Melting Points and Mixed Melting Points

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

A sample is put in the bottom of a 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.

Sample in the melting point tube.

Use a Thiele tube filled with mineral oil to heat your sample.

Attach the melting point tube to a thermometer.

Heat about 5o per minute until within about 10o of the melting point

Near the melting point heat at 1 - 2o per minute

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!

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

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.

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

Heat a acetic acid - water solution with a mole fraction water of 0.9

Heat a acetic acid - water solution with a mole fraction water of 0.2

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.

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?

Mixed Melting Points

Grind samples together to be sure they are mixed and then measure the melting point.

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