Acids and bases

Hello coylesebastian 2005,

There are several different definitions of acids and bases. The most typical that you will encounter, and are probably already familiar with, is the Arrhenius definition. These acids and bases will dissociate in water to form hydrogen cations and hydroxide anions, respectively. You'll certainly recognize the following dissociations for hydrochloric acid and sodium hydroxide:

HCl (aq) ⇌ H+ (aq) + Cl- (aq)

NaOH (aq) ⇌ Na+ (aq) + OH- (aq)

To identify acids and bases by the Arrhenius definition is going to require some basic knowledge and memorization of the periodic table of the elements as well as basic cations and anions!

For instance, in the dissociation of hydrochloric acid, we form H+ and Cl-, which are common ions. It is incredibly rare for you to see H- and Cl+; the ionization energies for Cl+ in particular is pretty high. You will always see Group 17 elements (the vertical column including Cl, F, Br, I, etc.) ionizing to an anion with -1 charge. This is pretty easy to understand because Group 17 elements all have a 7-electron valence and so readily pick up and extra electron to have a stable octet. This gives them a stable -1 charge.

A similar but opposite scenario occurs in Group 1. Group 1 elements include H, Li, K, Na. These compounds will readily lose an electron to become an octet by ionizing to a +1 cation. Hydrogen is the exception as it only has 1 electron to begin with, and so stripping away it's singular electron will not result in an octet, but rather a lone proton nucleus with +1 charge.

For the example you provided above, KOH will intuitively be basic because it will dissociate to a hydroxide anion. Furthermore, KO+ is not a common ion. However, K+ is a very common Group 1 +1 cation. So the dissociation would resemble that of sodium hydroxides (above), like this:

KOH (aq) ⇌ K+ (aq) + OH- (aq)

Once you memorize Group 1 and Group 17 (at least partly), you should easily recognize compounds to be acids (HNO3, HPO3, and even those which strongly dissociate 2H+ such as H2SO4) and some to be bases (NaOH, KOH, LiOH). If you become familiar with Group 2 elements which form +2 charge ions, such as Ca and and Ba, Ca(OH)2 and Ba(OH)2, you'll recognize that these will also be bases.

I should also offer another hint, which is that by convention ionic compounds should always be written in the order of cation → anion! If you remember this, you'll never see KOH and think it is an acid because the resulting formula would be KO-H+, which would be in the incorrect order: anion → cation.

If you have more questions about acids, there is much more to cover (including the Brønsted-Lowry acid-base theory, which you alluded to in your question stem). For now, your homework (sorry) should be familiarizing yourself with the periodic table (particularly, groups 1, 2 and 17, and perhaps the positions of N, O, P and S) and learning the common ions.

This list easily extends beyond the scope of what you would need to know, but I would at least focus on knowing all of the monoatomic ions and becoming very familiar with the polyatomic ions on the first page.

http://chalmerschemistry.weebly.com/uploads/5/5/7/1/5571931/common_ions_list.pdf

Again, if you have more questions, please let me know!