indicators & ph curves a guide for a level students knockhardy publishing
TRANSCRIPT
INDICATORS & INDICATORS & pH CURVESpH CURVES
A guide for A level studentsA guide for A level students
KNOCKHARDY PUBLISHINGKNOCKHARDY PUBLISHING
IndicatorsIndicators
INTRODUCTION
This Powerpoint show is one of several produced to help students understand selected topics at AS and A2 level Chemistry. It is based on the requirements of the AQA and OCR specifications but is suitable for other examination boards.
Individual students may use the material at home for revision purposes or it may be used for classroom teaching if an interactive white board is available.
Accompanying notes on this, and the full range of AS and A2 topics, are available from the KNOCKHARDY SCIENCE WEBSITE at...
www.argonet.co.uk/users/hoptonj/sci.htm
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CONTENTS• Acid-base indicators - theory
• Titration curves - introduction
• Titration curve; strong acid - strong base
• Titration curve; strong acid - weak base
• Titration curve; weak acid - strong base
• Titration curve; weak acid - weak base
• Titration curve; acid - carbonate
• Titration curve; phosphoric acid
• Check list
IndicatorsIndicators
Acid-base indicatorsAcid-base indicators
General Many indicators are weak acids and partially dissociate in aqueous solution
HIn(aq) H+(aq) + In¯(aq)
The un-ionised form (HIn) is a different colour to the anionic form (In¯).
Acid-base indicatorsAcid-base indicators
General Many indicators are weak acids and partially dissociate in aqueous solution
HIn(aq) H+(aq) + In¯(aq)
The un-ionised form (HIn) is a different colour to the anionic form (In¯).
Apply Le Chatelier’s Principle to predict any colour change
In acid - increase of [H+]- equilibrium moves to the left to give red undissociated form
In alkali - increase of [OH¯]
- OH¯ ions remove H+ ions to form water; H+(aq) + OH¯(aq) H2O(l)
- equilibrium will move to the right to produce a blue colour
Acid-base indicatorsAcid-base indicators
General Many indicators are weak acids and partially dissociate in aqueous solution
HIn(aq) H+(aq) + In¯(aq)
The un-ionised form (HIn) is a different colour to the anionic form (In¯).
Apply Le Chatelier’s Principle to predict any colour change
In acid - increase of [H+]- equilibrium moves to the left to give red undissociated form
In alkali - increase of [OH¯]
- OH¯ ions remove H+ ions to form water; H+(aq) + OH¯(aq) H2O(l)
- equilibrium will move to the right to produce a blue colour
Summary In acidic solution
HIn(aq) H+(aq) + In¯(aq)
In alkaline solution
COLOUR CHANGES OF SOME COMMON INDICATORS
Must have an easily observed colour change.
Must change immediately in the required pH rangeover the addition of ‘half’ a drop of reagent.
Acid-base indicatorsAcid-base indicators
PHENOLPHTHALEIN
LITMUS
METHYL ORANGE
1 2 3 4 5 6 7 8 9 10 11 12 13 14
CHANGE
CHANGE
CHANGE
pH
To be useful, an indicator mustchange over the “vertical” section of the curve where there is a large change in pH for the addition of a very small volume of alkali.
The indicator used depends on the pH changes around the end point - the indicator must change during the ‘vertical’ portion of the curve.
In the example, the only suitable indicator is PHENOLPHTHALEIN.
Must have an easily observed colour change.
Must change immediately in the required pH rangeover the addition of ‘half’ a drop of reagent.
Acid-base indicatorsAcid-base indicators
PHENOLPHTHALEIN
LITMUS
METHYL ORANGE
pH curvespH curves
Types There are four types of acid-base titration; each has a characteristic curve.
strong acid (HCl) v. strong base (NaOH) weak acid (CH3COOH) v. strong alkali (NaOH)
strong acid (HCl) v. weak base (NH3)
weak acid (CH3COOH) v. weak base (NH3)
In the following examples, alkali (0.1M) is added to 25cm3 of acid (0.1M)
End points need not be “neutral‘ due to the phenomenon of salt hydrolysis
pH 1 at the start due to 0.1M HCl
(strong monoprotic acid)
strong acidstrong acid (HCl) (HCl) v. strong base v. strong base (NaOH)(NaOH)
Very little pH change during the initial 20cm3
pH 1 at the start due to 0.1M HCl
(strong monoprotic acid)
strong acidstrong acid (HCl) (HCl) v. strong base v. strong base (NaOH)(NaOH)
Very little pH change during the initial 20cm3
Very sharp change in pH over the addition of less than half a drop of NaOH
pH 1 at the start due to 0.1M HCl
(strong monoprotic acid)
strong acidstrong acid (HCl) (HCl) v. strong base v. strong base (NaOH)(NaOH)
Very little pH change during the initial 20cm3
Very sharp change in pH over the addition of less than half a drop of NaOH
Curve levels off at pH 13 due to excess 0.1M NaOH
(a strong alkali)
pH 1 at the start due to 0.1M HCl
(strong monoprotic acid)
strong acidstrong acid (HCl) (HCl) v. strong base v. strong base (NaOH)(NaOH)
strong acidstrong acid (HCl) (HCl) v. strong base v. strong base (NaOH)(NaOH)
Any of the indicators listed will be suitable - they all change in the ‘vertical’ portion
PHENOLPHTHALEIN
LITMUS
METHYL ORANGE
Very little pH change during the initial 20cm3
Sharp change in pH over the addition of less than
half a drop of NH3
Curve levels off at pH 10 due to excess 0.1M NH3
(a weak alkali)
pH 1 at the start due to 0.1M HCl
strong acidstrong acid (HCl) (HCl) v. weak basev. weak base (NH (NH33))
PHENOLPHTHALEIN
LITMUS
METHYL ORANGE
strong acid strong acid (HCl) (HCl) v. weak basev. weak base (NH (NH33))
Only methyl orange is suitable - it is the only one to change in the ‘vertical’ portion
Steady pH change
Sharp change in pH over the addition of less than
half a drop of NaOH
Curve levels off at pH 13 due to excess 0.1M NaOH
(a strong alkali)
pH 4 due to 0.1M CH3COOH (weak monoprotic acid)
weak acid weak acid (CH(CH33COOH) COOH) v. strong basev. strong base (NaOH) (NaOH)
PHENOLPHTHALEIN
LITMUS
METHYL ORANGE
Only phenolphthalein is suitable - it is the only one to change in the ‘vertical’ portion
weak acidweak acid (CH (CH33COOH) COOH) v. strong basev. strong base (NaOH) (NaOH)
weak acidweak acid (CH (CH33COOH) COOH) v. weak basev. weak base (NH (NH33))
Types
Steady pH change
pH 4 due to 0.1M CH3COOH (weak monoprotic acid)
NO SHARPCHANGE IN pH
Curve levels off at pH 10 due to excess 0.1M NH3
(a weak alkali)
PHENOLPHTHALEIN
LITMUS
METHYL ORANGE
NOTHING SUITABLE
There is no suitable indicator- none change in the ‘vertical’ portion.The end point can be detected by plotting a curve using a pH meter.
weak acidweak acid (CH (CH33COOH) COOH) v. weak base v. weak base (NH(NH33))
Other pH curves - Other pH curves - acid v. carbonate
Sodium carbonate reacts with hydrochloric acid in two steps...
Step 1 Na2CO3 + HCl ——> NaHCO3 + NaCl
Step 2 NaHCO3 + HCl ——> NaCl + H2O + CO2
Overall Na2CO3 + 2HCl ——> 2NaCl + H2O + CO2
Other pH curves - Other pH curves - acid v. carbonate
Sodium carbonate reacts with hydrochloric acid in two steps...
Step 1 Na2CO3 + HCl ——> NaHCO3 + NaCl
Step 2 NaHCO3 + HCl ——> NaCl + H2O + CO2
Overall Na2CO3 + 2HCl ——> 2NaCl + H2O + CO2
There are two sharp pH changes
The second addition of HCl is exactly the same as the first because the number of moles of HCl which react with the NaHCO3 is the same as that reacting with the Na2CO3.
17.50cm3 35.00cm3
Other pH curves - Other pH curves - acid v. carbonate
Sodium carbonate reacts with hydrochloric acid in two steps...
Step 1 Na2CO3 + HCl ——> NaHCO3 + NaCl
Step 2 NaHCO3 + HCl ——> NaCl + H2O + CO2
Overall Na2CO3 + 2HCl ——> 2NaCl + H2O + CO2
First rapid pH change around pH = 8.5 due to the formation of NaHCO3 .
Can be detected using phenolphthalein
There are two sharp pH changes
Other pH curves - Other pH curves - acid v. carbonate
Sodium carbonate reacts with hydrochloric acid in two steps...
Step 1 Na2CO3 + HCl ——> NaHCO3 + NaCl
Step 2 NaHCO3 + HCl ——> NaCl + H2O + CO2
Overall Na2CO3 + 2HCl ——> 2NaCl + H2O + CO2
First rapid pH change around pH = 8.5 due to the formation of NaHCO3 .
Can be detected using phenolphthalein
Second rapid pH change around pH = 4 due to the formation of acidic CO2 .
Can be detected using methyl orange.
There are two sharp pH changes
Other pH curves - Other pH curves - polyprotic acids (Hpolyprotic acids (H33POPO44))
Phosphoric acid is triprotic; it reacts with sodium hydroxide in three steps...
Step 1 H3PO4 + NaOH ——> NaH2PO4 + H2O
Step 2 NaH2PO4 + NaOH ——> Na2HPO4 + H2O
Step 3 Na2HPO4 + NaOH ——> Na3PO4 + H2O
Other pH curves - Other pH curves - polyprotic acids (Hpolyprotic acids (H33POPO44))
There are three sharp pH changes
Each successive addition of NaOH is the same as equal
number of moles are involved.
Phosphoric acid is triprotic; it reacts with sodium hydroxide in three steps...
Step 1 H3PO4 + NaOH ——> NaH2PO4 + H2O
Step 2 NaH2PO4 + NaOH ——> Na2HPO4 + H2O
Step 3 Na2HPO4 + NaOH ——> Na3PO4 + H2O
Other pH curves - Other pH curves - polyprotic acids (Hpolyprotic acids (H33POPO44))
pH of H3PO4 = 1.5
Phosphoric acid is triprotic; it reacts with sodium hydroxide in three steps...
Step 1 H3PO4 + NaOH ——> NaH2PO4 + H2O
Step 2 NaH2PO4 + NaOH ——> Na2HPO4 + H2O
Step 3 Na2HPO4 + NaOH ——> Na3PO4 + H2O
Other pH curves - Other pH curves - polyprotic acids (Hpolyprotic acids (H33POPO44))
pH of NaH2PO4 = 4.4
pH of H3PO4 = 1.5
Phosphoric acid is triprotic; it reacts with sodium hydroxide in three steps...
Step 1 H3PO4 + NaOH ——> NaH2PO4 + H2O
Step 2 NaH2PO4 + NaOH ——> Na2HPO4 + H2O
Step 3 Na2HPO4 + NaOH ——> Na3PO4 + H2O
Other pH curves - Other pH curves - polyprotic acids (Hpolyprotic acids (H33POPO44))
pH of Na2HPO4 = 9.6
pH of NaH2PO4 = 4.4
pH of H3PO4 = 1.5
Phosphoric acid is triprotic; it reacts with sodium hydroxide in three steps...
Step 1 H3PO4 + NaOH ——> NaH2PO4 + H2O
Step 2 NaH2PO4 + NaOH ——> Na2HPO4 + H2O
Step 3 Na2HPO4 + NaOH ——> Na3PO4 + H2O
Other pH curves - Other pH curves - polyprotic acids (Hpolyprotic acids (H33POPO44))
pH of Na3PO4 = 12
pH of Na2HPO4 = 9.6
pH of NaH2PO4 = 4.4
pH of H3PO4 = 1.5
Phosphoric acid is triprotic; it reacts with sodium hydroxide in three steps...
Step 1 H3PO4 + NaOH ——> NaH2PO4 + H2O
Step 2 NaH2PO4 + NaOH ——> Na2HPO4 + H2O
Step 3 Na2HPO4 + NaOH ——> Na3PO4 + H2O
REVISION CHECKREVISION CHECK
What should you be able to do?
Recall the definition of a weak acid
Understand why indicators can be made from weak acids
Understand why indicators must change colour quickly over a small pH range
Recall and explain the shape of titration curves involving acids and bases
Explain why particular indicators are used for certain titrations
Explain the shape of the titration curve for phosphoric acid
Explain the shape of the titration curve for acid -sodium carbonate reactions
CAN YOU DO ALL OF THESE? CAN YOU DO ALL OF THESE? YES YES NONO
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