Buffers Flashcards
What is a buffer?
A solution that resists change in pH when small amounts of acid or alkali are added to it
Do buffers fully stop changes in pH?
No just resists change in pH when you add acid or alkali, but if you add too much will eventually change pH
What are the 2 types of buffer?
Acidic buffer
Basic buffer
What is an acidic buffer?
Formed from a week acid (carboxylic acid) and its salt (carboxylate salt)
What pH do acidic buffers aim to keep?
Under 7
What is a basic buffer made of?
A weak base + the salt it can form eg NH3 and NH4+
What pH do basic buffers aim to keep?
Over 7
What type of buffer does Edexcel require knowledge of?
Acidic buffers
What equations occur in an acidic buffer?
RCOOH <——> RCOO- + H+
RCOO-Na+ <——> RCOO- + Na+
How does a a buffer work if we add an acid?
We already have a large reservoir of carboxylate ions due to the dissociation of the weak acid’s salt + the weak acid
Adding acid = Increases the concentration of H+ ions
These react with carboxylate ions and shifts POE of first equation left to remove H+, until original is restored
How long is the position of equilibrium shifted left when an acid is added to an acidic buffer?
Until the original concentration of H+ is restored
How does a buffer work if we add an alkali?
Adding it Increases the concentration of OH-
This will react with H+ ions which decreases pH
Causes a shift in equilibrium to the right so carboxylic acid dissociates more and adds H+ ions to its original level
What are the important aspects of an acidic buffer solution we must have?
High concentration of carboxylate ion so it can react with additional H+ ions to resist pH change by removing H+
High concentration of ethanoic acid to dissociate to replace lost H+ ions
2 methods of making a buffer solution
Mixing a weak acid and its salt
Mixing an excess of weak acid and a small volume of strong base
How does making a buffer from weak acid + strong base work?
ALLL the strong base reacts so mol of strong base added = mol of salt formed
all Mol of salt formed dissociated into the carboxylate ion, so this forms the buffer
So amount of base added = amount of salt formed = amount of carboxylate (A-) ion
Assumption when calculating pH of a weak base made from strong base and weak acid
Mol of strong base reacts = mol of carboxylate ion A-
Calculating the pH of a buffer made of weak acid + strong base
Find start Mol of weak acid
Find mol of strong base = mol of carboxylate ion
Find left over mole of weak acid: original mole - mole of strong base
Calculate total volume and convert all moles to conc
Find H+ using the Ka expression using above concentrations
Calculating pH of a buffer made of weak acid + weak acid salt assumption
Weak acid salt dissociates fully so the moles of the weak acids salt = moles of carboxylate ion
Calculating pH of a buffer made of strong base + weak acid
Work out moles of weak acid in excess»concentration
Work out moles of hydroxide added=moles of carboxylate salt formed»concentration
Use Ka expression to find conc of H+
-log(H+)
Example of biological buffer
Carbonic acid-hydrogencarbonate equilibrium in the blood
Controls blood pH
The Carbonic acid-hydrogencarbonate equilibrium in the blood
HOCOOH <—–> HCO₃- + H+
How does the carbonic acid-hydrogencarbonate equilibrium work if blood becomes more acidic?
Position of equilibrium shifts left because [H+] increased so to counteract change and restore original ratio of HOCOOH:HCO₃-, the large reservoir of HCO₃- reacts with H+
So [H+] is lowered to increase pH back to normal
How does the carbonic acid-hydrogencarbonate equilibrium work if blood becomes more alkaline
OH- added reacts with H+ to form water so this decreases [H+] So position of equilibrium shifts right to produce more H+
Because of large concentration of HCO3-, ratio stays fairly constant and there is no change in pH
Work out new pH of buffer if an acid is added
Work out the moles of acid and salt in the initial buffer
Find moles of acid added, this reacts completely with moles of carboxylate salt so moles of carboxylate salt DECREASE by mol of acid added
So therefore by the same number of moles of weak acid increases
Use new values + Ka to find H+
