buffers and neutralisation

Question 1

what is a buffer solution

Answer 1

• system that minimises pH changes when a small amounts of acid or a base are added

Question 2

What are the two component buffer solutions contain to remove added acid or alkali

Answer 2

• weak acid
• conjugate base

Question 3

What does the weak acid and conjugate base do to act as a buffer

Answer 3

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

Question 4

what happens when alkalis and acids are added to a buffer

Answer 4

• two components in the buffer solution react and will eventually be used up
• as soon as one component has all reacted the solution loses its buffering ability towards added acid or alkalis
• as the buffer works the Ph does change but only by a small amount - not assume that pH stays completley constant

Question 5

what does a buffer solution based on a weak acid needs

Answer 5

• weak acid
• conjugate base

Question 6

describe the preparation of a buffer from a weak acid and its salt using ethanoic acid and sodium ethanoate giving an equation

Answer 6

• eg mix solution of ethanoic acid and one its salt eg sodium ethanoate
• when ethanoic acid is added to water the acid partially dissociates and the amount of ethanoate ions in the solution is very small
• ethanoic acid is the source of the weak acid component of the buffer solution
• salts of weak acids are ionic compounds and provide a convenient source of the conugate base
• when added to water the salt complelty dissolves
• dissociation into ions is complete and so the salt is the source of the conjugate base component of the buffer solution
• CH3COOH(aq) reversible sign H+(aq) + CH3COO-
• CH3COONa(s) +aq -> CH3COO-(aq) + Na+(aq)

Question 7

describe the preparation of buffers via partial neutralisation of the weak acid

Answer 7

• buffer can also be prepeared by adding an aqueous solution of an alkali such as NaOH (aq) to an excess of weak acid
• the weak acid is partially neutralised by the alklali forming the conjugate base
• some of the weak acid is left over unreacted
• resulting solution contains a mixture of salt and the weak acid and any unreacted weak acid

Question 8

describe the two resovoirs to remove added acid and alkali

Answer 8

• in eethanoic acid equilbrium the equilibrium position lies well towards ethanoic acid
• when CH3COO- ions are added to CH3COOH the equilibrium position shifts even further to the left reducing the small concentration of H+(aq) ions and leaving a solution containing two components CH3COOH and CH3COO-
• CH3COOH and CH3COO- act as two respvoirs that are able to act independently to remove added acid and alkali
• shifting the buffers equilbrium system either right or left

Question 9

what controls the pH in an acid buffer solution

Answer 9

• conjugate acid base pair HA(aq)/A-(aq)
• HA(aq) reversible sign H+(aq) + A-(aq)

Question 10

how does the conjugate base remove added acid

Answer 10

• on addition of an acid H+ (aq)
• [H+(aq)] increases
• H+ (aq) ions react with the conjugate base, A- (aq)
• the equilibrium position shifts to the left removing most of the H+ (aq) ions

Question 11

describe how the weak acid removes added alkali

Answer 11

• on addition of an alklai OH-(aq)
• [OH-(aq)] increases
• small concentrations of H+(aq) ions reacts with the OH- (aq) ions
• H+(aq) + OH- (aq) -> H2O(l)
• HA dissociates shifting equilibrium position to the right to restore most of the H+(aq) ions

Question 12

How do you choose the components for a buffer solution

Answer 12

• different weak acids result in buffer solutions that operate over different pH ranges
• a buffer is most effective at removing either added acid or alklai when they’re equal concentrations of the weak acid and its conjugate base
• [HA(aq)]=[A-(aq)]
• pH of the buffer solution is the same as the pKa value of HA
• operating pH is typically over about two pH units centred at the pH of the pKa value
• the ration of the conenctrations of the weak acid and its conjugate base can be adjusted to find tune the pH of the buffer solution

Question 13

why are buffer solutions inportant in living system

Answer 13

• well being of the human body relies on the precise pH control with different parts of the body requiring specific pH values for effective functioning
• enzymes are particularly sensetive and each has an optimum pH
• role of pH control in the body falls to buffer solutiosn eg plasma in the blood

Question 14

How does plasma control blood pH

Answer 14

• maintains between 7.35-7.45
• pH controlled by a mixture of buffers with the carbonix acid - hydrogencarbonate (H2CO3/HCO3-) buffer system being the mot important
• healthy blood should have the pH of 7.40

Question 15

What happens if the pH of the blood falls below 7.35

Answer 15

• if pH falles below 7.35 people can develop a condition called acidosis cause fatigue shortness of breath and in extreme cases, shock or death

Question 16

what happens if pH of blood rises above 7.45

Answer 16

• if pH rises above 7.45 the condiition is alkalosis causes muscle spasms, light headedness and nausea

Question 17

describe the addition of H+(aq) ions to the carbonic acid - hydrogencarbonate buffer solution

Answer 17

• [H+(aq)] increases
• H+(aq) ions react with the conjugate base HCO3- (aq)
• equilibrium position shift to the left removing most of the H+(aq) ions

Question 18

Describe the addition of an alkali to the carbonic acid - hydrogencarbonate buffer system

Answer 18

• [OH-(aq)] increases
• small concentration of H+ (aq) ions reacts with the OH- (aq) ions
• H+(aq) +OH-(aq) ->H2O(l)
• H2CO3 dissociates shifting equilbrium position to the right to restore most of the H+(aq) ions
  *

Question 19

does the body produces more acidic or more alkaline materials and how does the body get rid of this

Answer 19

• body produces far more acidic materials than alkaline
• conjugate base HCO3 - converts to H2CO3
• body prevents H2CO3 building up by converting it to carbon dioxide gas which is then exhaled by the lungs

Question 20

how are neutralisation reactions monitored

Answer 20

• acid base titrations use indicators to monitor neutralisation reactions accuratley
• the results are then analysed to find unknown information bout the acid or the base
• pH meter used to monitor change

Question 21

What is a pH meter

Answer 21

• an electrode dipped into a solution and connected to a meter that displays the pH reading
• pH meter typically records pH values to two decimal places

Question 22

describe the method for monitoring the pH as an aqueous base is added to an acid solution

Answer 22

• using a pipetter add a measured volume of acid to a conical flask
• place the elctrode of the pH meter in the flask
• add the aqueous base to the burette and add to the acid in the concical flask 1cm3 at a time
• after each addition swirl the contents record the pH and total volume of the aqueous base added
• repeat 3 and 4 until pH start to change more rapildy then add the aqueous base dropwise for each reading until the pH changes less rapidly
• now add the aquous base 1cm3 at a time again until an excess has been added and the pH has been basic with little change for several additions
• graph of pH against total volume of aqueous base added is then plotted curve for tritration between 0.1 moldm-3 solutions of a strong monobasic acid and a strong monobasic base
• For an alternative automatic method, you could attach the pH meter to a
  datalogger and use a magnetic stirrer in the flask.
• The aqueous base would
  then be added from the burette to the flask slowly, and the pH titration
  curve could be plotted automatically using the datalogger or appropriate
  software on a computer.