I’d like to think that every time a lab TA opens our lab book and sees anything stronger that 0.1 M anything in the materials section, they sigh loudly and bang their head against the table.
The reason I’d like to believe this is simple: every time we work with concentrated acids or bases in chem lab, someone gets hurt. No exceptions.
Last semester, our last lab entailed synthesizing aspirin. The reaction required a catalyst, which in our case was 16 M sulfuric acid (H2SO4). To give people not currently in a chemistry class some perspective, we run most of our acid/base labs with 0.1M H2SO4 or weaker.
Obviously, there were a lot of acids burns.
Yet, we also hear horror stories about bases. Bases are necrotic, and apparently there are repeated reports of students not realizing they’d gotten a concentrated base on themselves until they looked down and saw that it’d eaten a hole through their skin. Considering this, in the pre-lab for our pH lab this week, my safety information said, “Acids will burn you, and bases are the harbingers of death.”
I hope my TA didn’t actually read that, hehe…
We know a lot about acids and bases by virtue of being human beings. We deal with them all the time, in the form of carbonated sodas (H2CO3—carbonic acid), cleaners (NaOH—lye, or sodium hydroxide), and even citrus fruits (C6H8O7—citric acid). Yet, what exactly are acids and bases? What do they do, really?
There are several different ways to define what an acid or base is. The simplest definition that is still considered mostly accurate (and the definition often touted by chem lab TAs) is the Brønsted—Lowry definition, which simply says that an acid is a proton–donor and a base is a proton-acceptor.
All right, hang on. Consider the reaction of hydrochloric acid and sodium hydroxide, a relatively terrifying acid and an equally terrifying base that are both found in usefully high quantities in every chemistry lab.
HCl (aq) + NaOH (aq) → H2O (l) + NaCl (aq)
From this reaction, we can learn several things about the characteristics of acid-base reactions in general, as well as the characteristic of a strong acid-strong base reaction specifically. For today, I’m going to stick to the general reaction.
We start out with aqueous hydrochloric acid and sodium hydroxide and end with aqueous sodium chloride, or table salt. For the purpose of illustration, it might be helpful to think of each of these aqueous reagents as dissociated, or split up into their respective parts, like this:
H+ (aq) + Cl– (aq) + Na+ (aq) + OH– (aq) → H2O (l) + Na+ (aq) + Cl– (aq)
Here on the left side of the equation, we have a proton (a hydrogen without an electron is just a proton), a chloride anion (negatively-charged ion), a sodium cation (positively-charged ion) and a hydroxide anion. On the other side of the reaction, we have liquid water, the sodium cation and the chloride anion.
So, where to the proton and the hydroxide go? That’s the cool part—if you take the proton from the acid and stick it on the hydroxide from the base, you get water! Then you can rearrange the anion that was attached to the acid and the cation that was attached to the base to get a salt. In this case, we get our familiar friend, table salt.
Now we can see that the hydrochloric acid donated a proton, and the sodium hydroxide’s hydroxide group accepted it. This is essentially what we mean when we say that acids are proton-donors and bases are proton-acceptors.
Also, you’ll notice that the reaction of a terrifying acid and a terrifying base produces two innocuous and useful compounds, water and salt. In Gen Chem I, they tell you that this happens for all acid-base reactions—you get water and a salt. Turns out this isn’t entirely true, but neither is half of the stuff they teach you in general chemistry, apparently.
We have a basic grasp on what acids and bases are, but they aren’t all the same. Knowing the behaviors of weak and strong acids and bases is almost as important as understanding what acids and bases are in the first place.
Questions? Comments? Criticisms? You know where to put them.