Buffers and neutralisation

Question 1

What is a buffer solution

Answer 1

this is a mixture of a weak acid and conjugate base that minimises pH change on addition of small amounts of acid or base

Question 2

Describe a buffer solution

Answer 2

• it resists changes in pH during the addition of small amounts of acid or alkali, the changes are minimised for as long as there is a buffer solution

Question 3

Describe the ethanoic acid and sodium ethanoate system

Answer 3

• the weak acid CH3COOH partially dissociates
  CH3COOH == H+ + CH3COO-
• the salt dissociates completely generating the conjugate base CH3COO-
  CH3COO-Na+ == CH3COO- + Na+
• equilibrium mixture formed contains a high concentration of the undissociated weak acid, CH3COOH, and its conjugate base CH3COO-
• the high concentration of the conjugate base pushes the equilibrium to the left so the concentration of H+ ions is very small

Question 4

what is the alternative that you can make buffers from

Answer 4

• can be made from a weak acid and strong alkali

- in this situation an mixture of the salt and excess of a weak acid is formed

Question 5

How does a buffer control pH

Answer 5

HA == H+ + A-

• the weak acid HA removes added alkali
• the conjugate base A- removes added acid

when H+ is added to the solution

• H+ is increased
• the conjugate base A- reacts with H+ and removes the added acid
• the equilibrium moves to the left removing most of the added H+ ions

when OH- is added
- OH- is increased
- the small concentration of H+ ions reacts with the OH- IONS
H+ + OH- = H20
- HA dissociates shifting the equilibrium to the right to restore most of the H+ ions that have reacted

Question 6

How do you calculate the pH of buffer solutions

Answer 6

H+ = Kax[HA]/[A-]

Question 7

What are the two assumptions

Answer 7

[HA] equilibrium = [HA] undissoicated

Question 8

Describe the carbonic-acid hydrogencarbonate buffer system

Answer 8

• healthy human blood needs to have a pH between 7.35 and 7.45
• if the pH falls below 7.35 then acidosis can occur if the pH 7.45 then the condition is alkalosis
• pH is controlled by a mixture of buffers, the carbonic acid-hydrogencarboante ion buffer is the most important buffer
• increase in H+ ions then this is removed by the conjugate base HCO3 -, the equilibrium shifts left removing most of the hydrogen ions
• any increase in OH- ions and it is removed by the weak acid H2CO3 the small concentration of H+ ions react with the OH- ions
• controlled by changing rate of breathing, breathing more fastly removes more carbon dioxide
• carbonic acid is converted into aqueous carbon dioxide

Question 9

What is the equivalence point

Answer 9

this is the point in a titration at which the volume of one solution has reacted exactly with the volume of the second solution, this matches the stoichiometry of the reaction taking place

Question 10

what is the end point of a reaction

Answer 10

the end point is the point in a titration at which there are equal concentrations of the weak acid and conjugate base forms of the indicator, the colour at the end point is midway between the colours of the acid and conjugate base forms

Question 11

describe titrations needed to carry out this experiment

Answer 11

when carrying out a titration you are determining the volume of one solution that reacts exactly with a known volume of another solution, this is the equivalence point of the titration
- at this point the solution in the conical flask has reacted exactly with the solution in the burette

Question 12

describe the titration curve

Answer 12

section 1
- a slight increase in pH occurs as base is added, the increase is only slight because the acid is in excess, IT IS NOT HORIZONTAL
Section 2
- a sharp rise in pH occurs, the acid is no longer in excess so any base added has a large impact on the pH
- the equivalence point is the centre of the vertical section of the titration curve
Section 3
- slight increase in pH occurs as further base is added, the increase is only slight because the base is in excess now and extra base has little impact on the pH, it is rising slightly

Question 13

draw the titration curve for a strong base-strong base graph

Answer 13

DRAW IT

• vertical section covers a large change in pH, starting around pH 3 and ending at around pH 11 with an equivalence point at pH 7
• use methyl orange and phenolphthalein

Question 14

what are pH meters used for

Answer 14

they are used to measure the pH of the reaction mixture as solution from the burette is added over time

Question 15

How does the pH meter work

Answer 15

• pH gives more accurate reading than a universal indicator
• accurate reading to 2 decimal places
• contains an protected electrode which is placed in the solution and a small computer display gives the pH reading
• calibrate it first
• probe is removed from its storage solution and rinsed with deionised water
• the probe is blotted dry and then placed into a solution of a known pH often starting with pH 4 left to settle then checking that the pH 4 has been registered
• repeated with pH 7 and pH 10
• confirms that the pH probe is accurately measuring pH across a range of acidic neutral and alkaline values

Question 16

How do you chose a suitable indicator

Answer 16

• indicator is chosen so that the pH value of the end point is close as possible to the pH value of the titrations equivalence point
• suitable indicator occurs within the vertical section
• drop by drop
• indicator is not on the equivalence point because the equivalence point occurs halfway through an sharp rise or fall in pH, the moment at which the indicator passes through its end point is effectively the same as the moment that the equivalence point takes place

Question 17

Draw graph for strong acid - weak base titrations

Answer 17

• draw graph
• verticle section covers a smaller change in pH starting around pH 3 and ending around pH 7.5 with the equivalence point occuring at an more acidic value lower than 7
• methyl orange has an end point that falls within this pH range

Question 18

Draw graph for weak acid - strong base titrations

Answer 18

• vertical section covers a small range in pH and occurs towards the higher pH values
• starts at pH 6.5 and ends at around pH 11.5
• equivalence pH occurs at a more basic pH - this is a value above 7
• phenolphthalein has an end point that falls within this pH range

Question 19

Draw graph for weak acid - weak base titrations

Answer 19

• no real vertical section,
• neither indicator has an end point near the equivalnce point
• neither indicator is suitable
• • an indicator would change colour gradullay over a few cm3 of base added
• no indicators are suitable