All practical techniques Flashcards
How do you calculate percentage error?
Percentage error = (total uncertainty/final reading) x 100
Total uncertainty is how far you go in each direction, eg if the scale measured to the nearest 0.2g, the weight could be +0.1 or -0.1 from the answer so therefore the total uncertainty is 0.2
How can you reduce the percentage error?
Use more sensitive equipment
Use larger volumes / masses
How do you get an aldehyde, and then a carboxylic from a primary alcohol?
For an aldehyde (apple smelling), gently heat a primary alcohol with potassium dichromate solution and sulfuric acid in a distillation apparatus, so aldehyde is immediately distilled off. Colour change of orange to green
For a carboxylic acid (vinegar smell), vigorously heat a primary alcohol with potassium dichromate solution and sulfuric acid under reflux. Colour change of orange to green
How do you get a ketone from a secondary alcohol?
Reflux and heat secondary alcohol with pottassium dichromate solution, and sulfuric acid to produce a ketone. Colour change from orange to green
Can you oxidise a tertiary alcohol?
No
Describe refluxing and why it’s done?
So you don’t lose volatile compounds
The mixture is heated in a flask, fitted with a Liebig condenser, this means the mixture continuously boils and evaporates giving time for the reaction to occur
Describe distillation and why it’s done?
Gently heating a mixture in a distillation apparatus, the substances will evaporate out in order of boiling point
Means you can separate your products
Fractional distillation, allows you to separate out a product and it’s impurities
How can you remove any water soluble impurities from a compound?
Use separation, if the product is insoluble and want to remove any impurities that do dissolve in water
Once the reaction is complete, the mixture is poured into a separating funnel, and water is added
The funnel is shaken and then allowed to settle, the organic layer is less dense that the aqueous layer so should float on top
Any water soluble impurities should have dissolved in the lower aqueous layer, so can run off funnel into only left with the organic layer
The organic layer will now contain trace amounts of water, so it has to be dried
Add an anhydrous salt such as magnesium sulphate, which will act as a drying agent
Then filter the product to remove the solid drying agent
Describe how organic solids can be purified by recrystallisation?
Very hot solvent is added to the impure solid, until it just dissolves
This provides a saturated solution ( maximum amount of solid dissolved)
The solution is left to cool down slowly. Crystals of the product form as it cools, the impurities stay in solution, as they are in much smaller quantities so they take much longer to crystalise out
The crystals are removed via filtration under reduced pressure and washed with ice cold solvent
They are dried, leaving with you product much cooler than the orginal
What shoud you make sure about your solvent during crystallisation process?
The solid needs to be very soluble in the solvent when it’s hot, but very insoluble when the solvent is cold
Describe how you filter an organic solid under reduced pressure and why it’s done?
If your product is solid, and you want to separate it from any liquid impurities
Reaction mixture is poured into a Buchner funnel with a piece of filter paper in it. It is connected to a vacuum line, causing it to be under reduced pressure
Causing suction through the funnel which brings water quickly out leaving dry crystals of the product
Describe how the melting point and boiling point will differ the more impure a substance is?
The melting point is lowered more
The boiling point increases
