UNIT 4 - Practical Skills And Techniques

Question 1

What is a desiccator and what is it used for

Answer 1

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

Question 2

Weighing by difference steps

Answer 2

• 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

Question 3

What is refluxing

Answer 3

• 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

Question 4

How is refluxing carried out

Answer 4

• 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.

Question 5

Why are anti bumping granules added to reaction mixture when using refluxing

Answer 5

• 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.

Question 6

Refluxing :The condenser at the top of the flask

Answer 6

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

Question 7

What is Distillation used for

Answer 7

• 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

Question 8

What is distillation

Answer 8

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.

Question 9

In distillation what is the requirements of the liquids in the mixture

Answer 9

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.

Question 10

What can be done if the liquid product that is distil is very volatile (evaporates easily)

Answer 10

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.

Question 11

What is vacuum filtration , and why is it carried out

Answer 11

• 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.

Question 12

What apparatus is used in vacuum filtration and how is it carried out

Answer 12

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.

Question 13

What is done to the solid after filtration

Answer 13

• 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.

Question 14

What is recrystallisation used for

Answer 14

Recrystallisation is a very important technique used to purify an impure solids (by removing unwanted byproducts.)

Question 15

Steps of recrystallisation and requirements in each step

Answer 15

• 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

Question 16

How is the solvent chosen for recrystallisation

Answer 16

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

Question 17

What is solvent extraction

Answer 17

• another process used to purify a compound by using its different solubility between two solvents. This technique is used in organic chemistry for purification

Question 18

The crude product in solvent extraction

Answer 18

• 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)

Question 19

Solvent extraction - How can the desired product be extracted from a solution

Answer 19

• if the desired product is present in an aqueous mixture, it can be extracted from the mixture by the addition of a second solvent

Question 20

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 ______

Answer 20

• more
• immiscible
• solvent
• evaporated

Question 21

Solvent extraction : what will happen when the organic solvent is added ti the aqueous mixture containing the desired product

Answer 21

• 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

Question 22

What equipment is used for solvent extraction

Answer 22

Separating funnel

Question 23

Steps for solvent extraction

Answer 23

• 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

Question 24

Some of the product will still be present in the ________ layer, As an equilibrium will have been established between the two layers

Answer 24

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.

Question 25

What is done if not all product is extracted from the aqueous layer? How can we obtain a higher yield ?

Answer 25

• this layer is now mixed with a fresh volume of organic solvent in the separating funnel. This will extract more of the product remaining in the aqueous layer. This process is usually repeated two or three times to maximise the yield obtained.

Question 26

How can we get our final product

Answer 26

• the organic solvent with product is then treated with an anhydrous salt to removed any remaining water. The salt is then removed by filtration

Question 27

• A pure substance has a _______ melting point
• an impure substance melts over a _____ _______ of temperatures
• an impure substance melts at a ______ temperature than a pure substance

Answer 27

• fixed
• wide range
• lower

Question 28

• what is a melting point of a substance

- What can melting point determination do

Answer 28

• the melting point of a substance is the temperature range from when the solid first starts to melt and when all of the solid has melted
• melting point determination can be done to check the purity of a solid

Question 29

How is melting point determination carried out

Answer 29

• some of the solid us put in a melting point tube and this is placed in the melting point apparatus. It is heated slowly and the temperatures at which the solid starts to melt and the temperature at which it finished melting are recorded. The technique is repeated and an average of the temperature is taken. The average is compared with known melting point values in the data booklet.

Question 30

The greater the range of melting points, the more ________ are present

Answer 30

Impurities

Question 31

A range of less than _____ indicates a fairly pure substance

Answer 31

2degrees

Question 32

What is mixed melting point determination

Answer 32

Mixed melting point determination involves mixing a small quantity of the product with some of the pure compound (50% of each) and determining the melting point. If the melting point turns out to be short and close to the expected value, then the two substances must be identical. If the two substances are not similar, then the melting point of the mixture will be much lower and the melting range much broader. This results from the fact that each compound would act as an impurity of the other

Question 33

Percentage yield will always be less than_____

Answer 33

100%

Question 34

Why will percentage yield be less than 100%

Answer 34

• losses can occur during a separation technique
• may be due to the transfer of product from one flask to another
• purification by recrystallisation can result in losses if some product remains in the solution
• small yield may also be due to side reactions occurring
• due to incomplete reactions
• impure reactants
• position of equilibrium