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Chemistry Behind Acid Rain
Episode 817th June 2021 • Chemistry Connections • Hopewell Valley Student Publication Network
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Chemistry Connections

Episode #8  

Welcome to Chemistry Connections, my name is Barron Brothers , my name is James Huang and we are your hosts for episode #8, The Chemistry of Acid Rain. Today we will be discussing why acid rain is so harmful and its effects on the city and the ecosystem.

Segment 1: Introduction to Acid Rain

As the world population continues to increase, the resources required to maintain this population also increase, including factories to make products for the consumer, cars for transportation, and food. However, many do not consider the environmental implications just by living in today’s world as our environmental situation continues to decline. One of these effects is the increase of acid rain due to the amount of pollutants released by industrial processes and traditional power plants.

Acid rain is rain mixed with pollutants that lower the pH of the rain. A weak acid is an acid that dissociates little in water, versus a strong acid that dissociates almost completely. Ka values measure how much an acid dissociates in water. For weak acids, Ka<1, and Ka>1 for strong acids. A buffer is a solution made up of a weak acid and its conjugate base that resists changes in pH.

Segment 2: The Chemistry Behind Acid Rain

Non-polluted rainwater is slightly acidic (pH=5.6) because the evaporated water reacts naturally with the carbon dioxide in the air, forming carbonic acid. Carbonic acid then dissolves into hydronium ions (H3O+) and its conjugate base (the negative ion dissolved in the solution - in this case, HCO3-). In the case of carbonic acid, there is still a hydrogen atom available in the conjugate base of what we call mechanism 1. With this, HCO3- reacts with water again in the second mechanism. Here are some reactions to show you what we mean:

Pollutants from factories and car emissions contain gaseous sulfates and nitrates, which react with the evaporated water as well in reactions similar to carbonic acid.

Comparing the Ka values of the acids, we can see the effects of each additional pollutant on the acidity of the acid rain. As the Ka value increases, the amount of dissolved acid particles increases, lowering the pH of the rain more.

The Ka values of HNO3 and H2SO4 are relatively high compared to relatively low ones, such as H2CO3. Since CO2 is present naturally in the air, this explains why rainwater is slightly acidic. However, nitrates and sulfates are a result of unnatural pollution, such as from factories and fertilizers. Because of these Ka values, H2SO4 and HNO3 reduce the pH even further and have a greater effect than H2CO3, as the pH of acid rain ranges from 4.2 to 4.4. Also, the pH scale is logarithmic, meaning that the difference in acidity between water (pH=7) and rainwater is much lower than that of rainwater to acid rain, even if the pH difference is smaller. In addition, car and factory pollution forms additional CO2, forming more H2CO3 and decreasing the pH of acid rain further.

Acidic rain also reacts with building materials, such as limestone, aluminum, and steel, causing corrosion. Limestone (CaCO3) reacts with sulfuric acid to produce calcium sulfate (CaSO4), carbon dioxide, and water.

Therefore, the sulfuric acid in acid rain strips away at the calcium carbonate, leading to faster weathering compared to normal rain. This is very problematic, as limestone is used in both cement and concrete, so acid rain will have a severe effect on most buildings and structures around the world. In aluminum and steel, a similar process occurs, as various acids react with the metal to produce an aqueous solution, corroding the metal on certain buildings. Since steel is made up mostly of iron (97%), the iron is an appropriate representation of the decay of steel. For sulfuric acid, here are some reactions depicting the process:

Since many skyscrapers are made up of steel, this is a problem particularly in cities. Another effect of acid rain is how it takes away nutrients from the soil. The hydronium ions pull out vital nutrients from the clay, such as magnesium ions, and replace them with hydronium ones.

When the H3O+ in acid rain flows from the clay and mixes with lakes and other bodies of water, the pH of the water will drop. Most ecosystems are very sensitive to pH drops; i.e., most fish eggs will not hatch if pHwater<5. 

Resisting acid rain

Soils with calcium carbonate act as a buffer against acid rain. There are many ways to resist the effects of acid rain, one of which being the soil itself. Soil contains calcium carbonate (CaCO3), which can react with sulfuric acid to produce the weaker carbonic acid, as shown below. 

Calcium carbonate can also be used to reduce acid rain at its source. In factories where sulfur dioxide is produced, calcium carbonate can be injected into smokestacks, reacting with the sulfur dioxide to produce pH-neutral calcium sulfate (CaSO4). Carbon dioxide is still produced, but, using the same concept as the soil, the Ka of carbonic acid is lower than that of sulfuric acid. Therefore, this would lessen the factory’s impact on the pH of the rain, as a smaller Ka value has less impact on the environment.

Segment 3: Personal Connections

The issue of acid rain is paramount because today’s society is ignoring global warming and continuing to pollute. If excessive pollution continues, acid rain will become more common. This will cause more damage to ecosystems and buildings and endanger the safety and quality of life for many across the globe. We as the younger generation will grow up in this world, so we must pioneer the changes needed, such as regulating factory pollution, basing the economy on electric transportation rather than oil, and creating more green space free from chemical pollution. This is the only way to improve this world for us and all future generations.

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