Melting Point Flashcards
What is the characteristic of melting point?
- One of the characteristic physical properties of solids.
What is the purpose of melting point (How is it used? To find what?)
- ) Used to characterize a known compound; if you had to distinguish between several compounds). - The melting of these known compounds has to be different enough.
- ) Used to record a physical characteristic of an unknown compound for future characterization by others.
- ) To establish the purity of a known compound. - Impure compounds have a melting point that has a wider range and is lower than the pure compound.
Why does the melting point of an impure compound have a wider range and lower melting point than its pure counterpart?
- Energy is used to overcome the crystal lattice to melt the solid.
- Impure compounds disrupt the crystal lattice resulting in less energy needed to break the intermolecular forces of the solid.
What are the factors that impact melting point? Describe them.
- ) Intermolecular forces: Stronger the intermolecular forces, the higher the melting point (takes more energy to break the crystal lattice structure.)
- Ionic > dipole-dipole > hydrogen > VdW - ) How effectively molecules stack on one another (shape of the molecule).
- ) Efficient Stacking: How tightly can these molecules stack into the crystal lattice (Maximizes VdW forces).
- An unsaturated fat has a lower melting point than a saturated fat because it has a kink in one of its carbon chains which does not result in efficient stacking.
How do impurities impact staking abilities?
- Impurities disrupt stacking abilities, that is why their melting points are lower than their pure compound counterparts.
What are the criteria for filling melting point capillaries and the impact it has on melting point and range?
- ) Sample must be dry
- This is because wet solvents such as water act like impurities and cause a depression and a wider range in the melting point in the compound. - ) Sample must be finely powdered.
- Chunks of a compound will create air pockets within which cause slow heat transfer. Air pockets are not good conductors of heat. - ) Sample must be tightly packed
- Again, if not, it will cause air pockets.
- You do this by hitting the bottom of the test tube on a hard surface. - ) Sample must not be more than 2-3mm high
- If there is too much compound in the capillary then there will be different temperatures throughout the sample.
If steps 2-4 are not adhered to (they deal with heat transfer) then the melting point will have a wider range.
The upper melting point of the range will eventually match the actual melting point.
What are the definitions of Observed Melting Point and the Real Melting Point?
- ) Observed Melting Point: The temperature from when the first drop of liquid is observed in the sample to the temperature when the sample is completely liquefied.
- ) Real Melting Point: From the eutectic point to when the sample is completely liquefied.
What are characteristics that are not considered melting?
- Shrinking, sagging, color change, texture change, and sweating (where droplets appear on the outside surface of the sample, due to insufficient drying.
Label the melting point apparatus. What is the order of heating in the apparatus?
- ) The bottom is the heating block (Heated First)
- ) The sample holder is in the middle (Sample heated second)
- ) Thermometer bulb is heated last
Why must determination be slow?
- To give enough time for the heat to transferred equally from the heating block to the sample and finally to the thermometer bulb.
- If you go to fast, the temperature at the sample will be very different from the temperature on the thermometer. And the sample appears to have a wider range.
- Noting that the heat reaches the sample before the thermometer if you heat to quickly.
Do you expect the observed melting point to be high or lower than the expected melting point?
- Expect it to be lower.
Why is it suggested to make two samples of the pure unknown when taking the melting points of these unknowns?
- The first capillary is for approximation, while the second is for slow careful determination.
Why can’t you use a sample that you already melted to run another melting point determination?
- Because the sample may be decomposed which will lead to a lower melting point than the first.
What kind of molecules decompose before melting? What is the observation if this does occur?
- Ionic organic molecules (high melting points)
- Compounds with strong hydrogen bonds such as sucrose, polysaccharides, and proteins.
- A color change is the observation if this occurs.
How and why is calibration of melting point carried out?
- You take the melting point of a known compound and see how many deviations of the temperature of your thermometer is reading versus the actual melting point.
- You can add or subtract the deviation to the rest of your samples melting points to have more accurate results.
