Organic Chem Lab Quiz 1

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1

Melting Point Range

Temperature at which the first solid starts to melt to a temperature when the last solid dissolves to a liquid.

2

For what two purposes are melting points routinely used?

To identify the unknown and the purity of the organic compounds.

3

What two effects do impurities have on the melting-point range of an organic compound?

It lowers and broadens the melting point range.

4

Mixed Melting Point Techniques

-Determine the MP of the unknown and then compare it to the known table of MPs.
-Make a 50:50 mixture of the unknown and possible known.
If MP of unknown+ known= MP of known, then unknown=known
If MP of unknown + known not equal to MP of known, then unknown is not equal to known.

Rate of heating in melt-temps should be 1-2 degrees Celsius per min.

5

Three different organic solids A, B, C have the same melting point-->149-150C. A 50:50 mixture of A+B melts @ 130-139C. A mixture of A+C melts @ 149-150C.

Probable MP of a mixture of B+C will be 130-139C.

6

What is the purpose of the boiling chips in a distillation?

Provides irregular surface for bubble formation insures smooth boiling, prevents bumping.

7

Why is it dangerous to heat a liquid in a distillation apparatus that is closed and tightly at every joint and has no vent to the atmosphere?

It builds too much pressure and will explode.

8

Why is it important that the cooling water in the condenser enter from the lower end and exit at the upper and not vice-versa?

So the condenser jacket at the upper end will fill up all the way. It also provides adequate cooling.

9

Which procedure was more efficient in separating the mixture into its component?

Fractional distillation because a thermal gradient is created and it causes many vaporization/condensation cycles.

10

Why should a distillation flask not be filled much more than half-full?

Because it could boil over. Some undistilled material will be thrown up into the condenser.

11

Why is it important to perform a fractional distillation slowly? What happens if fractional distillation is done too quickly by rapidly raising the temperature of the heat source?

If it is done too quickly the 2nd liquid will boil off while the 1st one boils off as well. Our goal is to boil off the 1st liquid first then increase the temp to boil off the 2nd liquid.

12

Why was the boiling point of the distillate less than 100C?

In steam distillation one of the components is always water and its boiling point is less than 100C. The mixture will boil below 100C because its an immiscible mixture and won't behave like an ideal solution.

13

Why was the distillate cloudy throughout most of the distillation?

The co-distillate was not water soluble. It was a heterogenous mixture.

14

What properties are desireable in a recrystallization solvent?

1. For the compound to be purified
-high solubility @ high temp
-low solubility @ low temp
2. For impurities
-high solubility @ all temp
-low solubility @ all temp
3. Unreactive, inert and volatile

15

How are insoluble impurities removed during a recrystallization?

Hot filters

16

How are soluble impurities removed during a recrystallization?

Cold filters

17

What is the purpose of adding charcoal (activated Carbon) to the hot mixture during recrystallization?

Activates or decolorizes Carbon
Absorbs colored impurities.

18

What is the purpose of the washing the recrystallization material with fresh solvent?

It removes the rest of soluble impurities

19

Why is the wash solvent in previous question cold, not warm or hot?

Because it would dissolve the crystals

20

What was the purpose of adding 10% NaOH solution to the ether solution in the separatory funnel in part A of the experiment?

Since NaOH is a base, it is used to extract. NaOH added can be inverted from non-ionic organic soluble forms into water-soluble ionic forms by changing the pH, therefore NaOH was added.