Buffers Flashcards
What is a buffer solution?
A system that minimises pH change when small amounts of an acid or base are added
What are the 2 components of a buffer solution?
- The weak acid ,HA - this removes added alkali
- The conjugate base, A- - thus removes added acid
How can you prepare a buffer solution from a weak acid and a salt?
When ethanoic acid is added to water, the acid partially dissociates and the amount of ethanoate ions in solution is very small. Ethanoic acid is the source of the weak acid component in the buffer.
CH3COOH (aq) <=> H+ (aq) + CH3COO- (aq)
Salts of weak acids are ionic compounds and provide a conjugate base, When added to water, the salt completely dissolved. Dissociation into ions is complete and so the salt is the source of the conjugate base
CH3COONa (s) + aq -> CH3COO- + Na+ (aq)
How is a buffer prepared from an acid and alkali (neutralisation)?
By adding an aqueous solution of an alkali, such as NaOH (aq), to an excess of the weak acid. The weak acid is partially neutralised by the alkali, forming the conjugate base. Some of the weak acid is left over unreacted. The resulting solution contains a mixture of the salt of the weak acid and any unreacted weak acid
How can the control of pH in a buffer solution be explained?
The shift in the equilibrium position of the buffer solution
HA <=> H+ + A-
What happens to the equilibrium position for HA (aq) <=> H+ (aq) + A- (aq) on the addition of acid H+?
- [H+] increases
- H+ (aq) ions react with the conjugate base
- The equilibrium position is shifted to then LEFT, removing most of the H+ ions
What happens to the equilibrium position for HA (aq) <=> H+ (aq) + A- (aq) on the addition of alkali OH-?
- [OH-] increases
- The small concentration of H+ ions reacts with the OH- to form water
- HA dissociates, shifting the equilibrium position to the RIGHT to restore most of the H+ ions
When is a buffer most effective?
When there are equal concentrations of weak acid and its conjugate base
How do you calculate the pH of a buffer solution?
What is an example of strong acid - strong base titration?
HCl and NaOH
What is an example of a strong acid - weak base titration?
HCl and NH3
What is an example of a weak acid - strong base titration?
CH3COOH and NaOH
What is an example of a weak acid - weak base titration?
CH3COOH + NH3
What does the titration curve for a strong acid - a strong base look like?
What does the titration curve for a strong acid - weak base look like?
What does the titration curve for a weak acid - strong base look like?
What does the titration curve for a weak acid - weak base look like?
What is the end point?
The point in a titration where the indicate changes colour
What is the equivalence point?
The point during a titration when the amount of acid is exactly equal to the amount of base (full neutralisation occurs)
What are appropriate indicators for a strong acid - strong base titration?
Methyl orange and phenolphthalein
What is an appropriate indicator for a weak acid - strong base titration?
Phenolphtalein
What is an appropriate indicator for a strong acid - weak base titration?
Methyl orange
What is an appropriate indicator for a weak acid weak base titration?
There is no suitable indicator because there is no vertical section
What is the colour change of methyl orange on addition of OH- into acidic?
HA (aq) <=> H+ (aq) + A- (aq)
- OH- ions react with H+ in the indicator : H+ + OH- -> H2O
- The weak acid, HA, dissociates shifting the equilibrium to the right
- THe colour changes, first from red to orange at the end point, and finally to yellow when equilibrium is shifted right
What is the colour change of methyl orange on addition of H+ into basic?
HA (aq) <=> H+ (aq) + A- (aq)
- H+ ions react with the conjugate base A-
- Equilibrium position shifts to the left
- The colour changes from yellow to orange at the end point and finally to red when equilibrium has shifted to the left
What acts as a buffer in human blood?
HCO3 -
What is the equation/ buffer equilibrium in human blood?
CO2 (g) + H2O (l) <=> H+ (aq) + HCO3 - (aq)
What should the pH of human blood be?
7.35 - 7.45
What happens to the equilibrium in blood on addition of acid H+?
- [H+] increases
- H+ (aq) ions react with the conjugate base, HCO3 -
- The equilibrium position shifts to the left, removing most of the H+ ions
H2CO3 (aq) <=> H+ (aq) + HCO3 - (aq)
What happens to the equilibrium in blood on addition of an alkali OH-?
- [OH-] increases
- The small concentration of H+ ions react with the OH- ions to form water
- H2CO3 dissociates, shifting the equilibrium right to restore most of the H+ ions
H2CO3 (aq) <=> H+ (aq) + HCO3 - (aq)
Explain how a titration curve progresses?
- When the base is first added the acid is in great excess and the pH increases very slightly
- As the vertical section is approached, the pH starts to increase more quickly as the acid is used up more quick;y
- Eventually the pH increases rapidly during addition of a vary small volume of base, producing the vertical section
- After the vertical section, the pH will rise very slightly as the base is now in great excess
What is a pH metre made of?
An electrode that is dipped into a solution and connected to a meter that displays the pH reading
How do you measure pH with a pH meter?
1) Using a pipette, add a measured volume of acid to a conical flask
2) Place the electrode of the pH meter in the flask
3) Add the aqueous base to the burette and add to an acid in the conical flask, 1cm^3
4) After each addition, swirl the contents. Record the pH and the total volume of the aqueous base added
5) Repeat steps 3 and 4 until the pH starts to change more rapidly. Then add the aqueous base drop-wise for each reading until the pH changes less rapidly
6) Now add the aqueous base 1cm^3 at a time again until an excess has been added and the pH has been basic, with little change for several addition
7) Plot a titration curve
