Orgo Lab | Melting Point

The melting point is one of the ___________________________________________ of a solid.

characteristic physical properties

The melting point is one of the characteristic physical properties of a ____________________.

solid

What are the 3 uses for the measurement of the melting point?

1. to characterize a known compound

2. to record a physical characteristic for a new compound

3. to establish purity of a known compound

To characterize a known compound, melting points of these known compounds must be ______________________________.

different enough

Why do we want to record a physical characteristic for a new compound?

for future characterization by others

Impure compounds will melt over a _________________ range of temperatures and at a ____________________ melting point than the original. Explain why this is.

wider; lower

impurities disrupt crystal lattice, so less energy or heat is required to break intermolecular forces

What are the 2 primary factors that affect melting points?

1. intermolecular forces

2. how molecules stack or the shape of the molecule

What is meant by efficient stacking?

ability of molecules to pack tightly into a crystal lattice

An unsaturated fat has a ________________________ melting point than a saturated fat.

higher

A saturated fat has a ___________________________ melting point than an unsaturated fat.

lower

Before filling the melting point capillaries, you must ensure that the sample is what 4 things?

1. dry

2. finely powdered

3. packed tightly

4. no more than 2-3 mm high

Why must your sample be dry before filling the melting point capillaries?

moisture decreases the melting point andcauses the range to be wider

Why must your sample be finely powdered before filling the melting point capillaries?

if it's in chunks, it will not pack well and create air pockets that decrease heat transfer

Why must your sample be packed tightly before filling the melting point capillaries?

to avoid air pockets

Why must your sample be no more than 2-3 mm high before filling the melting point capillaries?

if too much of the sample is in capillary, then there will be a differential in temperature throughout the sample

If the sample is pure and dry, but it is chunky and not packed tightly, then the sample will appear to have a ____________________________________ range. And the _________________________ melting point will eventually match the ________________________ melting point.

wider melting point; upper; true

What is the melting range?

the temperature when the first drop of liquid is observed to the temperature when the sample has completely liquefied

What is the real melting range?

the temperature from the eutectic point to the temperature when the sample has completely liquefied

What is meant by sweating?

when droplets appear on the outside surface of the sample due to insufficient drying

Why can you not use a sample that you had already melted to run another melting point determination?

the sample may decompose so the second melting point will be lower than the first

What do you observe when molecules decompose before melting?

a color change

What has happened if the sample disappears as you heat it?

it has sublimed

How do you prevent a sample from undergoing sublimation?

use a sealed capillary

Give the order in which the sample holder, heating block, and thermometer bulb are heated.

1. heating block

2. sample holder

3. thermometer bulb

Why must the determination of a sample's melting point be a slow process?

to give enough time for heat to transfer equally from the heating block to the thermometer bulb

What 2 things will if you determine the melting point too quickly (heat too quickly)?

1. temperature at sample will be different from temperature at thermometer

2. sample will appear to have a wide melting point range

Noting that heat reaches the sample before the thermometer, when you heat a sample too quickly: do you expect the observed melting point temperature to be higher or lower than the expected temperature?

lower

When you take melting points of unknowns, in the procedure, it is suggested that you make 2 samples of the pure unknown. Why is this suggested?

A solution of X+Y has a lower ____________________________ than that of pure X, which results in a __________________________ melting point.

vapor pressure; lower

What is the difference between melting and dissolving?

melting is the breaking of intermolecular forces within the molecule

dissolving is the disrupting of attractive forces between 2 molecules and getting them to interact with one another

What is the eutectic temperature?

the lowest temperature at which a mixture at a certain concentration will start melting

What are the 2 definitions of eutectic point?

1. the lowest point on the melting point composition diagram where a mixture has a very narrow melting range

2. the point at which the 2 solids and their liquid solutions are in equilibrium

Why do insoluble impurities depress the melting point?

because they are poor heat conductors and act as air bubbles

If compounds A and B are different compounds but have the same melting point (120 C), then the mixture of A+B will have a ____________________ melting point than 120 C.

lower

To confirm the identity of an unknown compound, mix the unknown with a small amount of known compound with a similar melting point. If the melting point stays the same, then the unknown is ____________________________.

the known compound

To confirm the identity of an unknown compound, mix the unknown with a small amount of known compound with a similar melting point. If the melting point goes down, then the unknown is ____________________________.

not the known compound

What is 1 similarity and 1 difference between the eutectic point and melting point of a pure compound?

similarity: narrow melting point ranges

difference: eutectic point is lower than the pure melting point

Would you pick standards with higher or lower melting points than the unknown?

higher