Student Podcast, Episode 3

Chapter 3: Chemistry

On a Friday afternoon, someone spills a liter of a common industrial solvent on a lab floor. They close the windows, shut off the ventilation, and leave it for the weekend. On Monday a cleanup crew arrives.

Should they bring a mop or an air pump? Your textbook works this out, and the answer is the air pump. By Monday there is nothing left on the floor.

All of it has moved into the air of the room. Nobody pushed it there. The chemistry decided where it would end up, and that is what this chapter is about.

Part one: the rules

Chemistry asks two questions about any reaction: how fast, and where it stops. How fast is kinetics, which comes back with Chapter 4. This episode is about where it stops, which is equilibrium.

Start with bookkeeping. Atoms rearrange but never appear or vanish, so a balanced reaction has the same atoms of each element on both sides. Charge is conserved too, which is the half people forget.

Then thermodynamics. The first law says energy changes form but the total stays fixed. The second law tells you which way things go. A process runs on its own only if it lowers the system's free energy. Picture a ball in a valley. It rolls downhill without help, and it stops at the bottom.

That bottom is equilibrium. The equilibrium constant, K, tells you where the bottom is. The reaction quotient, Q, tells you where you are right now. Compare the two and you know which direction the reaction will move.

One subtlety. What actually reacts is not quite the concentration you measured. It is the activity, the effective concentration once other dissolved ions crowd around it. In most fresh water the difference is small enough to ignore. In seawater or brine it is not, and ions carrying bigger charges feel it most.

Part two: the reactions

Figure 3.4 lays out the major equilibrium processes side by side, each with its own constant. It looks like a list to memorize. It is one idea applied over and over: a chemical settling between two places, and something deciding the split.

Volatilization is a liquid or solid giving off vapor. Warmer temperatures push more into the air. That is the spill on the lab floor, and it also works at the scale of a continent. Persistent pesticides and industrial chemicals volatilize easily in warm regions, drift north on the wind, and condense in the cold Arctic. The book calls this the circle of poison. Seals, whales, polar bears, and Inuit communities carry the risk of chemicals produced and used far away, some of them banned in wealthy countries and still manufactured for export.

Air-water equilibrium is how a gas splits between air and water, governed by Henry's law. Oxygen dissolves better in cold water than in warm, about 14 milligrams per liter near freezing against about 9 at 20 degrees, which the book notes is one reason trout like cold streams. Run the same idea with carbon dioxide: more in the air means more dissolved in the ocean, forming carbonic acid. Ocean pH has dropped about a tenth of a unit since the Industrial Revolution. pH is logarithmic, so that is roughly a 25 percent increase in acidity, with carbonate shells and coral on the losing end. Watch the units here: the Henry's law constant changes form depending on which direction the reaction is written.

Acid-base chemistry is about hydrogen ions: acids give them up, bases take them. Most natural waters sit between 6 and 9, and even clean rain is around 5.6 because of dissolved carbon dioxide. pH also decides which form a chemical takes. Ammonia has two forms, one far more toxic to fish than the other, and pH sets the split.

Alkalinity is a water's capacity to neutralize acid. Many New England lakes have very little of it because the rock beneath them releases so little, which is why acid rain hit this region as hard as it did.

Precipitation and dissolution decide whether a solid dissolves or dissolved ions drop out as one. The white crust inside a kettle is calcium carbonate coming out of hard water. The same reaction running the other way dissolves limestone into caves, springs, and sinkholes, including karst terrain in western Massachusetts. Treatment plants run it on purpose, adjusting pH to pull metals out of water as solids.

Sorption is chemicals sticking to solids. Adsorption means onto a surface. Absorption means into the material. Sorption covers both. A molecule sits at lower energy on a surface than in water, so some of it moves there.

Whether a pollutant clings to soil or travels with groundwater is largely a sorption question. Every one of these reactions answers the same thing the spill did. For that liter on the lab floor, the answer was the air.