2. describe how reaction rates are quantified, and

3. identify where reaction kinetics are helpful in environmental geosciences.

References

1. Ch. 17 Kinetics - Chemistry: Atoms First | OpenStax