Practical Techniques Flashcards
Draw out a diagram for reflux and give the reaction conditions.
Reaction conditions: Heat Antibumping granules Alcohol Acidified potassium dichromate (oxidising agent)
What are the uses of anti bumping granules?
Used to prevent the formation of large bubbles and reduced boiling vigorously. Also prevents splash and therefore loss of mixture.
What happens when heating under reflux?
Mixture is heated to boiling point for a prolonged time.
Vapour is formed which escapes the evaporation mixture. Then this condensed back into a liquid and returns to the mixture.
Any original solution or part completion must be oxidised to ensure completion.
Draw the apparatus of distillation.
Explain the importance of the thermometer and the ice bath.
Thermometer- identifies the chemicals separated based on their boiling points.
Ice bath- ensures that the mixture once condensed doesn’t evaporate off.
Why is distillation used?
To separate an organic product from a reaction mixture.
Why do we use melting point analysis?
All pure chemicals have a distinct melting and boiling point.
Pure samples have a precise melting point.
Impure samples will have an average range.
Draw the apparatus for melting point analysis
Thiele tube
What are the steps taken in melting pint analysis?
- Place a few grains of organic product in a melting point tube ( melted capillary tube)
- Attach the capillary tube to a thermometer using a rubber band:
- Submerge the capillary tube and thermometer in the oil or a thiele tube (not the rubber band)
- Using a micro Bunsen burn r heat the side arm of the thiele tube. Heat slowly when approaching melting point.
- Note the temperature when the substance starts and stops melting.
- Compare melting point to data book value to determine purity.
What observations would you make when doing melting point analysis?
What is the main error that may occur?
Impure sample:
Have a wide range for melting point.
Melting point is below/before the true melting point.
Pure sample:
Melt at lower temperatures due to inaccuracies with the equipment.
( eg. thermometer reading a different temperature to the reaction mixture)
Temperature on the thermometer may not be reading the correct temperature for the actual sample in the melting point tube.
When do we carry out filtration under reduced pressure?
When we carry out synthesis of a chemical, the organic product will often crystallise out of the mixture.
Draw the apparatus for filtration under reduced pressure.
Büchner funnel
Filter paper
Side arm flask
What are the steps in filtration under reduced pressure?
- Dissolved crystals are placed in the minimum hot solvent
- Allow to cool/crystallise (scrape side of beaker to encourage crystallisation)
- Pour into funnel and rinse the beaker with distilled water. Allow vacuum to suck through the liquid.
- Rinse through their cold solvent to remove soluble impurities (repeat)
- Leave to suck through for a while to remove moisture.
- Put in a warm place or desiccator.
What is the purpose of doing recrystallisation?
To purify products after synthesis
What are the steps in recrystallisation?
- Dissolve the sample in the LEAST amount of HOT SOLVENT possible.
- Carry out gravity filtration as soon as possible on the hot solvent using filter paper.
- Allow to cool and crystallise, can use scratch beakers to encourage crystallisation./ place in an ice bath.
- Collect the product (purified) using vacuum filtration washing the crystals with cold solvent to remove and excess filtrate and impurities.
Explain the purpose of using:
LEAST amount of HOT SOLVENT
Filtered hot
Cooled in and ice bath
Washed with cold solvent/water
LEAST amount of HOT SOLVENT- do only the desired product is dissolved and no other impurities are ( to increase yield) and using hot solvent increases the solubility of the product.
Filtered hot- to remove any insoluble impurities
Cooked in an ice bath- increased number of crystals forming (encourage crystallisation)
Washing with cold solvent/water- remove any soluble impurities