Module 5 Chapter 21
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What are buffer solutions
They are systems that minimise PH changes on the addition of small amounts of an acid or base
What are buffers made out of
Weak acids and their conjugate bases
What do the weak acid and conjugate bases in the acid do
They will act to remove the acid or the alkali
What is the equation for the dissociation of weak acid HA
HA <-> H+ + A-
What is A- in the dissociation of a weak acid
It is a conjugate base that removes added acid
What happens when alkalis/acids are added to a bugger
Both components in the buffer solution react and are eventually used up
What do HA and A- act as
2 independent reserves that are capable of reacting with added alkalis or acids, respectively
When is a buffer most effective
When [HA] = [A-]
When is a buffer no longer effective
When one of the components is completely used up. And the solution loses its buffering ability towards added acid or alkalis.
What is a feature of the PH of a buffer
It varies slightly when acting with any added species, or not be assumed to be totally constant
What do buffers based on weak acids require
A weak acid and its conjugate bases
What are the two methods for preparing a buffer
1.Adding solutions of a weak acid and conjugate salt 2. Partial neutralisation of weak acids
What is the process of making a buffer by adding solutions of a weak acid and its conjugate salt
You add the weak acid to water, so it partially dissociates and the amount of A- ions is very small Salt of weak acid = conjugate base, when added to water, completely dissolves
What is a feature of the weak acid that has been dissolved in water and the salt
As the dissociation of weak acids is very low the concentration of the acid can assume to remain the same. As the salts are ionic they completely dissociate in solution so provide a concentration for the conjugate base
How can you prepare buffers by partial neutralisation of weak acid
Aqueous solution of alkali added to an excess of the weak acid The weak acid is partially neutralised forming conjugate base Some weak acid remains unreacted So the mixture contains salt of the weak acid and an unnamed weak acid
What is the position of Ka at equilibrium of ethanol acid and what occurs as a result
It is very much towards the acid So when the CH3COO- is added, the equilibrium position shifts more to the left reducing small [H+] ions Resulting in a solution with two reservoirs that act independently
What happens when you add acid to a buffer solution
• [H+] increases • Conjugate base (A-) reacts • Equilibrium shifts left • [H+] removed increasing ph back to normal
What happens when you add alkali to a buffer solution
• [OH-] increases • H+ reacts • Equilibrium moves to the right • PH lowers
What do different weak acids form
Buffer solution that work for different PH ranges
What is the ph of the buffer equivalent to
Pka of HA
What is the operating ph of a buffer typically over
-2Ph units, centred at the PH of the pKA value
What can the ratio of the weak acid and conjugate base be adjusted to
The pH of the buffer solution
How is the Ph for the buffer calculated
By rearranging the expression for ka
What is a difference between the calculation of the ph of a buffer and the calculate of the ph of a weak acid
When calculating the PH of a weak acid, the approximation: [H+] = [A-] is used. However this doesn’t hold true for the buffer, as A- has been added as one of the components of the buffer.