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Module 1: Practical chemistry for exams›PAG 7.1 - Identifying organic unknowns 1

PAG 7.1 - Identifying organic unknowns 1

An overview of PAG 7.1 - 7.1: Identifying organic unknowns 1
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In OCR Chemistry Required Practical 7 (PAG 7.1), the presence of haloalkane and alkene functional groups in molecules is investigated using qualitative tests.

The test for the alkene functional group is usually completed on:

  • Cyclohexane
  • Cyclohexene
  • Heptane
  • Limonene.

The test for the haloalkanes is usually completed on:

  • 1-chlorobutane
  • 1-bromobutane
  • 1-iodobutane.
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In OCR PAG 7.1: Identifying organic unknowns 1, there are several important safety requirements to be aware of (beyond normal lab practice such as wearing goggles).

  • Take care when handling hot apparatus.
  • Flammable reagents are used in this practical – ensure that there are no naked flames/sources of ignition.
  • Chemical vapours can be harmful – ensure that the laboratory is well ventilated.
  • Dispose of chemicals as directed by your teacher – take particular care with disposing of organic chemicals.
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Experimental method for OCR Chemistry Required Practical 7 (PAG 7.1)

Part 1: Identification of an alkene

EXPERIMENTAL METHOD OCR PAG 7.1 PART 1. 1. Add 10 drops of bromine water to four test tubes. Ensure the bromine water does not come into contact with light before the test. 2. Add the samples to the bromine water. Label the test tubes according to the chemical being tested. Add 10 drops of test chemical and shake the test tube gently to mix the liquids. Make and record your observations. Where an alkene functional group is present, the bromine water will decolourise.
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Experimental method for OCR Chemistry Required Practical 7 (PAG 7.1)

Part 2: Identification of haloalkanes

EXPERIMENTAL METHOD OCR PAG 7.1 PART 2. 1. Prepare the silver nitrate solution. Add 1 cm³ of ethanol as a solvent and 1 cm³ of 0.05 mol dm⁻³ silver nitrate solution to three labelled test tubes. Add 5 drops of the different haloalkane to a separate tube. Shake the tubes to mix the reactants. Place the tubes in a beaker of hot water for 3 minutes. 2. Add the haloalkane and warm. Add 5 drops of each haloalkane sample to its labelled tube. Shake the tubes to mix the reactants. Place the tubes in a beaker of hot water for 3 minutes. 3. Add the haloalkane and warm. Observe the changes occurring, as well as the order in which they happen. Record the observations in the results table. Silver chloride is a white precipitate, silver bromide is cream, and silver iodide is yellow. The precipitates form in accordance with the ease of nucleophilic substitution: R-I > R-Br > R-Cl. © Medify
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Wearing the appropriate personal protective equipment is essential to avoid direct contact with harmful chemicals and to ensure safety throughout the experiment.

A simple illustration of a pair of goggles and gloves.
Do

Wear goggles and gloves when handling any chemicals, particularly concentrated acids, ammonia, and metal salts, as they can be corrosive or irritating to the skin and eyes.

A person wearing no goggles or gloves whilst handling chemicals.
Don't

Skip protective equipment just because ‘it’s only a few drops’.

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It is crucial to use clean and dry glassware when performing analytical tests.

Five test tubes arranged in a row, with varying heights and shapes.
Do

Ensure all glassware is clean and dry before use to avoid contamination and ensure accurate results.

A test tube with a white body and brownish stains or markings on its surface.
Don't

Reuse glassware that has not been properly cleaned and dried, as this could lead to inaccurate readings or unintended chemical reactions.

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Make sure to handle the bromine water safely.

A window with a blue sky reflection, accompanied by a pair of blue gloves and a protective goggles.
Do

Ensure good ventilation, and use gloves and eye protection when working with bromine water. Bromine is toxic and corrosive, and its fumes can be harmful if inhaled. Proper ventilation reduces the risk of exposure.

A hand is shown with a wound on the wrist, and a bottle labeled with a skull and crossbones is pouring a brown liquid onto the wound.
Don't

Let bromine water come into contact with skin or eyes, or use in enclosed spaces. Bromine can cause severe irritation and burns. Its fumes are harmful if inhaled.

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Below is a summary of results from this PAG:

A table with three columns labeled 'Functional group', 'Test', and 'Positive observation'. Row 1: 'Alkene', 'Add bromine water', 'Bromine water changes from orange-brown to colourless'. Row 2: 'Chloroalkane', 'Warm with aqueous silver nitrate in ethanol', 'White precipitate forms'. Row 3: 'Bromoalkane', 'Warm with aqueous silver nitrate in ethanol', 'Cream precipitate forms'. Row 4: 'Iodoalkane', 'Warm with aqueous silver nitrate in ethanol', 'Pale-yellow precipitate forms'.
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