Bronsted Theory
Acid: A chemical that releases a proton
Base: A Proton acceptor
Lewis acid and Bases
Acid: Electron Pair acceptor
Examples: BeCl2, AlCl3
Base: Electron pair donor
Examples: NH3, OH–
Dissociation constants (Ka/Kb)
The dissociation ratio is a measure of how weak/strong an acid is. As any other equilibrium constant the bigger the number will show that the equilibrium is shifted to the products.
The dissociation constant for an acid is:
The dissociation constant for a base is:
pH
pH = -log[H+]
Example 1: Calculate the concentration of propanoic acid with a pH of 3.52 and Ka of 1.35 x10-5 mol dm-3. Answer
pOH
pOH = -log[OH-]
pKw
In a soln. of water at room temperature, the hydroxide concentration multiplied by the hydronium concentration is going to equal 1 x10-14. If you take the negative log of both sides of this equation it shows you that the pH added to the pOH is going to equal 14.
For more information on Kw and pKw click here.
Indicator
An indicator is a chemical that changes colour depending on the pH of the solution. This is due to the fact that the indicator itself is a weak base, and both the uncharged molecule and the salt have different colours.
Every indicator changes colour at a particular pH, and therefore the right indicator has to be used for the right reactions.
Titration curves
The equivalence point of titration reaction is when OH– = H+. This is normally the average between the pH of the acid and that of the base. The right indicator has to be used in order to be able to observe the turning point.
| Indicator | pKa | Effective Range | Acid Colour | Base Colour |
| Methyl Red | 5.0 | 4.2-6.2 | Red | Yellow |
| Bromothymol Blue | 7.1 | 6.0-7.8 | Yellow | Blue |
| Phenolphthalein | 9.5 | 8.3-10.0 | Colorless | Red |
Buffers
A buffer solution is a solution that can keep the pH at a constant level even with the introduction of small amounts of an acid or a base. A buffer solution is composed of either a weak acid and its salt or a weak base and its salt.
Acid buffer (ethanoic acid):
Base buffer (ammonia):
Adding a base to an acid buffer:
Adding a base will reduce the concentration of H+ and therefore the equilibrium will shift to the right to form more H+.
Adding an acid to an acid buffer:
Adding an acid will increase the concentration of H+ and therefore the equilibrium will shift to the left to remove H+.
Buffer solutions are possible because the addition of the salt will instantly shift the equilibrium to the left, due to the introduction of the salt, and therefore small changes in the H+ concentration would be able to counteract due to the excess salt concentration.
Finding the pH of a buffer solution
The pH of a buffer solution can be found using the following equation:
For the derivation of this buffer equation please click here.
Example 2:
Find the pH when 1mL of 10M HCl is added to a buffer solution made up of 1M ethanoic acid and 1M sodium ethanoate when the volume of the buffer solution is 1L and Ka is 1.74 x 10-5. Answer
Questions on Acids and Bases