Chemistry- Practical Techniques

Question 1

Reason for heating under reflux and describe apparatus

Answer 1

Reactions involving volatile products- stop escaping
Continued heating 
Condenser attached vertically 
Anti-bumping granules
No stopper to avoid pressure build up

Question 2

Reason for distillation and describe apparatus

Answer 2

Separate miscible liquids by boiling point 
Condenser attached to side arm
Thermometer bulb in pathway of vapour 
Collection vessel for distillate 
Anti- bumping granules

Question 3

Reason for using separating funnel and describe apparatus

Answer 3

Split organic product by splitting by density
Steps
1) pour in mixture and shake to mix
2) every now and then open stopper to release pressure
3) lower density layer at the bottom may be product or may be aqueous it depends

Dry and remove water by adding anhydrous sodium sulfate or anhydrous magnesium sulfate

Question 4

Reason for doing thin layer chromatography (TLC), describe + limitations

Answer 4

Used to separate solution and determine number of compounds if impure
Pencil line at the bottom
Suitable solvent below the line
Stationary phase- silica plate
Mobile phase- solvent
May not be visible so use iodine vapour( toxic) or UV light
Limitations- hard to locate point, not all compounds have known Rf values

Question 5

How to calculate Rf value

Answer 5

Distance travelled by compound / distance travelled by solvent front

Question 6

Reason for doing Recrystallisation

Answer 6

Purify solid crude organic products
Important: Suitable solvent required, high solubility —> high temperatures 
Low solubility —> low temperatures
1) Heat everything 
2) Filter when hot  
3) Pure crystals will form when cooling ( or add seed crystal to initiate)
4) vacuum pump to filter 
5) allow to dry

Question 7

Theory behind recrystallisation

Answer 7

Solid will have impurities trapped in the lattice
Dissolving into solvent will release those impurities
When it crystallises the the solid will be pure

Question 8

Reason for vacuum filtration

Answer 8

Separate solid from liquid quickly
Use a Büchner funnel with filter paper
Vacuum pump creates low pressure in flask so liquid is forced through
Leaving solid on filter paper
Note: Disconnect pump before switching off to avoid suck back
Dampen filter paper to seal
Hirsch funnel for less material

Question 9

How to determine melting point

Answer 9

Used as evidence for purity and identity of products
Method
1) seal end of melting point tube by heating
2) add some dry crushed solid into tube
3)fix tube to apparatus next to thermometer
4) heat liquid around the tube
5) melting range calculated by original melting point temp to end temp
Compare the results with known temp to work out organic compound
Impure would have a large melting range

Question 10

How and why is a standard solution formed?

Answer 10

Formed to have a solution with an accurate concentration
1) weigh out solute
2) put solute in beaker with some solvent to dissolve
3) re-weigh weighing boat to account for residue
4) transfer to 250vm3 volumetric flask
5) add washing’s of beaker and rod into flask
6) pipette in solvent up to the mark, bottom of meniscus
7) mix by inverting flask
Note: If gone above line start again
Calculate conc by mass over volume or moles over volume

Question 11

Describe an acid base titration.

Answer 11

1) full burette with solution up to mark and record reading
2) pipette 25cm3 of standard solution
3) put that in the beaker at the bottom
4) add a suitable indicator
5) add drops until colour change
6) record volume at colour change
7) repeat and calculate mean

Question 12

How is a redox titration different?

Answer 12

No indicator needed
Transfer of electrons
Solution will change colour
Useful oxidising agent is manganate (VII) purple and manganese ions are colourless

Question 13

How to calculate % error/ uncertainty

Answer 13

Uncertainty/ measured vol x100