Melting Point
VIDEO LINKS:
Technique Experiments (Write protocols and perform techniques based on these):
- Preparing Pure Sample for Melting Point Determination
- Melting Point Determination of Pure Compounds:2a) Melting Point Determination with Stuart SMP102b) Melting Point Determination with SRS Digimelt
- Mixed Melting Point
Lab Experiments (Write protocols and perform experiments based on these):
- Using the technique videos from above, write a detailed protocol for determining the melting point when your known compounds are: lauric acid, myristic acid, palmitic acid, and stearic acid.
- Using the mixed melting point technique video from above, write a protocol for determining the melting point for a pair of compounds (stearic acid and biphenyl; OR acetanilide and fluorene) that are mixed in 25:75, 50:50 and 75:25 ratios.
- Using the mixed melting point technique video from above, write a detailed protocol for determining the identity of your unknown compound given possible options.
The prelab must include Experimental Protocol, Chemical Table and Equipment Table.
The lab report requires all sections (including prelab sections) to be completed in one document.
Experimental Protocol
(Analysis) Watch the experiment videos. Take notes on the protocol for both the technique and the experiment. Stop the video and re-watch as necessary to acquire the details of the procedure. Write out the protocol for each part of the experiment. (It can be written in sequential steps. Complete sentences are not necessary.) This is the protocol you will follow, so be detailed.
Chemical Table
(Representation) Prepare your chemical table including the materials you will use in the experiment. Here is a general template that you may use. (reactants AND products)
| Chemical Name | Chemical Formula | Molar Mass (g/mol) | Hazards | reference | PPE |
| Sodium Chloride | NaCl | 58.5 | Skin irritation | https://fscimage.fishersci.com/msds/21105.htm | |
Equipment Table
(Analysis) Identify the equipment (type AND size) needed for the experiment and include the name and an image of each. Be sure to describe the equipment, how to use it, and why it is suitable for this use.
| Equipment Name | Equipment Picture | Intended Purpose |
Data Collection
1.(Acquiring competencies) Following your detailed protocol based on the videos, perform all the experiments. Record your observations and take pictures of your key steps in the process. Your observations and images need to be incorporated in your data section and this section should be as detailed as possible as you will use this information to complete your discussion.
2.(Acquiring competencies) Following your detailed protocol based on the videos. Record your observations for the Melting Point Determination of Known Compounds:
3.(Acquiring competencies) Following your detailed protocol based on the videos, record your observations for the Melting Point Determination of Unknown Compound:
4.(Acquiring competencies) Following your detailed protocol based on the videos, record your observations for the Melting Point Determination of the Unknown Compound Identification using Mixed Melting Point technique:
| Compound name | Melting point start temperature | Melting point end temperature | Observations |
| Unknown Compound | Estimated Melting Point | Observations |
| Mixture Components | Melting point start temperature | Melting point end temperature | Observations |
Data Processing
1.(Representation) Fill out the following table with your data.
1.(Analysis) Identify the difference in melting point range between the various molecules. Provide a supported argument for the variations observed.
2.(Representation) Create a plot of chain length of the investigated molecules (on the x–axis) vs melting point (on the y–axis).
3.(Analysis) Identify your unknown and provide a supported argument for your conclusion.
4.(Analysis) Identify the type of intermolecular forces between molecules for every compound you investigated in the experiments.
5.(Assumptions)
| Compound Name | Length of Carbon Chain | Melting Point Range | Literature Melting Point |
| Assumptions made | Testing the assumption | If assumptions are wrong ... |
| The samples were pure | Recrystallize | Melting point will change after recrystallization |
Technique
Answer the following questions.
- (Analysis) Give a supported argument for using the smallest possible sample size in a melting point determination.
- (Lab Skills) Describe the proper way to load a capillary tube, including why it is necessary to grind the sample to a fine powder, and why the sample should be well packed down at the bottom of the tube.
- (Lab Skills) Describe the proper way to use a melting point apparatus, including what you look for when determining a melting point range.
- (Analysis) Give a supported argument for using a slow ramp rate during a melting point determination.
- (Analysis) Give a supported argument for why the two samples in the mixed melting point technique needed to be well mixed.
- (Analysis) If your predicted melting point is 118 °C and the experimental value is found to be 121 °C, comment on the accuracy of your measurement and provide a reason for the observed difference.
- (Analysis) If your predicted melting point is 118 °C and the experimental value is found to be 101 °C, comment on the accuracy of your measurement and provide a reason for the observed difference.
- (Analysis) Describe what you expect to see, if your ramp rate increases from 0.5 to 5 °C/minute and provide a rationale for your selection.
Discussion
Technique (20 points)
Answer the following questions.- (Analysis) Give a supported argument for using the smallest possible sample size in a melting point determination.
- (Lab Skills) Describe the proper way to load a capillary tube, including why it is necessary to grind the sample to a fine powder, and why the sample should be well packed down at the bottom of the tube.
- (Lab Skills) Describe the proper way to use a melting point apparatus, including what you look for when determining a melting point range.
- (Analysis) Give a supported argument for using a slow ramp rate during a melting point determination.
- (Analysis) Give a supported argument for why the two samples in the mixed melting point technique needed to be well mixed.
- (Analysis) If your predicted melting point is 118 °C and the experimental value is found to be 121 °C, comment on the accuracy of your measurement and provide a reason for the observed difference.
- (Analysis) If your predicted melting point is 118 °C and the experimental value is found to be 101 °C, comment on the accuracy of your measurement and provide a reason for the observed difference.
- (Analysis) Describe what you expect to see, if your ramp rate increases from 0.5 to 5 °C/minute and provide a rationale for your selection.
Discussion (20 points)
Write a minimum one-page (12 font, single spaced) discussion on the experiment conducted this week. Address as many questions as you can from each category as fully as possible integrating the collected data, providing explanations for the observed trends, and evaluating whether your original assumptions about the experiment were validated by the results. The assignment will be graded on completeness, clarity of the explanations and the meaningful integration of the collected and calculated data. Correct grammar and appropriate format for the chemical formulae and chemical reactions is expected.
- (Existing knowledge, research, and views) List the type of intermolecular forces you expect for each of your compound samples based on their structural formula.
- (Existing knowledge, research, and views) Describe the characteristics that influence the strength of each identified intermolecular force type.
- (Existing knowledge, research, and views) Describe the relationship between the strength of intermolecular forces and the melting point of a compound.
- (Existing knowledge, research, and views) Describe how the mixed melting point technique was useful in identifying an unknown organic compound.
- (Analysis) A sample appears to vanish from a melting point tube in the melting point apparatus. Provide at least two supported arguments for the cause of this observation.
- (Analysis) Identify the difference between the melting point of a pure and an impure compound.
- (Analysis) Identify the difference between the melting point range of a pure and an impure compound.
- (Analysis) Predict the general trend between the melting points of hexane (an alkane), hexanal (an aldehyde), and hexanoic acid (a carboxylic acid). Provide a supported argument for your choice.
- (Existing knowledge, research, and views) Identify at least two structural characteristics of organic molecules that can affect their melting point and indicate the direction of the effect.
- (Analysis) If an unknown compound had a melting point of 81-86 °C; is it more likely for the compound to be vanillin (mp 81-83 °C) or 1,4-dibromobenzene (mp 87-89 °C) Explain your reasoning.
- (Existing knowledge, research, and views) Find and list the published melting points for each known compound you worked with in the lab. (Make sure to reference the sources you use.)
- (Analysis) For each compound, compare the melting point values and ranges you obtained to the literature melting point values. Comment on the accuracy of your measurement and provide a reason for the observed difference.
- (Interpretation) Interpret the graph of melting point vs chain length. Identify the trend and give a supported argument for it. Generalize your interpretation to create a rule.
- (Experimental design) Design an experiment to test the effect of branching in hydrocarbon chains on melting point.
- (Experimental design) Design an experiment to test the effect of different ratios of a mixture of two compounds (25:75, 50:50, 75:25) on the melting point of the mixture.
- (Analysis) Assume that you mixed two compounds with similar melting point ranges. Predict the outcome of the experiment.
- (Assumptions) What did you take for granted when performing the experiment?
Abstract (5 points)
5 sentences:- Importance and the problem addressed
- What is the experiment testing?
- What method was used in the study?
- What were the results?
- Why is this important?
Abstract
5 sentences:
-
- Importance and the problem addressed
- What is the experiment testing?
- What method was used in the study?
- What were the results?
- Why is this important?