Lab #3 Melting Points & Crystallization
Crushing a solid compound into a fine powder results in a _____ melting point range than melting one large crystal.
More accurate
#3
Practice recrystallization and melting point techniques 1. Recrystallization of sodium acetate 2. Recrystallization of impure benzoic acid
Because the presence of impurities lowers the melting point, the melting point can be a good indicator of purity: the __________ the material, the _________ its melting point, and the __________ the melting point range.
Purer, higher, narrower
Melting Point Measurements
- Of impure and pure Pure sample = very narrow melting point range
At which of the following temperatures would sodium acetate be LEAST soluble in water?
0 °C
Recrystallization of Impure Benzoic Acid
1. Purify single component 2. Impure benzoic acid contains sodium acetate that will be removed 3. Add hot water to it until all dissolved 4. Remove sample flask and set aside in hood undisturbed to cool slowly and form crystals of benzoic acid and then put in ice bath 5. Collect crystals via buchner funnel 6. Dry further in oven - Determine mass to calculate percent pure you recovered (compared to initial amount you had)
Maleic acid and malonic acid have very similar names, and coincidentally, very similar melting point ranges. Maleic acid melts between 134-136 °C, and malonic acid melts between 135-137 °C. If a student inadvertently returned some malonic acid to the maleic acid jar, which of the following melting point ranges would you expect to find of the maleic acid mixture now?
130-135 °C MP of mixtures is lowered.
Melting Point:
Can be used to characterize - Measured as ranges (frist liquify and completely melted) - Pure has narrow melting point range (1-2 *C)
A student performs a recrystallization similar to how we did it in lab to purify salicylic acid from sodium chloride using warm water as the solvent. After heating the mixture until all of the material dissolves, the solution cools and a white precipitate forms. The student then performs a vaccum filtration to separate the white precipitate from the solution. Where can the student find the salicylic acid?
Collected in the Büchner funnel
Mixed/Impure samples
Depressed & broadened melting ranges, compared to pure sample
If you mix compound A (melting point range: 43-44 °C) with compound B (melting point range: 44-45 °C), then you will observe the mixture to have a melting point range of 43-45 °C.
FALSE
What physical property of organic compounds does recrystallization exploit to achieve purification?
Solubility
Recrystallization
Take advantage of solubility differences between compounds & temperature dependence - Crystals grow from supersaturated solutions. - Saturate solutions while hot and then cool it, so the solubility will decrease and solution becomes supersaturated & crystals form. * Can also be used for purifying solids!!! Filtration will physically separate the desired compound from undesired.
Impurities should be more soluble in the solvent than the desired material.
True
What is the step we will use to physically separate and collect the recrystallized benzoic acid?
Vacuum filtration
Eutectic Point for Mixture
When you add salt to ice, the melting point is lowered. The precise composition of salt and ice to yield the lowest melting point temperature is with 23 wt% salt
