Unit 2) 6) Buffers And Indicators

Question 1

Small changes in pH can have a surprisingly _____ affect on a system. What are some examples ?

Answer 1

Small changes in pH can have a surprisingly large affect on a system.
- for example adding a small volume of lemon juice or vinegar to milk changes the protein structure and curdling occurs.

Question 2

Many processes , particularly in living systems, have to take place within a _______ pH range
Give an example

Answer 2

Precise

Should the pH of blood move 0.5 units outside its range , the person would become unconscious and die

Question 3

What is a buffer solution

Answer 3

A buffer solution is one in which the pH remains approximately constant even when small amounts of acid or base or water are added

Question 4

What are the two types of buffer and what do they contain

Answer 4

• acid buffer - consist of a solution of a weak acid and one of its salts
• basic buffer - consist of a solution of a weak base and one of its salts

Question 5

For a buffer to stabilise the pH of a solution it must be….

Answer 5

• capable of reacting with added OH- ions

- capable of reacting with added H+ ions

Question 6

Acid buffer

• what it consists of and how much
• example of how it works when acid is added and when alkali is added

Answer 6

• acid buffer consists of weak acid and a salt
• example : ethanoic acid and sodium
  ethanoate.
  Ethanoic acid is only partially dissociated:
• CH3COOH + H20 (equilibrium arrow) CH3COO- + H3O+
• CH3COONa (equilibrium arrow) CH3COO- + Na+
• there are many CH3COOH (ethanoic acid ) molecules present in the buffer since the acid it weak it only dissociates to a small extent
• there are many CH3COO- (conjugate base ) ions present in the buffer , since the salt fully dissociates

+ so, when an acid is added to this buffer solution the excess H3O ions from the acid added react with the CH3COO- ions (conjugate base) from the salt in buffer- forming more undissociated ethanoic acid molecules (hydrogen ions converts the the conjugate acid CH3COO- to weak acid CH3COOH ) the overall result is that the concentration of H3O+ ions stays the same , so pH remains unchanged.

+ when an alkali is added to this buffer solution , the excess OH- ions (added from alkali) react with the hydronium ions from the ethanoic acid to form water. More ethanoic acid molecules dissociate to replace these hydronium ions. (The weak acid converts the OH to water) The overall result is that the concentration of H3O+ ions stays the same , so the pH remains unchanged.

Question 7

Basic buffer

• what does it consist of
• example and how it works

Answer 7

• basic buffer consists of a solution of a weak base and one of its salts
  Eg ammonia solution and ammonium chloride
• NH3 + H20 (equilibrium arrow) NH4+ + OH-
• NH4Cl (equilibrium arrow) NH4+ Cl-

There are lots of ammonia (NH3) molecules since the base is weakly dissociated
The salt fully ionised so provides many ammonium ions(NH4) to react with

• when acid is added the weak base (NH3) ammonia , removes any added hydronium ions
• when alkali is added ,OH reacts with H+ to form water. the ammonium ions (conjugate acid ) from the salt replaces any hydronium ions removed when the alkali was added.

Question 8

Summary

Answer 8

• (look in notes)

Question 9

How can the pH of a buffer solution be calculated

Answer 9

Using : pKa - log10 [acid] / [salt]
From data booklet
Where brackets mean “concentration of”

Question 10

What is buffer capacity

Answer 10

• Buffers are at their most effective at their maximum buffer capacity - I.e when there are equal quantities of acid and conjugate base
• maximum buffer capacity is when pH = pKa
• when selecting a buffer for solution at pH 2 , the most efficient buffer will be made from an acid with a pKa =2 mixed in equal quantities with one of its salts

Question 11

What is an indicator

Answer 11

• indicators are solutions (usually weak acids) of dyes , in which the colour of the acids is different to that of its conjugate base
• are usually weak organic acids

( essentially -indicators are weak acids or bases that are able to give a measure of the pH of a solution by their colour ) - they are different colours are different pH values

Question 12

Why are indicators used in acid- base titrations

Answer 12

Indicators are used in acid- base titrations as they change colour at the end point of the reaction.

Question 13

Example of indicator : litmus

- how is its dissociation in water represented

Answer 13

For example , the acid form of litmus is red and the conjugate base is blue.
If we use HIn to represent litmus :

HIn(aq)+ H20 (l) (equilibrium arrow) H3O+ + In- (aq)

HIn (acid which is red )
In- (conjugate base which is blue)

Question 14

What happens upon adding acid / base to the equilibrium before (the indicator)

Answer 14

• when adding an acid , H3O + will increase , the equilibrium will shift left and the solution will becomes red ( more acidic pH of solution so becomes red )
• when adding an alkali , OH- ions increases removing the hydronium ions shifting the position of equilibrium to the right hand side - so solution becomes blue (more alkaline solution so becomes blue )

Question 15

What is the equilibrium constant of an indicator represented by

Answer 15

K in (small in)

Question 16

Equation used to calculate equilibrium constant of indicator - and how to get to it

Answer 16

Kin = [H3O] [In-] / [HIn]

- look in notes for rearranging of this

Question 17

What does the colour of the indicator in any solution depend on

Answer 17

The colour of the indicator in any solution depends on the relative concentrations or the ratio of the acid and conjugate base
( [In-[HIn+] ) which in turn is dependant on the pH of the solution

Question 18

When does colour change take place

Answer 18

Colour change takes place when pKin = pH

Question 19

Colour change can only be seen when [HIn] differs from [In-] by a factor of ___

Answer 19

10

Question 20

the pH range Over which a colour change can be observed is established using the expression …

Answer 20

pH = pKIn + or - 1

Question 21

What needs to be considered when choosing the indicator in a titration

Answer 21

• equivalence point

Question 22

What is the equivalence point in titration

Answer 22

The equivalence is the point in a titration where the amount of titrant added is enough to neutralise the analyte solution completely

Question 23

Titration curves

Answer 23

LOOK IN NOTES

Question 24

Using Titration curves -suitable indicator according to pH range

Answer 24

• strong acid + strong base : the vertical region occurs between pH3 and pH 10 , so a suitable indicator would change colour within this pH range
• weak acid + strong base - vertical region occurs between pH7 and pH10
• strong acid + weak base - vertical region occurs between pH3 and pH8
• weak acid + weak base - no sharp vertical region : this means the pH does not change rapidly at any point so no suitable indicator is available.

Question 25

What is used to measure the equivalence point for a weak acid +weak base titration

Answer 25

A pH meter is used to produce a titration curve - this curve is plotted in order to identify the equivalence point

Question 26

Buffers summary -
When acids are added
When alkali is added

Answer 26

• on addition of acid to a buffer solution , the extra H+ ions react with the conjugate base present to produce undissociated acid
• when alkali is added to buffer , the OH ions react with the H ions to form water
• the H ions are provided by more of the weak acid dissociating to maintain the equilibrium between acid , conjugate base and h ions