Module 5 Chapter 21

Question 1

What are buffer solutions

Answer 1

They are systems that minimise PH changes on the addition of small amounts of an acid or base

Question 2

What are buffers made out of

Answer 2

Weak acids and their conjugate bases

Question 3

What do the weak acid and conjugate bases in the acid do

Answer 3

They will act to remove the acid or the alkali

Question 4

What is the equation for the dissociation of weak acid HA

Answer 4

HA <-> H+ + A-

Question 5

What is A- in the dissociation of a weak acid

Answer 5

It is a conjugate base that removes added acid

Question 6

What happens when alkalis/acids are added to a bugger

Answer 6

Both components in the buffer solution react and are eventually used up

Question 7

What do HA and A- act as

Answer 7

2 independent reserves that are capable of reacting with added alkalis or acids, respectively

Question 8

When is a buffer most effective

Answer 8

When [HA] = [A-]

Question 9

When is a buffer no longer effective

Answer 9

When one of the components is completely used up. And the solution loses its buffering ability towards added acid or alkalis.

Question 10

What is a feature of the PH of a buffer

Answer 10

It varies slightly when acting with any added species, or not be assumed to be totally constant

Question 11

What do buffers based on weak acids require

Answer 11

A weak acid and its conjugate bases

Question 12

What are the two methods for preparing a buffer

Answer 12

1.Adding solutions of a weak acid and conjugate salt
2. Partial neutralisation of weak acids

Question 13

What is the process of making a buffer by adding solutions of a weak acid and its conjugate salt

Answer 13

You add the weak acid to water, so it partially dissociates and the amount of A- ions is very small

Salt of weak acid = conjugate base, when added to water, completely dissolves

Question 14

What is a feature of the weak acid that has been dissolved in water and the salt

Answer 14

As the dissociation of weak acids is very low the concentration of the acid can assume to remain the same. As the salts are ionic they completely dissociate in solution so provide a concentration for the conjugate base

Question 15

How can you prepare buffers by partial neutralisation of weak acid

Answer 15

Aqueous solution of alkali added to an excess of the weak acid
The weak acid is partially neutralised forming conjugate base
Some weak acid remains unreacted
So the mixture contains salt of the weak acid and an unnamed weak acid

Question 16

What is the position of Ka at equilibrium of ethanol acid and what occurs as a result

Answer 16

It is very much towards the acid
So when the CH3COO- is added, the equilibrium position shifts more to the left reducing small [H+] ions
Resulting in a solution with two reservoirs that act independently

Question 17

What happens when you add acid to a buffer solution

Answer 17

1. [H+] increases
2. Conjugate base (A-) reacts
3. Equilibrium shifts left
4. [H+] removed increasing ph back to normal

Question 18

What happens when you add alkali to a buffer solution

Answer 18

1. [OH-] increases
2. H+ reacts
3. Equilibrium moves to the right
4. PH lowers

Question 19

What do different weak acids form

Answer 19

Buffer solution that work for different PH ranges

Question 20

What is the ph of the buffer equivalent to

Answer 20

Pka of HA

Question 21

What is the operating ph of a buffer typically over

Answer 21

-2Ph units, centred at the PH of the pKA value

Question 22

What can the ratio of the weak acid and conjugate base be adjusted to

Answer 22

The pH of the buffer solution

Question 23

How is the Ph for the buffer calculated

Answer 23

By rearranging the expression for ka

Question 24

What is a difference between the calculation of the ph of a buffer and the calculate of the ph of a weak acid

Answer 24

When calculating the PH of a weak acid, the approximation: [H+] = [A-] is used. However this doesn’t hold true for the buffer, as A- has been added as one of the components of the buffer.

Question 25

Why are Ph levels controlled int he body

Answer 25

They are under precise control so that proteins etc can function effectively

Question 26

What is the PH of blood plasma

Answer 26

PH 7.35-7.45

Question 27

What controls the PH in the blood

Answer 27

The buffer mixture controls this PH, carbonic acid - hydrogen carbonate (H2CO3 —> HCO- 3)

Question 28

What can a PH below 7.35 result in

Answer 28

Acidosis

Question 29

What can acidosis result in

Answer 29

Fatigue, shortness of breath and/or shock/death

Question 30

What can a PH above 7.45 result in

Answer 30

Alkalosis

Question 31

What does alkalosis result in

Answer 31

Spasms, light headed and/or nausea

Question 32

What happens as the body produced acidic products to maintain blood PH

Answer 32

The [H+] ion concentration will increase The conjugate base reacts with H+ The equilibrium position will shift to the left removing H+ from the blood

Question 33

What happens if the acid levels continue to rise

Answer 33

The PH would eventually fall leading to acidosis and symptoms would show, this is because the [HCO3 -] could fall too far/be used up so the buffer system will not be able to function

Question 34

What happens in order to maintain blood pH when alkali is added

Answer 34

The [OH- (aq)] increases Small concentrations of H+ ions reacts with OH- ions H+ + OH- —> H2O H2CO3 dissocociates, equilibrium position shifts to the right to restore H+ ions.

Question 35

What is used to monitor PH changes

Answer 35

A PH meter

Question 36

How does a PH meter record changes in PH

Answer 36

It is an electrode that is dipped into a solution, connected to a meter then displays a PH reading.

Question 37

What is a comparison between a PH meter and an indicator

Answer 37

A PH meter gives a more accurate measurement than just comparing colour to indicator PH

Question 38

What is the process of doing a strong acid-base titration with a PH meter (1)

Answer 38

1) Pipette a measured volume of acid into a conical flask 2) Place electrode of PH meter into the flask and record PH 3) Add aqueous standard solution to a conical flask and unknown to burette 4) Add 1cm3 of solution in burette each time and then swirl and record the PH and total volume of base added

Question 39

What is the process of doing a strong acid-base titration with a PH meter (2)

Answer 39

5. Repeat step 4 until pH starts tot change rapidly 6. Now add the base drop wise until the pH starts changing less rapidly 7. Continue adding base 1cm^3 at a time until the acid is in excess pH hardly alters with additional volumes and is now basic 8. Plot the data on a graph of acid volume to PH, including a line of best fit

Question 40

What does the shape of a titration curve depend on

Answer 40

The substance being titrated Type of acid or base (strong/weak)

Question 41

What does the titration curve between a strong acid and base look like and annotate the different stages

Answer 41

DRAW IT

Question 42

What does the shape of the titration curve depend on

Answer 42

The substance being titrated ie. Acid or base Type of acid or base (strong/weak)

Question 43

What is the equivalence point

Answer 43

It is the volume required to have completely reacted the acid and base together.

Question 44

What does the PH of the equivalence point depend on

Answer 44

The type of acid and base used

Question 45

What is the equivalence point normally for the different acids being added to the bases

Answer 45

Strong acid + strong base = 7 Strong acid + weak base < 7 Weak acid + strong base >7

Question 46

Why is the equivalence point of a strong acid - strong base 7

Answer 46

All the H+ ions completely neutralised by OH- ions so only water and salt present i.e neutral

Question 47

Why is the equivalence point

Answer 47

A weak base - NH3, will have a strong conjugate acid, NH4+ which will react with water to produce H3O + therefore

Question 48

Why is the equivalence point >PH7 for a weak acid and a strong base

Answer 48

A weak acid will have a strong conjugate base A- which can react with water to produce OH - ions PH>7

Question 49

What are acid-base indicators

Answer 49

They are weak acids that have different coloured conjugate bases

Question 50

What happens to an indicator in acidic conditions and basic conditions

Answer 50

The indicator equilibrium is shifted towards the weak acid. As the system becomes more basic the equilibrium shifts towards the conjugate base, altering the colour.

Question 51

What is the end point

Answer 51

The end point is when there is a change in colour in the titration

Question 52

What is a feature of each indicator

Answer 52

They will have different pH values for the end point as they each have different Ka values. But how they work is the same

Question 53

How can the pH of the end point be determined

Answer 53

Forma the pKA value of HA as at end point [HA] = [A-} therefore Ka = [H+]

Question 54

How do you choose an indicator

Answer 54

You choose an indicator that has a colour change that coincides with the vertical section of the PH curve

Question 55

What are the range of colour changes for the different indicators

Answer 55

Phenolpthalein - colourless < 8.2, pink > 8.2 Bromethymol blue - yellow < 7, Blue > 7.6 Methyl orange - red PH < 3, yellow PH>4