Environmental Impacts of Acid Anhydrides

 As discussed here, non-metal oxides react with water to create acidic solutions. 

The reason that this is so important environmentally is that the combustion of fossil fuels, and especially coal, release non-metal oxides into the air. All fossil fuels are composed primarily of carbon and carbon chains and thus their combustion releases \(CO_2\) into the air. Here is the combustion of octane, the primary compound found in gasoline:

\(2 ~ C_8H_18 + 25 ~ O_2 \rightarrow 16 ~ CO_2 + 18~ H_2O\)

Coal is primarily carbon, but it contains both sulfur and nitrogen as impurities (as well as other elements such as mercury). When the sulfur and nitrogen in coal burn, they release \(SO_2, SO_3, and ~ NO_x\) (a mixture of a number of different nitrogen oxygen compounds). That means that any factory that is burning coal, and everything (factories, cars, etc.) that is burning fossil fuels is releasing non-metal oxides into the air. When those non-metal oxides meet clouds in the atmosphere, they form acids which then fall as rain.

That acid, falling as rain can cause damage to buildings and statues as seen here.

There is good news and bad news associated with this. 

The good news is that we are MUCH better at "scrubbing" what comes out of our smoke stacks than we used to be. At one point in the last century, rain was measured with a pH as low as 3.5, that 100x's as acidic as normal rain (with a pH of 5.5). That means that we are doing much less damage to statues and buildings than we used to.

The bad news is that we are adding more and more \(CO_2\) to the atmosphere. Although carbonic acid is a relatively weak acid and we are no longer seeing acid-rain associated damage in the way we did before, there is another, more worrisome impact.

The majority of our planet is covered with water. As we increase the amount of \(CO_2\) in the atmosphere, we increase the amount that dissolves in the ocean. Although the ocean is buffered and huge, we have already, in the past century, seen a change in the pH of the ocean. Before the industrial revolution, the pH of the ocean was about 8.2. It is now about 8.1. Although this seems like a tiny change, due to the logarithmic nature of the pH scale, it actually represents about a 30% increase in the acidity of the ocean (source).

This change in the pH of the ocean is especially worrying because the bottom of the ocean food web is comprised primarily of tiny crustaceans often lumped together as "krill". These creatures form their protective shells from carbonate found in the ocean. As the acidity of the ocean increases (as the pH goes down) the concentration of carbonate decreases according to the reaction:

\(CO_3^{-2} + H^{+1} \rightleftharpoons HCO_3^{-1}\)

Not only will this make it harder and harder for these creatures to form shells, but the increased acidity can even dissolve shells that have formed.

Of course, if you damage or remove the bottom of a food web, there is the potential for a complete collapse of the rest of the web. Given that 10% of the world's population depends on fisheries for their livelihood and 4.3 billion people are reliant on fish for 15% of their animal protein intake (source), this problem looms over everything we do.