Chapter 21 - Buffer And Neutralisation Flashcards
What is a buffer solution?
A solution that minimises pH change when small amounts of acid or base are added
What are the two components of a buffer solution?
A weak acid and it’s conjugate base
What is the role of the weak acid in a buffer solution?
Removes any alkali
What is the role of the base in a buffer solution?
Removes any acid
What are the two ways in which a buffer solution can be made?
Partial neutralisation of an acid and preparing the weak acid from its salt
How do you make a buffer solution using a weak acid and its salt?
Mix the two together
The acid partially dissolves in the solution and the salt fully dissociates. This means that the ions produced by the salt act as the conjugate base
How do you make a buffer solution by partially neutralising a weak acid?
An add excess of weak acid to a strong alkali. Once the two have reacted, you are still left with some weak acid because it was in excess
What is the function of the two reservoirs?
If you add acid, the H+ ions combine with the conjugate base ions to form more acid and shift the equilibrium to the left and reduce the H+ concentration so the pH stays fairly constant
If you add alkali, the OH- ions combine with the H+ ions in the acid to form water. This removes H+ ions from the mixture, so more weak acid has to dissociate to form more, shifting the equilibrium to the right
When is a buffer solution most effective?
When there are equal concentrations of weak acid and conjugate base
What can be observed when the concentrations of acid and base are equal?
The pH of the buffer solution is roughly the same as the pKa of the acid
The operating pH range is typically 2 units
How do you calculate the pH of a buffer solution?
[H+] = Ka x [HA]
[A-]
What assumptions have to be made to calculate the pH of a buffer solution?
The salt of the conjugate base is fully dissociated
HA is only slightly dissociated
What organic molecules are particularly sensitive to pH?
Enzymes
What happens when blood pH falls below 7.35?
A condition called acidosis can develop, which can lead to fatigue and shortness of breath
What happens when blood pH rises above 7.45?
A condition called alkalosis can develop, causing nausea and muscle spasms
What buffer system primarily controls blood pH?
The carbonic acid-hydrogencarbonate buffer system
What happens when H+ is added to the carbonic acid-hydrogencarbonate buffer system?
[H+] increases, so H+ ions react with the conjugate base (HCO3-)
This causes the equilibrium to shift to the left to remove the H+ ions
What happens when OH- is added to the carbonic acid-hydrogencarbonate buffer system?
[OH-] increases, so OH- ions react with H+ ions to produce water
H2CO3 dissociates to restore the H+ ions and the equilibrium shifts to the right
Describe the graph of a typical pH titration curve
1) When the base is first added, the increase in pH is very slight as the acid is still in excess
2) The equivalence point - the vertical section of the graph where the addition of a small amount of base results in a large increase in pH
3) Towards the end of the graph, the base is in excess, so the gradient decreases
What is the equivalence point?
The volume of one solution that reacts exactly with the volume of the other solution. It is the central point of the vertical section
What colour will the end point of a titration be when using methyl orange?
Orange
What happens when OH- ions are added to an acidic solution?
OH- ions react with H+ ions in the indicator
The weak acid dissociates, shifting the equilibrium to the right
The colour changes to yellow as the equilibrium shift
What happens when H+ ions are added to a basic solution?
H+ ions recap with the conjugate base
The equilibrium shifts to the left
The colour changes to red as the equilibrium shifts
How do you select an indicator to use for a titration?
It must have a colour change that coincides with the vertical section
