Acids and Bases I: Introduction
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.
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+.
Conjugate Acids and Conjugate Bases:
Conjugate acid and base simply means the complement of a base or acid, respectively. Let’s take the equilibrium of formic acid (HCOOH) in water:
It’s easiest to start with the component you know is an acid or base, in this case formic acid. We’ll label HCOOH as the acid. Because this is an acid/base reaction, we know that there must be a base to accept the hydrogen that the acid is donating, thus we can label water as the base.
Now we jump to the other side of the equation where we have H3O+ and HCOO-. Because this is an equilibrium, the reaction is proceeding both forward and backward, so on the right side of the equation there is also an acid/base reaction taking place. However, we don’t also classify these as an acid or base, instead they are designated conjugate acid and conjugate base. The conjugate acid will donate a proton and become the base, and the conjugate base will accept a proton to become the acid.
Note that the acid pairs with the conjugate base, and the base pairs with the conjugate acid (not acid with conjugate acid, and base with conjugate base). In this example, the conjugate base would be HCOO- and the conjugate acid would be the H3O+.
Water and KW:
In a neutral solution, the concentration of hydrogen ions is equal to the concentration of hydroxide ions. Water is typically considered to be neutral, and KW is used to demonstrate the acidity of water. KW has a value of 1.0x10-14 and is equal to the concentration of hydrogen ions multiplied by the concentration of hydroxide ions:
A solution of pure water at ideal conditions will always have equal parts H+ and OH- to maintain stoichiometry, thus [H3O+] = [OH-], therefore KW = [H3O+]2 = [OH-]2. By solving for either the concentration of hydronium ion or hydroxide ion, we take the square root of KW, 1.0x10-14, and find that these both are equal to 1.0x10-7. This is our baseline. If the concentration of hydrogen ions exceeds this, the solution will be acidic. If the concentration of hydroxide ions exceeds this, aka the concentration of hydrogen ions falls below this number, then the solution will be basic.
Note that at different conditions, KW will no longer equal 1.0x10-14 and the concentrations of hydroxide and hydrogen will no longer be equivalent.
pH and basic calculations:
We can’t discuss acids and bases without talking about pH. pH measures the acidity/basicity of a solution. Solutions with a pH below 7 are considered acidic, while solutions with a pH above 7 are considered basic. The pH number itself is just a simpler way to denote the concentration of hydrogen ions. In fact, “p” could be translated to “-log()”. Therefore, pH can be converted to –log(H), where H is just the concentration of hydrogen ions. This –log() trick can be used in other circumstances as well, for example pOH of pKa.
If the hydrogen concentration is 1.0x10-7, the pH = -log(1.0x10-7) = 7.0. If the concentration is 1.0x10-2, the pH = 2.0.
pH + pOH = 14. You’ll probably see a question that either requires you to calculate pOH or use pOH to calculate something else. The only change you need to make is use the hydroxide concentration instead of the hydrogen concretion, which can be found either using the Kw equation above or the equation listed at the beginning of this paragraph.