Buffers

Question 1

What is a buffer solution?

Answer 1

A system that minimises pH change when small amounts of an acid or base are added

Question 2

What are the 2 components of a buffer solution?

Answer 2

• The weak acid ,HA - this removes added alkali
• The conjugate base, A- - thus removes added acid

Question 3

How can you prepare a buffer solution from a weak acid and a salt?

Answer 3

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)

Question 4

How is a buffer prepared from an acid and alkali (neutralisation)?

Answer 4

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

Question 5

How can the control of pH in a buffer solution be explained?

Answer 5

The shift in the equilibrium position of the buffer solution

HA <=> H+ + A-

Question 6

What happens to the equilibrium position for HA (aq) <=> H+ (aq) + A- (aq) on the addition of acid H+?

Answer 6

• [H+] increases
• H+ (aq) ions react with the conjugate base
• The equilibrium position is shifted to then LEFT, removing most of the H+ ions

Question 7

What happens to the equilibrium position for HA (aq) <=> H+ (aq) + A- (aq) on the addition of alkali OH-?

Answer 7

• [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

Question 8

When is a buffer most effective?

Answer 8

When there are equal concentrations of weak acid and its conjugate base

Question 9

How do you calculate the pH of a buffer solution?

Answer 9

Question 10

What is an example of strong acid - strong base titration?

Answer 10

HCl and NaOH

Question 11

What is an example of a strong acid - weak base titration?

Answer 11

HCl and NH3

Question 12

What is an example of a weak acid - strong base titration?

Answer 12

CH3COOH and NaOH

Question 13

What is an example of a weak acid - weak base titration?

Answer 13

CH3COOH + NH3

Question 14

What does the titration curve for a strong acid - a strong base look like?

Answer 14

Question 15

What does the titration curve for a strong acid - weak base look like?

Answer 15

Question 16

What does the titration curve for a weak acid - strong base look like?

Answer 16

Question 17

What does the titration curve for a weak acid - weak base look like?

Answer 17

Question 18

What is the end point?

Answer 18

The point in a titration where the indicate changes colour

Question 19

What is the equivalence point?

Answer 19

The point during a titration when the amount of acid is exactly equal to the amount of base (full neutralisation occurs)

Question 20

What are appropriate indicators for a strong acid - strong base titration?

Answer 20

Methyl orange and phenolphthalein

Question 21

What is an appropriate indicator for a weak acid - strong base titration?

Answer 21

Phenolphtalein

Question 22

What is an appropriate indicator for a strong acid - weak base titration?

Answer 22

Methyl orange

Question 23

What is an appropriate indicator for a weak acid weak base titration?

Answer 23

There is no suitable indicator because there is no vertical section

Question 24

What is the colour change of methyl orange on addition of OH- into acidic?

HA (aq) <=> H+ (aq) + A- (aq)

Answer 24

• 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

Question 25

What is the colour change of methyl orange on addition of H+ into basic? HA (aq) <=> H+ (aq) + A- (aq)

Answer 25

- 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

Question 26

What acts as a buffer in human blood?

Answer 26

HCO3 -

Question 27

What is the equation/ buffer equilibrium in human blood?

Answer 27

CO2 (g) + H2O (l) <=> H+ (aq) + HCO3 - (aq)

Question 28

What should the pH of human blood be?

Answer 28

7.35 - 7.45

Question 29

What happens to the equilibrium in blood on addition of acid H+?

Answer 29

- [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)

Question 30

What happens to the equilibrium in blood on addition of an alkali OH-?

Answer 30

- [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)

Question 31

Explain how a titration curve progresses?

Answer 31

- 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

Question 32

What is a pH metre made of?

Answer 32

An electrode that is dipped into a solution and connected to a meter that displays the pH reading

Question 33

How do you measure pH with a pH meter?

Answer 33

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