CCEA A-Level Chemistry 1110
Formulae, Equations and Amounts of Substance
Atomic Structure
Bonding
Intermolecular Forces and Structure
Shapes of Molecules and Ions
Redox
The Halogens
Acid-Base Titrations
Qualitative Tests
Formulae and Amounts of a Substance
Nomenclature and Isomerism in Organic Chemistry
Alkanes
Alkenes
Halogenoalkanes
Alcohols
Infrared Spectroscopy
Energetics
Kinetics
Equilibrium
Group II Elements and Their Compounds
Lattice Enthalpy, Entropy and Free Energy
Rates of Reaction
Equilibrium and Acid-Base Equilibria
Isomerism
Aldehydes and Ketones
Carboxylic Acids
Derivatives of Carboxylic Acids
Aromatic Chemistry
Spectroscopy and Chromatography
Volumetric Analysis
Transition Metals
Electrode Potentials
Amines
Amides and Polymers
Amino Acids and Proteins
Aldehydes and Ketones
Distinguishing aldehydes from ketones, and reduction with lithal
- 3 things to know
- Unit A2 1
Overview
Observation with acidified potassium dichromate(VI) that identifies an aldehyde, appearance of Fehling's solution before it is used and why ketones are not oxidised by these mild reagents.
What you need to know
Answer each one before you reveal it.
Name the product formed when an aldehyde is oxidised by acidified potassium dichromate(VI).
State what is observed when an aldehyde is warmed with Fehling's solution.
Questions from this subtopic
Each one with the model answer underneath.
- 1
State the observation with acidified potassium dichromate(VI) that identifies an aldehyde.
Model answer
On warming, the orange solution turns green as the aldehyde is oxidised and the dichromate(VI) is reduced to chromium(III).
- 2
State what is observed when a ketone is warmed with acidified potassium dichromate(VI).
Model answer
There is no change and the solution stays orange, because a ketone is not oxidised.
- 3
Name the product formed when an aldehyde is oxidised by acidified potassium dichromate(VI).
Model answer
The corresponding carboxylic acid is formed.
- 4
Describe the appearance of Fehling's solution before it is used.
Model answer
It is a clear deep blue solution containing copper(II) ions held in an alkaline complex.
- 5
State what is observed when an aldehyde is warmed with Fehling's solution.
Model answer
A brick-red precipitate of copper(I) oxide forms and the blue colour fades.
- 6
State the change undergone by the copper species when Fehling's solution reacts with an aldehyde.
Model answer
The copper is reduced from Cu²⁺ to the copper(I) state.
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