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AH chemistry > Unit 2) 6) Buffers And Indicators > Flashcards

Unit 2) 6) Buffers And Indicators Flashcards

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1
Q

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

A

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.

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2
Q

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

A

Precise

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

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3
Q

What is a buffer solution

A

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

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4
Q

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

A
  • 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
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5
Q

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

A
  • capable of reacting with added OH- ions

- capable of reacting with added H+ ions

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6
Q

Acid buffer

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

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7
Q

Basic buffer

  • what does it consist of
  • example and how it works
A
  • 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.
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8
Q

Summary

A
  • (look in notes)
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9
Q

How can the pH of a buffer solution be calculated

A

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

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10
Q

What is buffer capacity

A
  • 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
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11
Q

What is an indicator

A
  • 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

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12
Q

Why are indicators used in acid- base titrations

A

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

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13
Q

Example of indicator : litmus

- how is its dissociation in water represented

A

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)

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14
Q

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

A
  • 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 )
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15
Q

What is the equilibrium constant of an indicator represented by

A

K in (small in)

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16
Q

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

A

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

- look in notes for rearranging of this

17
Q

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

A

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

18
Q

When does colour change take place

A

Colour change takes place when pKin = pH

19
Q

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

A

10

20
Q

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

A

pH = pKIn + or - 1

21
Q

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

A
  • equivalence point
22
Q

What is the equivalence point in titration

A

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

23
Q

Titration curves

A

LOOK IN NOTES

24
Q

Using Titration curves -suitable indicator according to pH range

A
  • 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.
25
Q

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

A

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

26
Q

Buffers summary -
When acids are added
When alkali is added

A
  • 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

AH chemistry (40 decks)

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