Chapter 21 Buffers And Neutralisation Flashcards
State what is meant by a buffer
A system which minimises pH changes on addition of small amounts of an acid or base
When can the pH of a buffer system change?
Adding large amounts of acid or base will change the pH of the buffer system
Describe how an acidic buffer is formed
-An acidic buffer can be formed from a weak acid (eg, ethanoic acid) and the salt of a weak acid (eg sodium ethanoate)
-The reaction between excess weak acid (eg, ethanoic acid) and a strong alkali (eg, NaOH)
Because the acid is in excess it will be able to react with the salt formed from the neutralisation reaction between the weak acid and strong base to form the buffer
What is an acidic buffer?
A system that buffers the pH below 7
Give the equation for the buffer system of ethanoic acid.
CH3COOH <—> CH3COO^- + H+
Describe how the buffer system works when more H+ is added
The negative ions (eg, ethanoate ions CH3COO^-) will combine with the extra H+ ions shifting equilibrium to the left forming more of the weak acid to restore pH.
Describe how the buffer system works when more base is added
The base will remove the H+ ions
This means that the weak acid has to dissociate more to replace the lost H+ ions causing equilibrium to shift to the right restoring pH.
What is the equation used to find [H+] of buffers?
Ka x [ACID]
[H+] = —————-
[SALT]
PH = -log [H+]
What is the equation used to find Ka of buffers?
[H+] [SALT]
Ka= —————-
[ACID]
What is the formula used to find Acid: Salt ratio?
Ka [SALT] [SALT]
——- = ———- = ———-
[H+] [ACID] 1
State what the normal pH of blood should be
The pH of blood is maintained at 7.40 by a buffer system
Describe how the buffer system of the blood counteracts an increase of H+ ions
The H+ will combine with the Hydrogen carbonate ions ( HCO3-) and shift equilibrium back to the left to restore pH forming more carbonic acid.
The carbonic acid will then dissociate into H2O and CO2 to prevent it from building up in the blood.
State the name that describes an increase of H+ ions in the blood
Acidosis where the pH lowers <7.35
Describe how the buffer system of the blood counteracts a decrease of H+ ions
Carbonic acid (H2CO3) will dissociate more shifting equilibrium to the right to produce more H+ ions.
State the name that describes a decrease of H+ ions in the blood
Alkalosis where pH rises >7.45
What type of buffer calculations may be asked?
- Direct combination of the weak acid and the salt forming the buffer
- From the combination of excess acid and a base where the excess acid will react with the salt formed (same number of moles as the limiting base) to form a buffer
What is the half equivalence point?
In the middle of the buffer region for weak acid and strong base pH titration curve
This is where there is equal concentration of weak acid and salt meaning it will have the maximum buffer capacity to deal with changes in OH- and H+
Name two indicators which can be used for acid- base titrations
Methyl orange
Phenolphthalein
How is the correct indicator selected for an acid base titration?
The indicator will need to change colour at the equivalence point of titration.
Therefore, the pH range of the indicator needs to lie within the pH range of the vertical section of the curve.
What is the indicator colour of methyl orange at low pH?
Red
What is the indicator colour of phenolphthalein at low pH?
Colourless
What is the approximate pH range for the colour change of methyl orange?
3.1 - 4.4
What is the approximate pH range for the colour change of phenolphthalein?
8.3 - 10
What is the colour change of methyl orange at high pH?
Yellow
What is the colour change of phenolphthalein at high pH?
Pink
What indicators can be used for strong acid/ strong base titrations?
Methyl orange
Phenolphthalein
What indicators can be used for weak acid/ strong base titrations?
Phenolphthalein only
What indicators can be used for strong acid/ weak base titration?
Methyl orange only
What indicators can be used for weak acid/ weak base titrations?
Neither indicators
PH meter needs to be used instead of
How do indicators work?
Indicators are weak acids and work as they have different coloured conjugate pairs.
As the pH of the solution changes during a titration, the equilibrium concentrations of the conjugate pairs also change. The colour will change depending on whether the indicator is protonated or deprotonated.
State what the vertical section of a pH titration curve shows
The volume of the substance in the burette needed to neutralise the substance in the conical flask.
