melting points and mixed melting points
DESCRIPTION
Melting Points and Mixed Melting Points. Experiment 1: Identify a compound by its melting point and mixed melting points. 113 - 115 o C 188 - 189 o C 183 - 186 o C 133 - 134 o C 135 - 137 o C 113 - 115 o C 110 - 113 o C 187 - 189 o C 133 - 135 o C. Acetamide - PowerPoint PPT PresentationTRANSCRIPT
Experiment 1: Identify a compound by its meltingpoint and mixed melting points.
Acetamide
p-Aminobenzoic acid
Camphoric Acid
trans-Cinnamic Acid
Malonic Acid
p-Nitrophenol
Resorcinol
Succinic Acid
Urea
113 - 115 oC
188 - 189 oC
183 - 186 oC
133 - 134 oC
135 - 137 oC
113 - 115 oC
110 - 113 oC
187 - 189 oC
133 - 135 oC
Melting Points and Mixed Melting Points
A sample is put in the bottom of a melting point tube.
Melting Point Tube
Put a small amount of the compound in the open end of the melting point tube.
Turn over and tape the closed end on the desk top until the compound falls to the bottom.
Fill
Sample in the melting point tube.
Use a Thiele tube filled with mineral oil to heat your sample.
Thiele Tube
Attach the melting point tube to a thermometer.
Attach Thermomete
r
Heat about 5o per minute until within about 10o of the melting point
Near the melting point heat at 1 - 2o per minute
Heat Rate
Mel-Temp in the lab
Temperature
Starts to melt Finished melting
m.p. = Start - Finish
Why is salt put on snow covered roads?
Ice melts at 0oC
What happens to the melting point if salt is added?
Ice melts!
Add Salt
Impurities such as salt lowers the melting point of water.
Putting salt on icy roads causes the ice to melt because it lowers the melting point of water.
Impure compounds usually melt lower than pure compounds so the melting point may be used as a measure of the purity of a compound
Impurities
Acetic Acid, CH3COOH, is a colorless liquid that melts at 16.6oC.
Let’s look at the melting point of mixtures of water and acetic acid.
Mp of Mixtures
Plot of melting point vs. mole fraction water for mixtures of water and acetic acid.
Eutectic point
Cool a acetic acid - water solution with a mole fraction water of 0.9
AcOH - Water
Heat a acetic acid - water solution with a mole fraction water of 0.9
AcOH - Water
Heat a acetic acid - water solution with a mole fraction water of 0.2
AcOH - Water
Pure compounds usually melt over a narrow temperature range, often 1o or less.
Impure compounds melt lower than pure compounds and over a wider temperature range.
Pure Compounds
m.p. = 115o - 119o m.p. = 118o - 120o m.p. = 121o - 122o
Melting points are a measure of purity
m.p. = 120o - 122o m.p. = 120o - 122o m.p. = 120o - 122o
Two of these bottles contain benzoic acid and one m-nitrophenylacetic acid.
How do you tell what is in each bottle?
Grind samples together to be sure they are mixed and then measure the melting point.
Mixed Melting Points
m.p. = 114o - 117o
m.p. = 120o - 122o
m.p. = 115o - 118o
Mixed 1 and 2
Mixed 1 and 3
Mixed 2 and 3
1 2 3
Results
1. Measure the melting point of your unknown
2. Run mixed melting points to confirm identification
Procedure