Acids and Bases are the basic cornerstones of life on earth. Many of the elements on the earth have acidic or basic parts to them. These elements have been studied for hundreds of years, shown by the word acid comes from the Latin word acidus, meaning sour. "Alkali," another word for base, is derived from the Arabic word for the ashes that come from burning certain plants.
This section is to give you an overview of the chemistry of acids and bases to assist you with your work or play. Please do not use any of these sections by themselves instead of the classroom, as they are not complete enough to supply all information for your particular class.
Definitions of Acids and Bases
I.)Arrhenius
The definition of an Arrhenius acid is a substance that ionizes in water and produces Hydrogen Ions (H+).
HCl ---> H+ + Cl-
Arrhenius also defined a base as a substance that ionizes in water to produce Hydroxide Ions (OH-).
NaOH ---> Na+ + OH-
II.)Bronsted-Lowry
These two men chose to define acids and bases by their ability to donate or accept protons. An acid would therefore be a substance that is capable of donating a proton where a base would be a proton acceptor.
HC2H3O2 + H2O <---> C2H3O2- + H3O+
Here Bronsted-Lowry would say
HC2H3O2 and H3O+
Are the Acids
H2O and C2H3O2-
Are the Bases
At this point it is also important to introduce the term Conjugate Pair. In this case the species with the H+ ion is the acid and the one without is the base. Here would be the Conjugate Pairs for the above reaction.
HC2H3O2 and C2H3O2-
H2O and H3O+
III.)Lewis
Before I mentioned that Arrhenius looked at Hydrogen and Hydroxide ions for acids and bases, and Bronsted-Lowry looked at proton donations. Now Lewis focuses on electrons, therefore covering the most broad spectrum of acids and bases. In Lewis' definition an acid is an electron pair acceptor, and a base is an electron pair donor. It is very hard to illustrate this definition on a web page, so please look in your textbook for further information.
pH
When using acid and base chemistry, it is very important to understand the concept of pH. Meaning Power of Hydrogen it is just a way of taking very small or large numbers and making them manageable. The only trick to mastering pH (or pOH in some cases) is to understand the log function.
p(Anything) = -log(Anything)
pH = -log[H+]
pOH = -log[OH-]
In a solution the following is true about pH
When [H+] = [OH-], the solution is neutral and pH = 7
When [H+] > [OH-], the solution is acidic and pH < 7
When [H+] < [OH-], the solution is acidic and pH > 7
Weak Acids
When weak acids are placed in water there will be a very small fraction of it dissociated into Hydrogen ions(H+) or its conjugate Base ions(B-). The dissociation constants, Ka and Kb measure the strength of an acid or base. These are just equilibrium constants, click to read more about equilibrium.
[H+] [B-]
Ka = ----------
[HB]
[H+] = molar concentration of hydrogen ions
[B-] = molar concentration of conjugate base ions.
[HB] = molar concentration of undissociated acid molecules
Strong Acids
Strong acids are those who dissociate completely in water, therefore the reaction goes to completion and never reaches equilibrium. Because strong acids never reach equilibrium, they can never have an equilibrium constant. Because there are so many weak acids and bases, you should take time to memorize important strong acids and bases, and most everything else would be weak.
Strong Acids
HCl, HBr, HI, HNO3, HClO4, H2SO4
Strong Bases
LiOH, NaOH, KOH, Ba(OH)2, Sr(OH)2
It is also important to realize that the conjugate of a strong acid would be an extremely weak base, the same is also true in the case of strong bases. Also, if the acid is an Oxoacid, an acid that contains oxygen, then the greater number of oxygen atoms attached to the central acid, the stronger the acid becomes due to the weakining attraction of the H+ ion.
Kw
The folowing is the equilibrium reaction for water, the w in Kw.
H2O <--> H+ + OH-
Kw = 1 X 10-14 = [H+][OH-]
pH + pOH = 14
This information comes in very handy when trying to calculate the Ka or Kb in an equation when the other is given, because:
Kw = 1 X 10-14 = KaKb
Acid and Base Salts
There are four rules that should be memorizied when trying to determine if a salt is acidic or basic.
I.) If a salt is composed of conjugates of a strong base and a strong acid, its solution will be neutral.
II.) If a salt is composed of the conjugates of a weak base and a strong acid, the solution will be acidic.
III.) If a salt is composed of the conjugates of a strong base and a weak acid, its solution will be basic.
IV.) If a salt is compose of the conjugates of a weak acid and weak base, the pH of the resulting solution will depend on the equilibrium constants of the acid and base.
Buffers
A buffer is a solution with a stable and known pH. It is composed of a large ammount of a weak acid or base with its conjugate. When calculating the pH of a buffer, you should use the following equation:
pH = pKa + log ([A-] / [HA])
[HA] = conc. of weak acid
[A-] = conc. of conjugate base
Titration
When titrating it is common to see the graph in the following text. This section will provide a very basic understanding of the graph, please consult your textbook or teacher for following information.
The only important things to realize here is that the Equivalence Point is when exactly enough base has been added to neutralize all of the acid. After you find this, the Half-Equivalence point is half of the base added (x-axis) of the Equivalence point. Here is the important part, at this point the pH of the solution (around 2.1 here) is equal to the pKa. Now you can easily find the Ka of the solution.
Special thank you to the princeton review and data from AP Chemistry labs to provide information on this section.