Organic Chemistry: Melting Point Determination

Before we start, always remember the following: separate all of the data into specifically labeled sections; be as concise as possible; full sentences are only required in the section labeled Discussion/Conclusion.

Your professor should provide you with a sample lab report with specific headings. For this report, the following section labels will be used: Title, Purpose, Reaction Sheme and Mechanism, Reagents and Safety Data, Procedure, Data and Observations, Calculations, Discussion.

The first couple sections will be found on your lab handout. Just be specific, summarize, and do not worry about writing full sentences.

Title: Melting Point Determination
Purpose: to identify unknown compounds by comparing their melting points to the literature melting points of pure known compounds.
Reaction Scheme and Mechanism: (this section is mainly for chemical reactions and may be left blank).
Reagents and Safety Data: pure urea, pure cinnamic acid, 80/20 urea/cinnamic acid mix, 50/50 urea/cinnamic acid mix, 20/80 urea/cinnamic acid mix.
Procedure: (summarize the instructions as succinctly as possible).
1) Grind each sample and pack 2mm of each into a separate capillary tube.
2) Use Mel-Temp apparatus to determine the melting points of each (note: each mixture will have a lower melting point than either of the pure substances).
3) Fill out loan slip and retrieve a set of unknowns and knowns.
4) Grind unknown and pack into first capiallary tube.
5) Divide unknown into two portions; add known #1 to first half and known #2 to the second half. Grind and pack into second and third capillary tubes.
6) Determine melting point of all three.

The next section is pure data collection. You must read through all of the instructions given in order to find out what information is important and what needs to be collected. For example, standard melting points for pure substances can be found in the CRC Handbook of Chemistry and Physics (which should be found in your lab room). The melting point ranges for the unknowns will need to be found using the Mel-Temp apparatus.

Data and Observations: (know what data you need to look up and what to collect)
Standard Melting Points (according to the CRC Handbook)
Pure Urea 133.3 degrees Celsius
Pure Cinnamic Acid 133 degrees Celsius

Experimental Melting Point Ranges (collected through experimentation)
Pure Urea 131.0 – 133.1 degrees Celsius
Pure Cinnamic Acid 131.0 – 133.5 degrees Celsius
80/20 U/CA Mix 101.5 – 128.5 degrees Celsius
50/50 U/CA Mix 94.4 – 98.5 degrees Celsius
20/80 U/CA Mix 95.1 – 109.5 degrees Celsius

Standard Melting Points (according to the CRC Handbook)
Known #1 – Biphenyl 68.93 degrees Celsius
Known #2 – Stearic Acid 69.3 degrees Celsius

Experimental Melting Point Ranges (collected through experimentation)
Pure Unknown 60.1 – 65.2 degrees Celsius
Unknown/Known #1 56.1 – 60.1 degrees Celsius
Unknown/Known #2 60.1 – 65.2 degrees Celsius

The next section involves personal calculations using the data collected. There will almost always be a specific set of calculations needed, but not always.

Calculations: See Phase Diagram (learn to use Excel spreadsheets to make graphs). The starting temperature for each range is the blue line. The ending temperature for each range is the red line. The eutectic point is found by following the slopes down to a common point.

The final section requires complete sentences. This is where you will interpret the data you collected and draw specific conclusions.

Discussion: Melting points are used as a way to identify compounds. The pure compounds had an experimental temperature range that contained the standard melting point as given by the CRC Handbook of Chemistry and Physics. The pure urea had a range of 131.0 to 133.1 degrees Celsius and a standard melting point of 133.3 degrees Celsius. The pure cinnamic acid had a range of 131.0 to 133.5 degrees Celsius and a standard of 133 degrees Celsius. The impure compounds have temperature ranges that are lower and broader than standard. The mixtures (part urea, part cinnamic acid) differed greatly: 80/20 melted at 101.5 – 128.5 degrees Celsius; 50/50 at 94.4 – 98.5 degrees Celsius; 20/80 at 95.1 – 109.5 degrees Celsius.

By plotting both high and low temperatures (see Phase Diagram), I estimate the eutectic point (lowest melting temperature of mixtures) to be around 90.0 degrees Celsius at 40/60 urea/cinnamic acid mix.

I think my pure unknown (#7) was stearic acid. When my unknown was mixed with biphenyl, it melted at range 56.1 – 60.1 degrees Celsius; the standard melting point of pure biphenyl is 68.93 degrees Celsius. When mixed with stearic acid, it melted at range 60.1 – 65.2 degrees Celsius. My pure unknown melted at the same range as my unknown/stearic acid mixture (only 4 degress off), which indicates that both are pure. The impure mixture melted at a much lower range (8.83 degres off), twice that of the pure compound.

* For this lab report, I recieved a 20 out of 20 points.


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