Lesson

It is a known issue that, in an attempt to meet our energy needs, humans have put a lot of pollution into Earth and its global systems. There have been many changes and much research over recent years to clean up the pollution we’ve already created and to find alternative methods that are less polluting. In this activity, students will investigate three major pollutants that result from the burning of fossil fuels to generate electricity. These are CO₂, SO₂, and NO_x. The “x” in NO_x is a scientist’s shorthand to designate any of the group of binary nitrogen-oxygen compounds, such as N₂O, NO, and NO₂.

Acid Rain

Acid rain occurs when certain nonmetal oxides in the atmosphere dissolve and react within water while still in the atmosphere. These nonmetal oxides can react with small amounts of moisture and stay in the air for a while, or they can dissolve in the rain passing through the atmosphere to reach Earth as acid rain. You may recall from your study of acids and bases that metal oxides can react with water to form bases and nonmetal oxides can react with water to form acids.

Questions

1. Both SO₂ and NO_x undergo some chemical transformations in the atmosphere before they react with water. The new species are SO₃(g) and NO₂(g), respectively.
   1. For each of these new molecules, and for CO₂(g), write a balanced chemical equation that shows how each forms an acid upon reaction with water. Include symbols to show the states.
   2. Write the proper chemical name for each of the three acids.
   3. One of the three acids is a monoprotic strong acid. Identify this acid, then calculate the pH of a 0.015-M solution of this acid.
2. Sulfuric acid and carbonic acid are both diprotic acids. This means they will undergo acid dissociation in water in two steps.
   1. For sulfuric acid, write the acid dissociation equation for each step.
   2. Identify the K value for each of the steps you wrote above as being “very high” (K>>1) or “very low” (K<<1) and give a brief explanation for your answers.
   3. In the beaker below, draw the relative amounts of all species that would be present in a 0.10-M H₂SO₄ solution.

• Assume a total of 10 H2SO4 molecules prior to any ionization
• You do not need to show any H₂O molecules
• Your image will not be quantitatively accurate but should be qualitatively accurate

Of the three acids discussed, carbonic acid is the weakest, and thus has the least impact on acid rain, even though CO₂ is more abundant in the atmosphere than the other two pollutants. To understand why carbon dioxide can have an effect on the ocean’s pH, we must look beyond acid rain and consider how the atmosphere interacts with the ocean.

The ocean is a vast and variable mixture of many substances. Though it is not a true solution due to the many large and tiny particles floating within it, there are still a lot of substances that dissolve in the ocean. For this activity, treat the ocean like it is a typical aqueous solution, where water is the solvent and there are many different solutes. Ignore any undissolved substances except when they contribute to relevant dissolved particle amounts through equilibrium factors.

The temperature of the ocean varies, both around the globe and from day to day in any one area. For this activity, we will assume a constant and standard temperature while analyzing the system.

Interactive Questions

Open the simulation at: https://acswebcontent.acs.org/ChemistryInContextSuite/applets/OceanAcidification/oceanAcid.html

3. Use the slider in the simulation to find the pH for every 100 ppm CO₂, for the range of concentration shown on the scale. In the space below, sketch a plot of CO₂ concentration vs ocean pH.
   1. Is the relationship linear?
   2. Describe the pattern that you see, in terms of the plotted variables.
   3. According to the data, what effect does carbon dioxide have on the acidity of the ocean?
4. Consider the equation you wrote in question #1 for how carbon dioxide reacts with water. This reaction can actually be considered as happening in two different steps, each with their own equilibrium positions.
   1. Write the chemical equation that represents the equilibrium between atmospheric carbon dioxide (CO₂(g)), and dissolved carbon dioxide (CO₂(aq)).
   2. Write the chemical equation that represents the equilibrium between dissolved carbon dioxide and carbonic acid.
   3. What species exists in both equilibrium equations?
   4. Use equilibrium concepts to explain why there is a correlation between the amount of carbon dioxide in the atmosphere and the amount of carbonic acid in the ocean.

Click the button that says, “Show Graph”. When the graph appears, click the button that says, “Carbonates in a Closed System.” To imagine our ocean equilibria in a closed system, pretend you could box in a specific amount of ocean, so that nothing enters or escapes, and nothing else interacts with the system. Assume that equilibrium levels related to the atmosphere had already been established, and there are no additional interactions with the atmosphere.

5. You can see that this graph shows three different species as part of the carbonate system. Using what you know about acid dissociation reactions, write all chemical equations necessary to show how each species forms, starting from the dissolved carbonic acid.
6. Use the graph to identify the pH ranges where each of the species is most dominant (pH is the x-axis, and each line represents a different carbonate species):
   1. Carbonic acid
   2. Bicarbonate ion
   3. Carbonate ion
7. Use Le Châtelier’s principle to explain why carbonate is dominant in the pH range you specified for this system.

Click the button, “Carbonates in an Open Ocean.” This graph shows a constant concentration of carbonic acid, while the other species vary according to pH. This is different from the closed system because in an open system the carbonic acid is in equilibrium with the carbon dioxide in the atmosphere. When the atmospheric CO₂ remains constant, the concentration of carbonic acid remains constant. This graph shows that the proportion between the other two species in the carbonate system varies with pH.

Buffer- A system of chemical species that resist changes in pH because each species is involved in equilibrium reactions that will partially reverse any addition of hydronium or hydroxide.

8. Assume that the carbonate system is the major buffering system of the ocean.
   1. Explain how increasing the amount of CO₂ dissolved in the ocean will decrease the amount of the carbonate species in the ocean.
   2. The shells of many ocean organisms are made primarily of calcium carbonate (CaCO₃), which the organisms build from the calcium and carbonate ions in the ocean.

• Write the chemical equation that represents building these shells.
• Use equilibrium principles to explain what will happen to shelled organisms in the ocean if atmospheric CO₂ continues to rise.