On a Whim

by Mat in Vadnais Heights

Back in June of 2019, I wrote an article about gravitation in which I made a passing reference to equilibrium, with an aside that “one day the concept of equilibrium is gonna get an article all its own.” Well, hang on tight, my friends, because for good or bad, that day has arrived. 

In the simplest sense, equilibrium is just a state in which all applied forces have reached a balance. It is the reason, for example, that stars are spherical. In a star, the explosive power of nuclear fusion is pushing everything outwards from the center in all directions. The attractive force of gravity is pulling everything towards the center. At one specific distance, those forces will be precisely balanced. So, all points that distance from the center in any direction make up the surface of the star. And one of the properties of a sphere is … that all points on its surface are equidistant from its center. If the star expands a bit, it cools, and the fusion pressure goes down. That allows gravity to take over and causes it to contract again. If contraction continues too far, then the temperature goes up, and so does fusion pressure, triggering expansion. The star can never stray far from the balance point. 

There are lots of varied examples, some less cosmic than others. When cruising at a constant speed in your car, the force of the engine attempting to accelerate is precisely balanced by air drag and friction with the ground, resulting in uniform motion. Ever wonder why a damp towel won’t hang dry when the air is too humid? It’s not because the air is too saturated with water to allow evaporation. The water evaporates just fine, thank you. The problem is that water from the air is condensing on the towel as fast as the water in the towel can evaporate into the air, leaving no net change. 

Things start to get really interesting when we look at chemical reactions. Now, many of the chemical reactions you encounter on a day-to-day basis are irreversible. That is, they only proceed in one direction. Say you mix together hydrochloric acid (HCl) and sodium hydroxide, aka lye (NaOH). The former is, as its name states, an acid, and the latter is a base. In common terms, we say that when they’re mixed, they “cancel out,” leaving something neither acidic nor basic. In detail, what happens is that the hydrogen in the HCl and the sodium in the NaOH swap places, rearranging themselves into water and salt. Formally, HCl + NaOH → H2O + NaCl. The products of this reaction are stable. And drinkable! Well, drinkable if you get the amount of each reactant precisely correct. If you don’t, it’ll kill you, quickly and nastily. Seriously, don’t drink it. Anyway, that’s a one-way or irreversible reaction. 

 By contrast, reversible reactions can proceed in either direction. Consider your favorite carbonated beverage. We all know that “carbonation” is carbon dioxide dissolved in the water, but what does that mean, exactly? Let’s look at a bottle of soda with the cap firmly in place. Within the soda are molecules of carbonic acid, or H2CO3. With no prompting, carbonic acid will convert to water + carbon dioxide. In chemical terms, H2CO3 → H2O + CO2. That means the soda gets a bit watered down, and a little CO2 winds up in the air space at the top of the bottle. But the reaction runs just as well the other way! The CO2 in the air reacts with the water in the soda, creating carbonic acid. H2O + CO2 → H2CO3. As long as the CO2 stays trapped in the bottle, there’s always about the same amount of carbonic acid in the soda. Some is always leaving and some always arriving, so the soda exists in a state of dynamic equilibrium. If you remove the bottle cap and let it sit for a while, the carbonic acid will continue to turn into water plus CO2, but the CO2 floats away. Since there’s comparatively little of it in the air, the reverse reaction that re-creates the carbonic acid cannot proceed at anything like the same rate, and the acid disappears. That’s why flat soda tastes sickly-sweet — you’re missing the acidic zing as well as the bubbles. 

This dynamic equilibrium is actually crucial to life as we know it. How does oxygen get from the air in your lungs to other sites in your body where it can be burned to do work? Well, in the high-pressure environment of your lungs, the hemoglobin in your blood will bind with oxygen. That is, Hb + O2 → HbO2. When this newly-created oxyhemoglobin reaches the lower-pressure acidic environment in your other tissues, the reaction runs the other way, and oxygen is released to do its thing: HbO2 → Hb + O2. The newly freed hemoglobin returns to the lungs, and the cycle repeats. 

There are many examples of this kind of thing in biology, including the osmotic balance that regulates the amount of water in cells and the ATP ← → ADP reactions your body uses to store and release energy. If all such reactions just ran one way to completion, then at a minimum, you’d need to consume and excrete an enormous amount of all kinds of stuff just to keep ahead of the chemistry and stay alive. More likely that’s just straight up impossible, and you’d drop dead in about two minutes flat. That being the case, I think equilibrium did warrant an article all its own. That’s my story, and I’m sticking to it.