4 Acids and Bases#
Introduction#
An acid-base reaction is one in which a hydrogen ion, \(\ce{H+}\), is transferred from one chemical species to another. Such reactions are of central importance to numerous natural and anthropogenic processes, ranging from the chemical transformations that take place within human bodies, lakes and oceans, to industrial-scale production of fertilizers, pharmaceuticals, and other substances essential to society.
In this chapter we explore the basic geochemistry of acid-base reactions. Below are some examples of where acids and bases are encountered in natural and anthropogenic-influenced environments.
Atmospheric \(\ce{CO2}\) and \(p\ce{H}\) in Natural Water#
Pure water containing no dissolved ions or gases is considered to be “neutral” or neither acidic not basic. However, rainwater is not pure water since it comes in contact with Earth’s atmosphere which contains many gases. While atmospheric gases such as and are relatively nonreactive in water, other gases such as are very reactive. 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 \(\ce{H+}\) are the source of acidity to rainwater. The presence of \(\ce{H+}\) typically lowers \(p\ce{H}\) of typical rainwaters to \(\approx 5.7\). As we learned from thermodynamic principles, increasing \(\ce{CO2(g)}\) concentrations in these sequence of reactions favors formation of more \(\ce{H+}\) or the increase in acidity of water. This increase in acidity in ocean water has been attributed to suppressing of the carbonate ion (\(\ce{CO3^2-}\)) concentrations - an ion that is critical for marine calcifying organisms such as corals. For more information, see Ocean Acidification | National Climate Assessment (globalchange.gov). Thermodynamic principles predict that lowering will also lower the formation acidity in water.
Acid Rain#
Human activities - mainly fossil fuel-based power generation and automobile emissions - are the main cause of acid rain. Fossil fuels typically contain \(\pu{1-3 \%}\) of sulfur along with nitrogen and other minerals. Coal and fuel combustion releases large concentrations of nitrogen oxides (\(\ce{NO_x (g)}\), a combination of \(\ce{NO (g)}\) and \(\ce{NO2 (g)}\)) and sulfur dioxide (\(\ce{SO2}\)) into the atmosphere. These gases mix and react with rainwater, \(\ce{O2 (g)}\), and other chemicals to form more acidic pollutants, known as acid rain. Sulfur dioxide and nitrogen oxides dissolve very easily in water and can be carried very far 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 dissolution of \(\ce{CO2(g)}\) in water causing 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 been 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. 35)
Fig. 35 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 is any surface water or groundwater that drains from an active or abandoned mining operation. Mine drainage can be high-quality similar to natural waters or contaminated by leftover materials. Polluted mine drainage can be extremely acidic and often contains high concentrations of toxic, heavy metals. In general, the more acidic the water, the more likely it is to be harmful to living organisms.
Mine drainage forms from a chemical reaction between water and rocks containing sulfur-bearing minerals. The resulting waters become rich in sulfuric acid and dissolved iron as shown in the reactions below. As the iron settles out of the water, it can form red, orange, or yellow sediments in the bottom of streams. The acidic runoff further dissolves heavy metals such as copper, lead, mercury into groundwater or surface water. The rate and degree by which acid-mine drainage proceeds can be increased by the action of certain bacteria.
Fig. 36 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