4 Acids and Bases#
Introduction#
An acid-base reaction involves the transfer of a hydrogen ion (\(\ce{H+}\)) from one chemical species to another. These reactions are crucial to a wide range of natural and human-made processes. They play a significant role in the chemical transformations occurring within the human body, lakes, and oceans and in the industrial production of fertilizers, pharmaceuticals, and other substances essential to society.
In this chapter, we explore the fundamental geochemistry of acid-base reactions. Below are examples of where acids and bases are found in natural and human-influenced environments.
Atmospheric \(\ce{CO2}\) and \(p\ce{H}\) in Natural Water#
Pure water with no dissolved ions or gases is considered “neutral,” meaning it is neither acidic nor basic. However, rainwater is not pure because it comes into contact with the Earth’s atmosphere, which contains various gases. While some atmospheric gases are relatively nonreactive in water, others are highly reactive. For instance, carbon dioxide (\(\ce{CO2}\)) reacts with water to form carbonic acid (\(\ce{H2CO3}\)), which makes rainwater acidic.
The sequence of \(\ce{CO2(g)}\)-caused rainwater acidification reactions is as follows. These important reactions will be explored further in this chapter.
The produced \(\ce{H+}\) ions are the source of acidity in rainwater. Typically, the presence of (\(\ce{H+}\)) lowers the \(p\ce{H}\) of rainwater to approximately 5.7. As we learned from thermodynamic principles, increasing concentrations of (\(\ce{CO2(g)}\)) in these reactions favor the formation of more \(\ce{H+}\) ions, leading to increased water acidity. This rise in acidity in ocean water has been linked to a decrease in the concentration of carbonate ions (\(\ce{CO3^2-}\)), which are crucial for marine calcifying organisms, such as corals. For more information, see Ocean Acidification | National Climate Assessment (globalchange.gov). Thermodynamic principles also predict that a decrease in \(p\ce{H}\) will result in lower acidity in water.
Acid Rain#
Human activities, primarily those related to fossil fuel-based power generation and automobile emissions, are the leading causes of acid rain. Fossil fuels typically contain 1-3% sulfur, nitrogen, and other minerals. The combustion of coal and fuel releases significant amounts of nitrogen oxides (\(\ce{NOx}\), which includes both \(\ce{NO}\) and \(\ce{NO2}\)) and sulfur dioxide (\(\ce{SO2}\)) into the atmosphere. These gases mix and react with rainwater, \(\ce{O2}\), and other chemicals to form acidic pollutants, classified as acid rain. Sulfur dioxide and nitrogen oxides dissolve quickly in water and can be transported over long distances by the wind.
The sequence of \(\ce{SO2(g)}\)-caused rainwater acidification reactions is as follows:
Sequence of \(\ce{NO_x}\)-caused rainwater acidification reactions:
The release of \(\ce{H+}\) into water is the cause of increased acidity in water. These reactions produce more acid than the dissolution of \(\ce{CO2(g)}\) in water, causing a drop of \(p\ce{H}\) to alarming levels in affected areas.
Whatever happened to acid rain? - Joseph Goffman | TED-Ed
Extraction of metals from ore materials (typically oxides or sulfides of metals) usually involves heating the ore at high temperatures to separate the base metal from the remaining elements. This process is called smelting and has probably been applied by human civilizations as far back as 5500 BCE. Smelting of sulfide minerals (\(\ce{CuS}\), \(\ce{PbS}\), \(\ce{FeS2}\), etc.) releases into the atmosphere causing acid rain, if the emissions are not controlled (e.g., see Fig. 36)
Fig. 36 Rampant copper mining and smelting operations during the 19th century caused widespread acid rain and environmental damage in many parts of the world. In the above image, a train is bringing copper ore out of the mines in Ducktown, Tennessee. Sulfuric acid fumes from copper smelting copper destroyed all vegetation and denuded the landscape. Image source: Ducktown, Tennessee - Wikipedia#
Acid Mine Drainage#
Acid mine drainage refers to surface water or groundwater that flows from an active or abandoned mining operation. This drainage can vary in quality; it may be similar to natural waters or contaminated with leftover materials from mining activities. Polluted mine drainage is often highly acidic and may contain elevated levels of toxic heavy metals. Generally, the higher the acidity of the water, the more harmful it is to living organisms.
Mine drainage occurs from a chemical reaction between water and rocks that contain sulfur-bearing minerals. This reaction produces water rich in sulfuric acid and dissolved iron, as illustrated in the reactions below. When the iron settles out of the water, it forms red, orange, or yellow sediments at the bottoms of streams. The acidic runoff can also dissolve heavy metals such as copper, lead, and mercury, contaminating groundwater or surface water. The activity of certain bacteria can accelerate the rate and extent of acid mine drainage.
Fig. 37 Acid mine drainage can change the color of a stream into red or orange. Image source: Mine Drainage | U.S. Geological Survey (usgs.gov)#
Learning Goals#
Learning Goals
The main goals for this chapter are to:
learn to identify acids and base properties of different chemical compounds
apply acid and base properties to describe \(p\ce{H}\) of aqueous solutions
understand the role weak acids play in natural environments