Acids and Bases

Summary

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:

blank picPka

The dissociation constant for a base is:

Pkb

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.

water

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.

indicator

Every indicator changes colour at a particular pH, and therefore the right indicator has 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.

titration curve

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

Endpoint vs Equivalence Point

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):

acid buffer

Base buffer (ammonia):

base buffer

Adding a base to an acid buffer:

Adding a base will reduce the concentration of Hand 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:

salt

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

 

Advertisements