OCR A level Chemistry Practical techniques

What does distillation do?

separates liquids with different boiling points

What does a separating funnel do?

Separates water form organic liquids after distillation

What does drying do?

Remove traces of water

Name 2 drying agents

MgSo4

CaSo4

What does redistillation do?

Used to separate 2 organic liquids with the same/ similar boiling points

What is filtration?

the process that separates a solid from the liquid

What does recrystallization do?

Used to purify solid product after filtration

How do you separate an organic liquid?

Distillation then separating funnel then drying then redistillation

How do you separate a solid?

Filtration then recrystallisation then filtration

What does condensation equipment do in reflux?

It stops organic vapour from escaping by condensing it back to liquid

How is vapour converted to liquid in reflux?

The vapour rises up the inner tube until it meets the outer jacket containing cold water so the vapour condense and drips back into the flask

Why is reflux used?

It is used as heating under reflux enables a liquid to be continually boiled whilst the reaction occurs and it also prevents volatile Components from escaping and stops the flak boiling dry

Why are anti-bumping granules used in reflux?

As they ensure small bubbles arise and prevents vigorous uneven boiling

Why is grease applied in reflux?

A think grease layer is applied to the ground glass joint to ensure a good seal

How do you clamp the condenser in reflux?

Loosely as it is fragile

Why should the condenser never have a stopper?

If it did it would create a closer system and pressure would build up inside as the heated air expands

Where does water enter in reflux?

Water enters from the bottom and leaves at the top to ensure the jacket is full

How do you connect the rubber tubing to the reflux equipment?

Rubber tube is used to connect the inlet of the condenser to the tap and the outlet to the sink

Where should water enter in distillation?

From the bottom to endure sufficient cooling

What equipment is used in distillation?

Pear shaped flask
Condenser
Rubber tubing
Heat source
Clamp stand
Screw tap adaptor
Receiver adaptor
Still head
Thermometer

How does distillation work?

-The different liquids in the mixture will have different boiling points.

-The liquid with the lowest boiling point is the most volatile and will boil first.

-The vapour moves out of the flask up into the other parts of the apparatus, leaving behind the less volatile components of the mixture.

-When the vapours reach the cold condenser, they condense and become a liquid.

-This liquid then drips into the collecting flask.

How do you identify organic products after distillation?

-When preparing samples of organic liquid water may be obtained along side

-This will bring about the appearance of two liquid layers
-One will be organic one and one will be the aqueous one
-To identify which one is which you add some water to the mixture the larger that increases in size is the aqueous layer

How do you separate organic layers and aqueous layers?

-Ensure tap of separating funnel is closed
-Pour liquid mixture in separating funnel and place a stopper on the top of the funnel and invert the contents
-Add water to see which layer gets bigger and this one is the aqueous layer
-Place a conical flask under the separating funnel, remove stopper and open tap until the whole of the lower layer has left the funnel
-Place a second conical flask under the separating funnel to collect the other layer
-The layers are now separated in the flasks
-Label which is which

Why is titration used?

Used to find a solution of unknown concentration by reacting it with another solution of known concentration

What are the steps of a titration?

-Fill a burette with the acid of known concentration.

-Accurately measure an amount of the alkali using a calibrated pipette and pipette filler.

-Add the alkali to a conical flask with a few drops of a suitable indicator.

-Run in acid from the burette until the colour just changes showing that the solution in the conical flask is now neutral.

-Repeat the procedure adding the acid dropwise.

How can you remove acid impurities?

add aqueous sodium carbonate
shake
acid will react and release CO2
add solution to separating funnel
slowly open tap whilst holding separating funnel upside down
remove aqueous sodium carbonate layer
wash organic layer and run into a flask

What are the results of using phenolphthalein

Alkali turns pink

Acid turns colourless

What are the results of using methyl orange?

Alkali turns yellow

Acid turns orange
End point turns orange

Draw reflux apparatus.

Include:- Liebig condenser.
- COLD water in (bottom right).
- COLD water out (top left)
- Pear-shaped flask.
- Anti-bumping granules.
- Open top.
- HEAT.

Draw the gas collection apparatus?

Draw the titration equipment (1)

Draw the gas syringe equipment

Draw the distillation equipment (1)

Draw the filtration equipment

How do you calculate percentage error for equipment?

error given x amount of time equipment used / measurement for apparatus

How do you calculate maximum percentage error?

add all the individual equipment errors

Give 2 ways in which you can reduce error in titrations

replace measuring cylinders with pipettes/ burettes as they are more precise

to reduce burette reading errors make the titre a larger volume

give 2 ways in which you can make burette titre a larger volume

increase the volume and concentration of substance in conical flask

decrease the concentration of substance in burette

Give 3 ways in which you can reduce error in measuring mass

use more accurate balance
use larger mass
measure by difference

How do you measure by difference?

weigh the sample before and after addition and then work out the difference in mass

Give 4 ways in which you can improve a gas syringe experiment

Minimize apparatus error
Make sure the gas syringe is large enough to collect predicted volume
Put metal in beaker, shake and tip to start reaction preventing gas leakage
Allow it to cool before measuring volume as gas expands when hot

How do you minimize error in a gas syringe experiment?

Decrease uncertainty by using apparatus with finer scale divisions

Increase size of measurements made

When can you say that the discrepancy in results is due to non apparatus errors?

If the percentage error due to the apparatus is less than the percentage difference between the actual and predicted value

When can you say that the discrepancy in results is due to apparatus errors?

If the percentage error due to apparatus is greater than the percentage difference between the actual and predicted value

How do you calculate uncertainty?

Graduation/ 2

name three drying agents

CaCl2
CaSo4
MgSo4

What is the procedure for a drying a liquid?

-Add organic liquid to conical flask
-Use a spatula and add some drying agent to the liquid and gently swirl the contents to evenly distribute them
-Place a stopper on the flask to prevent your products from evaporating away
-If this solid has all stuck together in a lump this means water is present
-Add more drying agent until the solid on the solution is as fine powder
-Decant the liquid from the solid into a flask and it should be dry if it's clear

How can you prepare aspirin?

Reflux salicylic acid, ethanoic anhydride and glacial ethanoic acid using quick fit apparatus

Solid is formed which has to be separated before being purified

How can you prepare benzoic acid?

Alkaline hydrolysis
Quickfit apparatus
Reflux methyl benzoate with aqueous sodium hydroxide
Now carry out acidification

Solid is formed which has to be separated before being purified

What is the difference between quickfit apparatus and normal apparatus?

Designed to be easily connected
Involves:
Round bottom flask
Pear shaped flask
Condenser

What does it mean to filter at reduced pressure?

Technique for separating solid from a solvent or liquid

What apparatus do you need to filter at reduced pressure?

Büchner flask
Büchner funnel
Pressure tubing
Filter paper
Access to filter or vacuum pump

How do you filter at reduced pressure?

-connect one end of pressure tubing to vacuum outlet whilst attaching other end to Büchner flask
-fit Büchner funnel to flask ensuring tight fit
-switch on vacuum pump that is attached to filter pump
-place a filter paper inside funnel that is wet with the same solvent used to prepare solid
-filter sample by pouring mixture into centre of filter paper
-rinse beaker with solvent so all solid crystals collect into funnel
-rinse crystals into funnel with more solvent and leave them to dry

How do you carry out recrystallisation?

-pour solvent into conical flask
If the solvent is flammable, warm solvent over water bath
If solvent is water place flask on tripod and gauze over Bunsen paper
-put impure sample into a second flask
-slowly add solvent into impure sample until dissolved.
Add minimum required to dissolve
-allow solution to cool so crystals or desired product form
-after no more crystals form filter under reduced pressure to obtain dry solid

What is the melting range?

Difference between the temperature at which sample starts to melt hand the temp at which melting is complete

How can you use melting range to indiciate purity of a compound?

Impure compounds have have a wild melting range

Pure compounds have a very sharp melting range

How do you determine the melting point of a compound?

-take glass capillary tube and hold one end to a flame and rotate until the end is sealed
-fill tube with crystals
-place tube into a sample hole of a melting point apparatus and place thermometer in thermometer hole
-using rapid heating settings heat up sample whilst observing it through magnifier
-once the solid starts to melt record melting point
-repeat

Why do you heat the sample slowly during melting point determination?

As samples melt over a range of temperatures heating too quickly will result in the thermometer recording a higher temperature than actual melting range

Why do you allow the mixture to cool back to room temperature before re-filtering I'm recrystallisation?

If the solvent is still warm then some of solid will dissolve and so not all solid will have crystallised so the yield will be lower

During re-crystallisation why is the solid washed with ice cold solvent?

Because in hot solvent the solid is soluble so will dissolve and not form crystals

What should you rinse a burette with before it is used

The substance that will be placed in it

Why do you rinse out a burette before you use it ?

It could react with residual water in the burette or with substances from a previous titration concentration of substance in the burette would be lower a larger volume would be required

What must you ensure about jet space in the burette and why

It is filled with solution no air bubbles if air is in the apparatus errors will occur when filling and a larger titre will be delivered

Why use a white tile in titration

Improve contrast makes colour change more obvious

Overall purifying an organic solid

1. Dissolve impure compound in a minimum volume of hot solvent
2. Hot filter solution through filter paper
3. Cool filtered solution by placing the beaker in ice
4. Filter under pressure with bunched flask to separate crystals
5. Wash crystals with distilled water
6. Dry crystals on absorbent paper

Why dissolve in minimum volume of hot solvent ?

Must dissolve compound and impurities and be stable when cold // not dissolve

Role of filtration under pressure /by Jen flask

Speeds up filtration

Why would you wash crystals with distilled water ?

Remove SOLUBLE IMPURITIES

Why may you lose yield in filtration under pressure ?

- crystals lost when filtering or washing
- products may remain in solution after recrystallisation
- other side products / reaction occurring