Melting Points and Mixed Melting Points

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Presentation transcript:

Melting Points and Mixed Melting Points Experiment 1: Identify a compound by its melting point and mixed melting points. 113 - 115 oC 188 - 189 oC 183 - 186 oC 133 - 134 oC 135 - 137 oC 110 - 113 oC 187 - 189 oC 133 - 135 oC Acetamide p-Aminobenzoic acid Camphoric Acid trans-Cinnamic Acid Malonic Acid p-Nitrophenol Resorcinol Succinic Acid Urea

Melting Point Tube 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. Fill 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. Attach Thermometer

Heat Rate 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 Finished melting Starts to melt m.p. = Start - Finish

Why is salt put on snow covered roads?

Ice melts! Ice melts at 0oC Add Salt 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

Mp of Mixtures 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 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 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 AcOH - Water 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 AcOH - Water Heat a acetic acid - water solution with a mole fraction water of 0.2

Pure Compounds 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.

Melting points are a measure of purity m.p. = 115o - 119o m.p. = 118o - 120o m.p. = 121o - 122o

How do you tell what is in each bottle? Two of these bottles contain benzoic acid and one m-nitrophenylacetic acid. m.p. = 120o - 122o m.p. = 120o - 122o m.p. = 120o - 122o 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.

Results m.p. = 120o - 122o Mixed 1 and 2 m.p. = 114o - 117o 3 m.p. = 120o - 122o Mixed 1 and 2 Mixed 1 and 3 m.p. = 114o - 117o m.p. = 115o - 118o Mixed 2 and 3

Procedure 1. Measure the melting point of your unknown 2. Run mixed melting points to confirm identification