Determining Acids and Bases

[Alyssa Gonzalez 2G]

How do you determine whether a chemical is an acid or a base? Also, how do we distinguish which acids are strong and which bases are strong? Additionally, how do we know when an acid or base will then dissociate completely and what's the difference in approaching a problem if you have a weak base versus strong base?

[Jonason Wu 3I]

Acids tend to contain H+ while bases tend to contain OH-. I am not sure if there is a way to tell if something is a strong/weak acid or base, but there is a list of them in the textbook.

[Jayson Hall 2H]

Hello, I will try my best to go through your questions.

1) How do you determine if something is an acid or a base?

There are two main definitions of an acid/base that I am aware of. Brønsted-Lowry acids are proton (H+) donors, which means they will have a spare H+ to give. This will look like HCl, or HBrO3, or CH3COOH, or NH4+. Conversely, Brønsted-Lowry bases are proton acceptors, so stuff like F- and CO3 2-. The other definition is Lewis acids/bases, which revolves around electron pairs. A Lewis acid will accept an electron pair. An example is BF3 because boron in this case only has 6 valence electrons, which is stable, but it’s also cool with gaining a lone pair from water (creating H3O+ when there is a large amount of water and thus acidifying the solution). Conversely, Lewis bases are electron pair donors, things like NH3 has a lone pair that it can give to water, making NH4+ and OH- (yes, something can be both a Brønsted-Lowry and a Lewis acid/base). If you are still confused, I would recommend the Focus Topics provided on the Lavelle class website, or going to office hours.

2) How do we know which acids/bases are strong?

We can sort of guess based on structure and stability of anion, but generally the strong acids are memorized. There are 6 (?) or so, depending on who you ask, and they are HCl, HI, HBr, HClO4, H2SO4, HNO3. Sometimes HClO3 is included too, but I would stick to memorizing these main ones.

Strong bases show up less commonly so I’m less certain, but usually they’re an alkali / alkaline earth metal paired with OH. Some examples that come to mind are NaOH, KOH, Ba(OH)2.

Also generally, if a Ka/Kb is provided, it’s good to assume that the acid or base in question is weak, because usually the reason we call something “strong” is that the Ka or Kb is too high to be useful (and we just assume it dissociates like an ionic compound in water).

3) How do we know when something dissociates completely?

If it’s strong, it dissociates completely. Otherwise, it doesn’t.

4) How do you approach solving a weak base vs. strong base problem?

Basically, when a base is strong, the problem is much simpler. This is because you can assume that the concentration of OH- is directly proportional to the concentration of the base. For example, if you dissolve 0.1 M KOH in water, your [OH-] is 0.1 M because there is 1 mol OH for every mol of KOH. Similar concept for something like 0.1 M Ba(OH2), but since there are 2 OH- for every mol of Ba(OH2), the [OH-] would be 0.2 M.

If you have a weak base, you would need to use the ICE box table and Kb and all the equilibrium stuff we have been learning in class to calculate the [OH-]. I won’t go over it in detail here because there are other Chemistry Community posts and other places where you can review this topic.

We usually care about calculating [OH-] (or more typically [H3O+]) because we can know the pH of a solution containing a certain concentration of the acid/base, which is important for multiple reasons. One reason I can think of is that if we were to test a biological enzyme in a solution, we would need to know if the pH of the solution matched its optimal pH for activity in order to get the best yield out of the experiment.

Hope this helps!

[405696752]

I believe there is a list of acids and bases in the book. But the ones we have been reviewing in class are HCI, CH_3COOH, NH4+ and HBrO3 because they are able to give a H+ thats what you call a proton. when they have an extra and are able to spare it as In for bases they are the opposite of protons they don't give they accept and they're mostly like f- and CO3 2-. as in for how do you know which one is an acid or base you can memorize them or write it down in somewhere you can go back and look. it's in the notebook and they're easier to have them in hand.