exp 2 post lab

Question 1

SALICYLIC ACID

• occurs as a ___ compounds in ____
• physical properties?
• solubility
• chemical formula
• functional groups (2)
• molar mass
• exhibits ____ bonding

Answer 1

• natural compound in plants
• odorless white to light tan solid
• soluble in water at 25 C: 2.24 g/mL
• HOC6H4COOH
• carboxylic acid and hydroxyl/phenol
• 138.12
• hydrogen bonding

Question 2

Melting point of pure salicylic acid

Answer 2

158 - 159 C

Question 3

4 components of SALICYLIC ACID mixture

Answer 3

1. Desired compound (pure SA)
2. Totally insoluble impurities
3. Totally soluble impurities
4. Hot recrystallizing solvent

Question 4

How are INSOLUBLE IMPURITIES separated from the mixture?

Answer 4

hot gravity filtration

Question 5

How are SOLUBLE IMPURITIES separated from the mixture?

Answer 5

after hot gravity filtration, cool the hot mixture to room temp

the desired pure compound of SA will start to crystallize

through SUCTION FILTRATION, soluble impurities + majority of the solvent will be removed

Question 6

How to remove excess recrystallizing solvent and traces of soluble impurities from the pure desired compound?

Answer 6

Wash with cold solvent + suction filtration

Question 7

The 3 recrystallizing solvent choices for the experiment:

+ which is the appropriate?

Answer 7

1. water (appropriate)
2. 95% ethanol
3. acetone

Question 7

Effect of using too much recrystallization solvent on crude sample

in terms of recovery and purity

Answer 7

• leads to unsaturation/dilution (which may make recrystallization difficult to occur)
• lowers recovery but no effect on purity

Question 8

How much recrystallization solvent to dissolve sample? + why (1)

Answer 8

minimum amount

• ensure maximum yield of crystal

Question 8

Criteria for choosing the APPROPRIATE SOLVENT for RECRYSTALLIZATION (6)

Answer 8

1. Desired substance must be soluble in the solvent at ELEVATED temperature
2. Solvent should not react with desired substance
3. Impurities should totally soluble or insoluble (not partially)
4. Boiling Point of solvent should be less than the melting point of the desired substance
5. Solvent must be relatively volatile
6. Solvent must be safe, non-toxic, and inexpensive

Question 8

After washing with cold solvent, how to remove traces of the cold recrystallizing solvent?

Answer 8

air dry

Question 9

Effect of using too less recrystallization solvent on crude sample

Answer 9

• lead to premature recrystallization
• higher recover but lower purity

Question 10

why should hot gravity filtration be HOT?

Answer 10

• prevents premature recrystallization
• avoids impurities

Question 11

FLUTED FILTER PAPER

importance (2)

Answer 11

• maximizes the surface area for filtration
• minimizes contact with walls of the funnel

Question 12

STEMLESS FUNNEL

importance (2)

Answer 12

• avoid clogging
• prevent rapid cooling of solution

Question 13

DECOLORIZING CARBON / ACTIVATED CARBON

• what type of molecules does it attract?
• pure sample is colored, should this be used?
• pure sample is colorless, should this be used?
• pure sample is polar, should this be used?
• pure sample is nonpolar, should this be used?

Answer 13

• nonpolar molecules kasi pure CARBON siya
• no, kasi baka matanggal din color ng sample (or small amounts)
• yes, most effective
• yes, most effective
• no, didikit yung pure sample sa carbon at maiiwan sa hot gravity filtration

Question 14

What is added to fluted filter paper in hot gravity filtration and why?

Answer 14

CELITE

• used as a filtration medium
• prevent penetration of decolorizing carbon while maintaining enhanced carbon

Question 15

RECRYSTALLIZATION is based on the principle that the ___ of most solids ___ with ___ temperature

Answer 15

• solubility
• increases
• increased

Question 16

RECRYSTALLIZATION

• involves _____ ___ of filtrate placed in an _____ bath

+ why need gawin?

Answer 16

• slow cooling
• ice

slow cooling muna kasi pag linagay sa ice bath agad, pwede mabasag glass ware or mag crystallize agad which can lead to impurities

Question 17

which will crystallize more assuming there were more of the former than the latter:

crude desired substance
impurities

Answer 17

crude desire substance

Question 18

the slower the rate of cooling, the ____ the crystals formed

Answer 18

larger

Question 19

in cases, why does recrystallization not occur rapidly?

how to initiate recrystallization?

Answer 19

• due to a kinetic barrier to crystal formation

1. Scratching inner walls of beaker or glass container (release tiny fragments of glass and provide energy from vibrations)
2. Addition of seed crystal (small crude or pure solid)

Question 20

INITIATION of RECRYSTALLIZATON

WHAT does the 2 ways provide?

• it is the initial process that forms a site upon which additional particles are deposited as the crystal grows
• where mixture particles can palibot to initate their recrystallization

Answer 20

NUCLEATION SITE

Question 21

What happens if suction filtration is used instead of hot gravity filtration?

Answer 21

rapid cooling due to cold air from vacuum -> premature recrystallization

Question 22

when is suction filtration used?

Answer 22

after cooling of sample (pagtapos ng hot gravity filtration) and may crystals na na form suction filtration is used to remove the majority of recrystallization solvent + soluble impurities then wash with cold solvent afterward to remove traces of soluble impurities

Question 23

what happens if crystals are not washed with cold solvent? what if hot solvent is used? *in terms of recovery and purity

Answer 23

- increase recovery, lowers purity - decrease recovery, no effect on purity

Question 24

MELTING POINT - temperature in which the ____ and ____ states of a substance are in ____ or are _____- _____ - purpose? - what bath is used to determine this? - what is the initial step done?

Answer 24

- solid and liquid - equilibrium - co-existing - to establish a pure substance's identity - oil-bath method - pulverize on watch glass

Question 25

why cant the pure crystals (product) be pulverized in the filter paper directly?

Answer 25

kasi may "lints" ang filter paper na pwedeng dumikit sa crystals and decrease impurity

Question 26

MELTING POINT RANGE??

Answer 26

- temperature in which solid starts to melt - temperature in which the last trace of solid turns to liquid and the average of the 2

Question 27

3 CAUSES of ERRORS in melting point determination + why

Answer 27

1. Rapid heating - higher melting point than literature value 2. Presence of Impurities - lower melting point than literature value 3. Loose, uneven packing of crystals - widens melting point since more solids will be melted