Acids and Bases I: Introduction
Updated: Jan 9, 2020
The following will be covered in this article:
Strong vs. weak acids and bases
Conjugate acids and conjugate bases
Basic pH calculations
Different scientists have defined acids and bases in different ways. You’ve probably heard of the three most common: Arrhenius, Lewis, and Bronsted-Lowry.
Arrhenius Definition: When dissolved in water, an acid will donate H+ to solution and a base will donate OH- to solution.
Lewis Definition: An acid will accept an electron pair, a base will donate an electron pair.
I’m Bronsted-Lowry: When dissolved, an acid will donate H+ to solution and a base will donate OH- to solution.
As you can see, the Arrhenius and Bronsted-Lowry definitions are basically the same, the only difference being that the Arrhenius definition requires the solvent to be water whereas the BL definition does not. There’s a high chance you’ll have to determine which acid or base belongs to each definition.
Strong vs. weak acids and bases:
There are two classifications comparing the strengths of acids and bases, weak or strong. There are typically considered 6 strong acids and 6 strong bases.
These acids will ionize completely when in solution, or in other words, if you dissolve 1 mole of hydrochloric acid, HCl, in water, there will be 1 mole of H+ ions and 1 mole of Cl- ions. The same would be true for any of the bases as well – 1 mole of KOH dissolved in water would result in 1 mole of potassium ions and 1 mole of hydroxide ions. However, be careful to watch the stoichiometry. 1 mole of H2SO4 would produce 2 moles of hydrogen ions and 1 mole of sulfate ions.
Weak acids and bases on the other hand don’t dissociate 100%. The extent to which they ionize is dependent on the specific weak acid or base, and is denoted by the Ka value. A “stronger” weak base may dissociate 30% or 40%, and a “weaker” weak base may dissociate only 10%, 1%, or even less. For a weak acid that only dissociates 10%, we could imagine a solution of water with 10 molecules of acetic acid (CH3COOH) dissolved in it. If only 10% dissociates, that means in solution there would be 9 molecules of CH3COOH, 1 molecule of CH3COO- and 1 molecule of H+.
When a question asks what the major species in a solution are, it’s asking what compounds are going to significantly dissociate into ions. For example, if you were to dissolve table salt in water, the major species would be Na+, Cl-, and H2O. The same concept applies to acids and bases – if you were to dissolve HCl in water, the major species would be H+, Cl-, and H2O. However, what if instead of HCl, the question asked about acetic acid, CH3COOH? Because this is a weak acid, it will only partially dissociate. Because of this, in solution you will have water, CH3COO-, H+, and also CH3COOH still in its acid form.
What components are major, and what are minor? As a quick trick that works majority of the time, you can assume that strong acids will dissociate into major species, while weak acids will dissociate into minor species. Using the acetic acid example, this means that the major species would be water and CH3COOH, while the minor products would be CH3COO- and H+.