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Chemistry of Acid Rain 2
Episode 1317th June 2021 • Chemistry Connections • Hopewell Valley Student Publication Network
00:00:00 00:08:20

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Chemistry Connections

Episode #13  

Welcome to Chemistry Connections, my name is Tejas and I am your host for episode #13 called The Chemistry of Acid Rain. Today we will be discussing the causes and effects of acid rain and the chemistry behind it. 

Segment 1: Introduction to The Chemistry of Acid Rain

If you’ve studied chemistry, and even if you haven’t, you might have heard of the term pH. A pH of 7 means the solution is neutral, a pH over 7 means a solution is basic, and a pH under 7 means that a solution is acidic. So, from the words acid rain, you might guess that it means rain that has a pH much lower than 7, and you’d be right — that’s exactly what acid rain is. But how does it form? 

Acid rain is formed when sulfur dioxide and nitric oxides (NOx for short) react with water in the atmosphere to form acids. Then, the sulfuric and nitric acids that were formed fall to the ground mixed with water in a process called wet deposition. This is what you probably think of when you hear the words acid rain, but acid rain also includes dry deposition. This is when acids don’t have the moisture to come down as rain, and instead attach to surfaces and form even larger acidic properties. Then, the next time it rains, these particles get washed into the water and travel through the ground, damaging plants and animals and potentially entering lakes or rivers. 

While acid rain is a natural phenomenon, as natural sources such as volcanoes also emit Nox and sulfur dioxide. However, most of the time, the problem is man-made. Two thirds of SO2 and one fourth of NOx in the atmosphere come from electric power generators, which burn fossil fuels to generate electricity. Cars and other vehicles also emit these gases, and so do oil refineries and other pieces of equipment used in the manufacturing industry. 

Because acid rain can harm humans and kill wildlife, it’s important to understand the chemistry behind it. Once we understand the causes of acid rain and why it occurs, we can start trying to limit the amount of sulfur dioxide and NOx we put into the air. 

Segment 2: The Chemistry Behind Acid Rain

Sulfur dioxide and nitric oxides are produced by the combustion of fossil fuels. When these gases rise up into the atmosphere, they can react in a few different ways to produce acids. 

Two molecules of sulphur dioxide can react with diatomic oxygen gas to produce two molecules of sulfur trioxide. Then, each of those sulfur trioxide molecules reacts with liquid water from cloud droplets to produce H2SO4, or sulphuric acid. This is the acid that then falls to the ground with water as acid rain.

Alternatively, 2 molecules of nitrogen monoxide can react with diatomic oxygen gas to produce two molecules of nitrogen dioxide. Then, those two molecules react with water to produce nitric acid, HNO3, and nitrous acid, HNO2. 

So we have these three end products, H2SO4, HNO3, and HNO2. What makes these acids? According to the Bronsted-Lowry theory, any compound that can transfer a proton, or an H+ ion, to another compound is an acid. As you can see from the makeup of these products, they all have hydrogen atoms ready to be given away. However, two of these products are more important than the other. These are nitric acid and sulfuric acid. Both of these are strong acids; this means they are more stable when they have donated an H+ ion. Strong acids dissociate fully to completion, so when nitric acid and sulfuric acid dissolve in water, they donate an H+ ion to H20 to form large amounts of H30+. This is important because H30+ is what makes things acidic; therefore, when nitric acid and sulfuric acid dissolve in water, they produce highly acidic solutions. This is what makes up acid rain and makes it dangerous. 

Now that we’ve talked about how acid rain forms and why the emission of sulfur dioxide and NOx produce highly acidic solutions, we should talk about how this acidity is measured. This goes back to the beginning of our podcast when I mentioned pH — a pH of 7 is neutral, over 7 is basic, and under 7 is acidic. But what exactly is pH? Well, a pH is an easy way to understand exactly how much H30+ is in a solution. To find pH, you take the negative log of the concentration of hydronium in the solution. The higher the concentration of H3O+, the more acidic the solution and the lower the pH. As we just learned, acid rain is formed when the strong acids nitric acid and sulfuric acid dissolve in water. This means that acid rain has a very low pH. More specifically, any precipitation with a pH lower than 5.6 is classified as acid rain. 

Segment 3: Personal Connections

So why is this important, and why did I choose to do a podcast about this? What made acid rain interesting to me is that most people have heard of acid rain, but they don’t really know what it is or how it works. When the topic of pollution and saving the environment is brought up, you hear a lot about how the burning of fossil fuels traps carbon dioxide in the atmosphere, but not as much about the emission of sulfur dioxide and NOx. Hopefully after listening to this podcast, you understand the relationship between fossil fuels and acid rain — it's yet another reason why we have to switch to clean energy if we want to save our planet.

Thank you for listening to this episode of Chemistry Connections. For more student-ran podcasts and digital content, make sure that you visit www.hvspn.com

Sources:

https://en.wikipedia.org/wiki/Acid_rain

https://www.epa.gov/acidrain/what-acid-rain#:~:text=Acid%20rain%20results%20when%20sulfur,before%20falling%20to%20the%20ground

https://letstalkscience.ca/educational-resources/stem-in-context/what-acid-rain 

Music Credits

Warm Nights by @LakeyInspired 

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