UNIT 4 - Practical Skills And Techniques Flashcards
(34 cards)
What is a desiccator and what is it used for
A desiccator is a closed vessel that contains a desiccant ( a drying agent) in its base. The main desiccant is self indicating silica gel: it is blue when dry and turns pink when it absorbs moisture. An airtight seal is maintained in the desiccator by lightly greasing the ground glass surfaces on the lid and base.
- a desiccator will allow a crucible to cool down without a absorbing any more moisture
Weighing by difference steps
- weighing boat is weighed empty
- weighing boat and contents are measured and recorded (put contents into flask/ whatever used in experiment)
-weighing boat and residual material is weighed.
The accurate mass of sample transferred is the difference between the two recorded masses - or weigh by taring
; but the weighing bottle in and press tare - then put the weighing bottle with the solid in and record mass
What is refluxing
- refluxing is a technique that allows substances to be heated for a prolonged period of time without the volatile reactants and/or products escaping into the atmosphere
How is refluxing carried out
- the liquid reaction mixture is placed in a round bottomed or pear shaped flask, along with some anti bumping granules, with a condenser at the top.
Why are anti bumping granules added to reaction mixture when using refluxing
- when a reaction mixture is heated there is a tendency for it to boil violently (safety hazard) as large bubbles of superheated vapour suddenly erupt from the mixture. Anti bumping granules are added to minimise the tendency for violent boiling.
Refluxing :The condenser at the top of the flask
A condenser is placed in the mouth of the flask and connected to cold tap water (water goes in at the bottom and out the top) the flask is then heated and the vapours produced are immediately condensed back into liquids when they reach the cool condenser
What is Distillation used for
- distillation can be used for identification and purification if organic compounds. The boiling point of a compound of a compound (which can be determined by distillation is well defined and thus is one of the physical properties of a compound by which it is identified
- distillation can be used to purify a compound by separating it from less volatile substances in a reaction mixture
What is distillation
Distillation is the process of heating a liquid or a mixture of liquids, until boiling and the collecting and cooling the vapours. The liquid that distils over within a certain temperature range should be collected in the receiving flask. the method used their different boiling points (well defined physical property) to separate the more volatile from less volatile material.
Summary: a process of heating and cooling the liquids so they can be separated from each other.
In distillation what is the requirements of the liquids in the mixture
A difference of 15 degrees Celsius between the boiling points of two liquids is necessary to make sure that the maximum amount of desired product is isolated from the mixture.
What can be done if the liquid product that is distil is very volatile (evaporates easily)
If the liquid product is particularly volatile, it is good practice to place the receiving flask in an ice/water bath and to ensure the received adapter on the condenser extends well into the flask. This measure will be,o minimise the loss of product through evaporation.
What is vacuum filtration , and why is it carried out
- Vacuum filtration is a technique used for separating a solid product from a liquid
- this filtration is carried out under reduced pressure and it produced a faster method of separating a precipitate from a filtrate than traditional gravity filtration methods.
What apparatus is used in vacuum filtration and how is it carried out
Apparatus :
- Either a : Büchner , Hirsch, sinter glass funnel
- Büchner flask
:the mixture of solid and liquid is poured though filter paper into the Büchner funnel. The solid is trapped by the filter funnel and the liquid is drawn through the funnel into the flask below by a vacuum.
What is done to the solid after filtration
- the pure recrystallised solid product collects on the filter paper. The filtrate contains a solution of soluble impurities. The solid product should now be washed with a small volume of cold solvent. This removes any soluble impurities that are mixed in with the pure solid.
What is recrystallisation used for
Recrystallisation is a very important technique used to purify an impure solids (by removing unwanted byproducts.)
Steps of recrystallisation and requirements in each step
- dissolve impure crystals in the minimum volume of hot solvent. The hot solvent is added to the impure crystal, and mixture is heated. The solvent that is chosen so that the compound being purified is readily soluble at high temperatures but only sparingly soluble at low temperatures.
- filter the hot solution to remove impurities (this is usually carried out as a hot filtration- footer funnel and conical flask used in the filtration are heated before use)this prevents the solvent mixture from cooling prematurely (before it should) and forming crystals on both the filter paper and stem of funnel)
- now allow the solution to cool /crystallise
- filter off crystals using vacuum filtration and wash with cold solvent
- allow crystals to dry in oven and then cool in desiccator
How is the solvent chosen for recrystallisation
The solvent for recrystallisation is chosen so that the compound being purified is completely soluble at high temperatures and only sparingly soluble at low temperatures
What is solvent extraction
- another process used to purify a compound by using its different solubility between two solvents. This technique is used in organic chemistry for purification
The crude product in solvent extraction
- the crude product is extracted from the reaction mixture by solvent extraction - the crude product is the name given to the mixture left over after a reaction , which has the desired product (as well as impurities such as unreacted reactants and products of side reaction)
Solvent extraction - How can the desired product be extracted from a solution
- if the desired product is present in an aqueous mixture, it can be extracted from the mixture by the addition of a second solvent
Solvent extraction (choosing a solvent
- the desired product has to be _____ soluble in the chosen solvent than in the original mixture (its own solution it sits in) (water most likely)
- the solvent must be ________ with water, so when they are mixed they form separate layers
- the desired product must not react with the _____
- solvent must be readily ______
- more
- immiscible
- solvent
- evaporated
Solvent extraction : what will happen when the organic solvent is added ti the aqueous mixture containing the desired product
- when the organic solvent is added to the aqueous mixture , the product will move out the aqueous layer into the solvent layer. It can be separated from the solvent , a separating funnel is used for this technique
What equipment is used for solvent extraction
Separating funnel
Steps for solvent extraction
- first aqueous layer with desired product is added to separating funnel. The organic solvent is then added. With stopped in place the funnel is inverted. Tap is opened to release any pressure build up due to vapour rising. Repeat this over the course of s few minutes - this ensembles more of the product to move from the aqueous layer into the organic solvent layer due to increased contact between them
- after shaking the layers are allowed to separate, the organic solvent is usually less dense than water so will form the top layer , the lower aqueous layer can then be removed by turning the tap. The tap gets closed once the organic layer starts to get close to the opening. The top organic layer is then collected in a separate flask
Some of the product will still be present in the ________ layer, As an equilibrium will have been established between the two layers
Product in aqueous solvent (equilibrium arrow) product in organic solvent
The equilibrium may be such that 70% of the product is extracted into the organic solvent. This means that 30% of the product will remain in the aqueous layer.
