What is a redox titration ?

A redox titration is a titration in which an oxidising agent reacts with a reducing agent, with electrons transferred between species. The concentration of one species is determined from the known concentration of the other.

Why is dilute sulfuric acid used (rather than HCl or HNO 3 ) in the KMnO 4 titration?

Dilute sulfuric acid does not react with MnO 4 - ions and does not itself act as an oxidising agent under these conditions. Hydrochloric acid would be oxidised by manganate(VII) and nitric acid is itself an oxidising agent, both of which would interfere with the titration.

In the KMnO 4 titration, the endpoint is reached when the solution turns .......... , showing an excess of .......... .

In the KMnO 4 titration, the endpoint is reached when the solution turns pale pink , showing an excess of MnO 4 - .

What is the molar ratio of MnO 4 - to Fe 2+ in the manganate(VII) titration?

The ratio is 1:5 (one mole of MnO 4 - reacts with five moles of Fe 2+ ). This comes from the balanced ionic equation: MnO 4 - + 8H + + 5Fe 2+ → Mn 2+ + 4H 2 O + 5Fe 3+ .

True or False? KMnO 4 acts as a self-indicating reagent in redox titrations.

True. The purple MnO 4 - is reduced to the near-colourless Mn 2+ during the reaction. A permanent pale pink colour at the endpoint signals that all Fe 2+ has been oxidised and MnO 4 - is now in excess.

An iron tablet weighing 0.960 g required 28.50 cm 3 of 0.0180 mol dm -3 KMnO 4 to reach the endpoint. What is the percentage by mass of iron in the tablet?

Moles KMnO 4 = (0.0180 × 28.50)/1000 = 5.13 × 10 -4 mol Moles Fe 2+ = 5 × 5.13 × 10 -4 = 2.565 × 10 -3 mol Mass Fe = 56.0 × 2.565 × 10 -3 = 0.1436 g Percentage by mass = (0.1436/0.960) × 100 = 15.0%

In the manganate(VII) titration, Fe 2+ is the .......... agent and is oxidised to .......... .

In the manganate(VII) titration, Fe 2+ is the reducing agent and is oxidised to Fe 3+ .

Why should a burette with white numbering be used for KMnO 4 titrations?

The deep purple colour of potassium manganate(VII) solution makes it very difficult to read burette markings with black numbering. White numbering provides sufficient contrast to read the titre accurately.

State two uses of the manganate(VII) titration.

Determining the percentage purity of iron supplements Determining the formula of a sample of hydrated ethanedioic acid

What is an iodometric titration ?

An iodometric titration is a redox titration in which an oxidising agent liberates iodine from iodide ions, and the iodine produced is then titrated against a known concentration of sodium thiosulfate solution.

What is the equation for the reaction between thiosulfate and iodine?

2S 2 O 3 2- (aq) + I 2 (aq) → 2I - (aq) + S 4 O 6 2- (aq) Thiosulfate is the reducing agent , oxidised to tetrathionate, while iodine is reduced to iodide.

Why is starch added near the endpoint rather than at the start of a thiosulfate–iodine titration?

Starch is added when the solution turns a pale straw colour , just before the endpoint. Added too early with high iodine concentrations, starch forms a very stable complex with iodine, making it difficult to detect the endpoint accurately.

In the thiosulfate–iodine titration, the endpoint is reached when the .......... colour disappears, indicating all the iodine has reacted.

In the thiosulfate–iodine titration, the endpoint is reached when the blue-black colour disappears, indicating all the iodine has reacted.

In the bleach analysis worked example, 25.0 cm 3 of diluted bleach required 25.20 cm 3 of 0.05 mol dm -3 Na 2 S 2 O 3 . What is the mole ratio of ClO - to S 2 O 3 2- ?

The ratio is 1:2 (ClO - : S 2 O 3 2- ). One mole of ClO - produces one mole of I 2 , which reacts with two moles of Na 2 S 2 O 3 .

True or False? The thiosulfate–iodine titration can be used to find the concentration of an oxidising agent.

True. The oxidising agent liberates a known amount of I 2 from iodide ions, and titrating that iodine with Na 2 S 2 O 3 lets you calculate the concentration of the original oxidising agent .

Why is starch indicator added during the iodine-thiosulfate titration, and when is it added?

Starch is added when the solution reaches a straw yellow colour to clarify the endpoint. It reacts with the remaining iodine to form a blue/black colour, which disappears at the endpoint when all iodine has been reduced to iodide ions.

Describe the colour change sequence as thiosulfate is added to iodine solution in this titration.

The iodine solution starts brown/yellow , gradually becoming paler. When starch is added at the straw-coloured stage, the solution turns blue-black . At the endpoint, the colour disappears completely to give a colourless solution.

State the general calculation sequence for a redox titration involving iodine.

Write half equations for oxidant and reductant. Deduce the overall equation. Calculate moles of thiosulfate used. Use the mole ratio to find moles of I 2 . Use the mole ratio to find moles of the original oxidising agent . Calculate its concentration in the original solution.

What complex ion is formed when ammonia is added to copper(II) sulfate solution?

The [Cu(NH 3 ) 4 (H 2 O) 2 ] 2+ complex ion is formed, giving a characteristic dark blue solution.

What is Buchner filtration ?

Buchner filtration is a technique that uses a Buchner funnel and flask connected to a vacuum to filter under reduced pressure, allowing faster and more efficient recovery of solid crystals than gravity filtration.

Why is ethanol added to the copper–ammonia solution in step 7 of the transition metal complex preparation?

Ethanol is less polar than water, so ionic compounds are less soluble in it. Adding ethanol reduces the solubility of Cu(NH 3 ) 4 SO 4 •H 2 O, causing it to precipitate as dark blue crystals.

In the synthesis of the copper–ammonia complex, concentrated ammonia must be handled in .......... while wearing .......... .

In the synthesis of the copper–ammonia complex, concentrated ammonia must be handled in a fume cupboard while wearing gloves .

Write the equation for the preparation of tetraamminecopper(II) sulfate monohydrate.

CuSO 4 •5H 2 O + 4NH 3 → Cu(NH 3 ) 4 SO 4 •H 2 O + 4H 2 O The product Cu(NH 3 ) 4 SO 4 •H 2 O forms as dark blue crystals.

True or False? A percentage yield greater than 100% in this preparation indicates a very pure product.

False. A yield above 100% indicates the crystals have not been completely dried . The remaining water adds to the measured mass, making it appear greater than the theoretical yield.

Why is cold ethanol used (rather than hot ethanol) to wash the crystals in step 8?

Cu(NH 3 ) 4 SO 4 •H 2 O is less soluble in cold ethanol , so using cold ethanol removes impurities without dissolving the product. Hot ethanol would dissolve the crystals and reduce the yield.

Given M r of CuSO 4 •5H 2 O = 249.6 and M r of Cu(NH 3 ) 4 SO 4 •H 2 O = 245.6, the moles of CuSO 4 •5H 2 O in 1.5 g = .......... mol.

Given M r of CuSO 4 •5H 2 O = 249.6 and M r of Cu(NH 3 ) 4 SO 4 •H 2 O = 245.6, the moles of CuSO 4 •5H 2 O in 1.5 g = 0.00601 mol.

Name two reasons why the actual yield of the copper–ammonia complex might differ from the theoretical yield.

Losses: the reaction may not go to completion and some product may remain in solution rather than crystallising out. Gains: the crystals may be impure or incompletely dried, increasing the measured mass above the true yield.

What reagents are used to test for sulfate ions, and what is the positive result?

The sample is acidified with dilute hydrochloric acid , then a few drops of aqueous barium chloride are added. A positive result is a white precipitate of barium sulfate (BaSO 4 ).

What are the colours of the precipitates formed when Cl - , Br - and I - ions react with acidified silver nitrate solution?

Cl - gives a white precipitate (AgCl) Br - gives a cream precipitate (AgBr) I - gives a yellow precipitate (AgI)

To test for ammonium ions, sodium hydroxide is added and the mixture is warmed. A positive result is .......... gas turning damp .......... litmus paper blue.

To test for ammonium ions, sodium hydroxide is added and the mixture is warmed. A positive result is ammonia gas turning damp red litmus paper blue.

What reagent distinguishes aldehydes from ketones using a silver mirror test?

Tollens' reagent (ammoniacal silver nitrate) is used. When warmed with an aldehyde , a silver mirror forms. A ketone gives no reaction as it cannot be oxidised by Tollens' reagent.

How is the presence of a carbonyl group (aldehyde or ketone) confirmed in qualitative analysis?

The compound is mixed with 2,4-dinitrophenylhydrazine (2,4-DNPH) . A positive result is a deep-orange precipitate , which can be purified by recrystallisation and identified by its melting point.

True or False? Bromine water decolourises when shaken with an alkene.

True. Alkenes are unsaturated and undergo an addition reaction with bromine water, removing the orange/yellow colour to give a colourless product. A saturated compound would not react.

What is the test for carbonate ions, and what is the positive result?

Dilute hydrochloric acid is added and the gas produced is bubbled through limewater . A positive result is the limewater turning milky (cloudy), indicating carbon dioxide has been produced.

When Fehling's solution is warmed with an aldehyde, a .......... precipitate is formed. With a ketone, .......... reaction occurs.

When Fehling's solution is warmed with an aldehyde, a brick-red precipitate is formed. With a ketone, no reaction occurs.

How are primary and secondary alcohols distinguished from tertiary alcohols in qualitative analysis?

Acidified potassium dichromate(VI) (or KMnO 4 ) is added. Primary and secondary alcohols cause the solution to change from orange to green (or purple to colourless). A tertiary alcohol produces no colour change as it cannot be oxidised.

How is the presence of a carboxylic acid confirmed in qualitative analysis?

Solid sodium carbonate or aqueous sodium hydrogen carbonate is added. A positive result is effervescence (bubbles of carbon dioxide gas).

What is recrystallisation ?

Recrystallisation is a purification technique in which an impure solid is dissolved in a minimum volume of hot solvent, then allowed to cool so that the desired product crystallises out, leaving impurities in solution.

What are the reactants and catalyst used in aspirin synthesis?

Salicylic acid and ethanoic anhydride react together, with a few drops of concentrated sulfuric acid as catalyst, to produce aspirin (acetylsalicylic acid) and ethanoic acid.

In the aspirin synthesis, 6.0 g of salicylic acid is heated with ethanoic anhydride at .......... °C for about .......... minutes.

In the aspirin synthesis, 6.0 g of salicylic acid is heated with ethanoic anhydride at 60 °C for about 20 minutes.

What does a broad melting point range indicate about the purity of a solid product?

A broad melting point range indicates an impure product. Impurities disrupt the crystal lattice and lower the melting point, causing the solid to begin melting below the expected value and over a wider temperature interval.

What is the melting point of pure aspirin, and how is it used to assess purity?

Pure aspirin melts at 135 °C . The measured melting point of the product is compared to this value: a value close to 135 °C with a sharp, narrow range indicates high purity.

True or False? Slow cooling during recrystallisation produces larger, well-defined crystals.

True. Slow cooling allows more ordered crystal growth, producing larger crystals that are easier to filter and dry than the small crystals formed by rapid cooling.

After heating, the aspirin mixture is added to .......... water and the crude product is recovered using .......... filtration.

After heating, the aspirin mixture is added to cold water and the crude product is recovered using Buchner filtration.

Why is ethanoic anhydride used in aspirin synthesis rather than ethanoic acid?

Ethanoic anhydride is a more reactive acylating agent than ethanoic acid. It reacts more readily with the hydroxyl group of salicylic acid, giving a better yield under milder conditions.

Name two key skills practised in Core Practical 16.

Any two from: using a water bath for controlled heating, recrystallisation to purify a solid product, using Buchner filtration to recover crystals, and melting point analysis to assess purity.

International A Level (IAL)ChemistryEdexcelFlashcards6. Practical Skills in Chemistry IIAdvanced Inorganic & Organic Chemistry Core Practicals

Advanced Inorganic & Organic Chemistry Core Practicals (Edexcel International A Level (IAL) Chemistry): Flashcards

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FrontRedox Titration - Iron(II) & Manganate(VII)
What is a redox titration?

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What is a redox titration?
A redox titration is a titration in which an oxidising agent reacts with a reducing agent, with electrons transferred between species. The concentration of one species is determined from the known concentration of the other.
Why is dilute sulfuric acid used (rather than HCl or HNO₃) in the KMnO₄ titration?
Dilute sulfuric acid does not react with MnO₄^- ions and does not itself act as an oxidising agent under these conditions. Hydrochloric acid would be oxidised by manganate(VII) and nitric acid is itself an oxidising agent, both of which would interfere with the titration.
In the KMnO₄ titration, the endpoint is reached when the solution turns .......... , showing an excess of .......... .
In the KMnO₄ titration, the endpoint is reached when the solution turns pale pink, showing an excess of MnO₄^- .
What is the molar ratio of MnO₄^- to Fe²⁺ in the manganate(VII) titration?
The ratio is 1:5 (one mole of MnO₄^- reacts with five moles of Fe²⁺). This comes from the balanced ionic equation: MnO₄^- + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 4H₂O + 5Fe³⁺.
True or False?
KMnO₄ acts as a self-indicating reagent in redox titrations.
True.
The purple MnO₄^- is reduced to the near-colourless Mn²⁺ during the reaction. A permanent pale pink colour at the endpoint signals that all Fe²⁺ has been oxidised and MnO₄^- is now in excess.
An iron tablet weighing 0.960 g required 28.50 cm³ of 0.0180 mol dm^-3 KMnO₄ to reach the endpoint. What is the percentage by mass of iron in the tablet?
Moles KMnO₄ = (0.0180 × 28.50)/1000 = 5.13 × 10^-4 mol
Moles Fe²⁺ = 5 × 5.13 × 10^-4 = 2.565 × 10^-3 mol
Mass Fe = 56.0 × 2.565 × 10^-3 = 0.1436 g
Percentage by mass = (0.1436/0.960) × 100 = 15.0%
In the manganate(VII) titration, Fe²⁺ is the .......... agent and is oxidised to .......... .
In the manganate(VII) titration, Fe²⁺ is the reducing agent and is oxidised to Fe³⁺.
Why should a burette with white numbering be used for KMnO₄ titrations?
The deep purple colour of potassium manganate(VII) solution makes it very difficult to read burette markings with black numbering. White numbering provides sufficient contrast to read the titre accurately.
State two uses of the manganate(VII) titration.
1. Determining the percentage purity of iron supplements
2. Determining the formula of a sample of hydrated ethanedioic acid
What is an iodometric titration?
An iodometric titration is a redox titration in which an oxidising agent liberates iodine from iodide ions, and the iodine produced is then titrated against a known concentration of sodium thiosulfate solution.
What is the equation for the reaction between thiosulfate and iodine?
2S₂O₃^2- (aq) + I₂ (aq) → 2I^- (aq) + S₄O₆^2- (aq)
Thiosulfate is the reducing agent, oxidised to tetrathionate, while iodine is reduced to iodide.
Why is starch added near the endpoint rather than at the start of a thiosulfate–iodine titration?
Starch is added when the solution turns a pale straw colour, just before the endpoint. Added too early with high iodine concentrations, starch forms a very stable complex with iodine, making it difficult to detect the endpoint accurately.
In the thiosulfate–iodine titration, the endpoint is reached when the .......... colour disappears, indicating all the iodine has reacted.
In the thiosulfate–iodine titration, the endpoint is reached when the blue-black colour disappears, indicating all the iodine has reacted.
In the bleach analysis worked example, 25.0 cm³ of diluted bleach required 25.20 cm³ of 0.05 mol dm^-3 Na₂S₂O₃. What is the mole ratio of ClO^- to S₂O₃^2-?
The ratio is 1:2 (ClO^- : S₂O₃^2-). One mole of ClO^- produces one mole of I₂, which reacts with two moles of Na₂S₂O₃.
True or False?
The thiosulfate–iodine titration can be used to find the concentration of an oxidising agent.
True.
The oxidising agent liberates a known amount of I₂ from iodide ions, and titrating that iodine with Na₂S₂O₃ lets you calculate the concentration of the original oxidising agent.
Why is starch indicator added during the iodine-thiosulfate titration, and when is it added?
Starch is added when the solution reaches a straw yellow colour to clarify the endpoint. It reacts with the remaining iodine to form a blue/black colour, which disappears at the endpoint when all iodine has been reduced to iodide ions.
Describe the colour change sequence as thiosulfate is added to iodine solution in this titration.
The iodine solution starts brown/yellow, gradually becoming paler. When starch is added at the straw-coloured stage, the solution turns blue-black. At the endpoint, the colour disappears completely to give a colourless solution.
State the general calculation sequence for a redox titration involving iodine.
1. Write half equations for oxidant and reductant.
2. Deduce the overall equation.
3. Calculate moles of thiosulfate used.
4. Use the mole ratio to find moles of I₂.
5. Use the mole ratio to find moles of the original oxidising agent.
6. Calculate its concentration in the original solution.
What complex ion is formed when ammonia is added to copper(II) sulfate solution?
The [Cu(NH₃)₄(H₂O)₂]²⁺ complex ion is formed, giving a characteristic dark blue solution.
What is Buchner filtration?
Buchner filtration is a technique that uses a Buchner funnel and flask connected to a vacuum to filter under reduced pressure, allowing faster and more efficient recovery of solid crystals than gravity filtration.
Why is ethanol added to the copper–ammonia solution in step 7 of the transition metal complex preparation?
Ethanol is less polar than water, so ionic compounds are less soluble in it. Adding ethanol reduces the solubility of Cu(NH₃)₄SO₄•H₂O, causing it to precipitate as dark blue crystals.
In the synthesis of the copper–ammonia complex, concentrated ammonia must be handled in .......... while wearing .......... .
In the synthesis of the copper–ammonia complex, concentrated ammonia must be handled in a fume cupboard while wearing gloves.
Write the equation for the preparation of tetraamminecopper(II) sulfate monohydrate.
CuSO₄•5H₂O + 4NH₃ → Cu(NH₃)₄SO₄•H₂O + 4H₂O
The product Cu(NH₃)₄SO₄•H₂O forms as dark blue crystals.
True or False?
A percentage yield greater than 100% in this preparation indicates a very pure product.
False.
A yield above 100% indicates the crystals have not been completely dried. The remaining water adds to the measured mass, making it appear greater than the theoretical yield.
Why is cold ethanol used (rather than hot ethanol) to wash the crystals in step 8?
Cu(NH₃)₄SO₄•H₂O is less soluble in cold ethanol, so using cold ethanol removes impurities without dissolving the product. Hot ethanol would dissolve the crystals and reduce the yield.
Given M_r of CuSO₄•5H₂O = 249.6 and M_r of Cu(NH₃)₄SO₄•H₂O = 245.6, the moles of CuSO₄•5H₂O in 1.5 g = .......... mol.
Given M_r of CuSO₄•5H₂O = 249.6 and M_r of Cu(NH₃)₄SO₄•H₂O = 245.6, the moles of CuSO₄•5H₂O in 1.5 g = 0.00601 mol.
Name two reasons why the actual yield of the copper–ammonia complex might differ from the theoretical yield.
Losses: the reaction may not go to completion and some product may remain in solution rather than crystallising out.
Gains: the crystals may be impure or incompletely dried, increasing the measured mass above the true yield.
What reagents are used to test for sulfate ions, and what is the positive result?
The sample is acidified with dilute hydrochloric acid, then a few drops of aqueous barium chloride are added. A positive result is a white precipitate of barium sulfate (BaSO₄).
What are the colours of the precipitates formed when Cl^-, Br^- and I^- ions react with acidified silver nitrate solution?
1. Cl^- gives a white precipitate (AgCl)
2. Br^- gives a cream precipitate (AgBr)
3. I^- gives a yellow precipitate (AgI)
To test for ammonium ions, sodium hydroxide is added and the mixture is warmed. A positive result is .......... gas turning damp .......... litmus paper blue.
To test for ammonium ions, sodium hydroxide is added and the mixture is warmed. A positive result is ammonia gas turning damp red litmus paper blue.
What reagent distinguishes aldehydes from ketones using a silver mirror test?
Tollens' reagent (ammoniacal silver nitrate) is used. When warmed with an aldehyde, a silver mirror forms. A ketone gives no reaction as it cannot be oxidised by Tollens' reagent.
How is the presence of a carbonyl group (aldehyde or ketone) confirmed in qualitative analysis?
The compound is mixed with 2,4-dinitrophenylhydrazine (2,4-DNPH). A positive result is a deep-orange precipitate, which can be purified by recrystallisation and identified by its melting point.
True or False?
Bromine water decolourises when shaken with an alkene.
True.
Alkenes are unsaturated and undergo an addition reaction with bromine water, removing the orange/yellow colour to give a colourless product. A saturated compound would not react.
What is the test for carbonate ions, and what is the positive result?
Dilute hydrochloric acid is added and the gas produced is bubbled through limewater. A positive result is the limewater turning milky (cloudy), indicating carbon dioxide has been produced.
When Fehling's solution is warmed with an aldehyde, a .......... precipitate is formed. With a ketone, .......... reaction occurs.
When Fehling's solution is warmed with an aldehyde, a brick-red precipitate is formed. With a ketone, no reaction occurs.
How are primary and secondary alcohols distinguished from tertiary alcohols in qualitative analysis?
Acidified potassium dichromate(VI) (or KMnO₄) is added. Primary and secondary alcohols cause the solution to change from orange to green (or purple to colourless). A tertiary alcohol produces no colour change as it cannot be oxidised.
How is the presence of a carboxylic acid confirmed in qualitative analysis?
Solid sodium carbonate or aqueous sodium hydrogen carbonate is added. A positive result is effervescence (bubbles of carbon dioxide gas).
What is recrystallisation?
Recrystallisation is a purification technique in which an impure solid is dissolved in a minimum volume of hot solvent, then allowed to cool so that the desired product crystallises out, leaving impurities in solution.
What are the reactants and catalyst used in aspirin synthesis?
Salicylic acid and ethanoic anhydride react together, with a few drops of concentrated sulfuric acid as catalyst, to produce aspirin (acetylsalicylic acid) and ethanoic acid.
In the aspirin synthesis, 6.0 g of salicylic acid is heated with ethanoic anhydride at .......... °C for about .......... minutes.
In the aspirin synthesis, 6.0 g of salicylic acid is heated with ethanoic anhydride at 60 °C for about 20 minutes.
What does a broad melting point range indicate about the purity of a solid product?
A broad melting point range indicates an impure product. Impurities disrupt the crystal lattice and lower the melting point, causing the solid to begin melting below the expected value and over a wider temperature interval.
What is the melting point of pure aspirin, and how is it used to assess purity?
Pure aspirin melts at 135 °C. The measured melting point of the product is compared to this value: a value close to 135 °C with a sharp, narrow range indicates high purity.
True or False?
Slow cooling during recrystallisation produces larger, well-defined crystals.
True.
Slow cooling allows more ordered crystal growth, producing larger crystals that are easier to filter and dry than the small crystals formed by rapid cooling.
After heating, the aspirin mixture is added to .......... water and the crude product is recovered using .......... filtration.
After heating, the aspirin mixture is added to cold water and the crude product is recovered using Buchner filtration.
Why is ethanoic anhydride used in aspirin synthesis rather than ethanoic acid?
Ethanoic anhydride is a more reactive acylating agent than ethanoic acid. It reacts more readily with the hydroxyl group of salicylic acid, giving a better yield under milder conditions.
Name two key skills practised in Core Practical 16.
Any two from: using a water bath for controlled heating, recrystallisation to purify a solid product, using Buchner filtration to recover crystals, and melting point analysis to assess purity.

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