Lab 1: Melting Point and Melting Point Depression

melting point

the temperature at which a solid changes into a liquid at standard atmospheric pressure

for a pure compound, it is the temp at which the vp of the solid and liquid are equal

thermal energy becomes sufficient to overcome the IMFs holding the particles in a rigid structure

melting point depression

the phenomenon where the melting point of a substance decreases when it is impure or a mixture

impurity must be soluble to lower mp

eutectic point

the specific composition and temperature at which a mixture of two or more substances has the lowest mp

why is melting point determined?

if the compound is known, it helps to characterize the sample

if the compound is new, it is recorded to allow future characterization by others

the range of the mp is indicative of the purity of the compound

how is melting point determined?

IMFs

order of IMF strength

London Dispersion Forces

Pi-Pi Stacking

Dipole-Dipole

Hydrogen Bonding

Ion-Dipole

Ionic Attractive Forces

London Dispersion Forces (Van der Waals Forces)

Organic molecules that contain only carbon and hydrogen (hydrocarbons) are weakly attracted to each other by London Dispersion forces

Increase as molecular size increases

The larger the molecule, the greater the attractive force for neighboring molecules and the greater the energy required to get two molecules to move apart

Temporary

Occur due to random movement of electrons

Strength increases with size, surface area, and number of electrons

Organic hydrocarbons

Pi-Pi Stacking

Type of noncovalent interaction that occurs between aromatic rings due to the overlap of their pi bonds

Dipole moment between two aromatic rings

Generally considered weak to moderate in terms of strength depending on other IMFs

Important in DNA structure

Dipole-Dipole

The attractive forces between molecules increases when functional groups containing electronegative atoms, such as chlorine, oxygen, and nitrogen are present because these atoms are more electronegative than carbon

Electronegative atoms pull electrons toward themselves, making their end of the bond slightly negatively charged (δ-) and leaving the carbon slightly positively charged (δ+)

Electronegative atoms pull electrons towards themselves

Permanent dipole

Hydrogen Bonding

Occurs with organic molecules containing O–H groups (alcohols and carboxylic acids) or N–H groups (amines or amides)

The hydrogen in these groups is attracted to the unshared pair of electrons on the O or N of another molecule, forming a hydrogen bond, often symbolized by a dashed line

Ion-Dipole

Electrostatic attractions between an ion and a polar molecule, where the oppositely charged ends of the ion and the dipole are attracted to each other

Ionic Attractive Forces

The electrostatic force that occurs when oppositely charged ions are attracted to each other, forming an ionic bond

Form when a metal atom (which tends to lose electrons to become a positive ion, or cation) and a nonmetal atom (which tends to gain electrons to become a negative ion, or anion) transfer electrons

The strong, permament electrostatic force of attraction that hold together oppositely charged molecules

Usually solids with high melting points due to the strong attractive forces between them

what type of compounds have really high melting points?

ionic (salts)

what effect would poor heat circulation have on the observed mp?

Inaccurate observed melting point

Potentially higher or lower than the true value

Causes non-uniform heating and poor heat transfer

Uneven melting and sample decomposition

Three test tubes, labels A, B, and C, contain substances with approximately the same melting points. How could you prove that the test tubes contain three different chemical compounds?

• Mixed melting point test

  • Mixing two substances and performing melting point will lead to a broader and lower range than their pure melting points

• Spectroscopy

  • NMR

  • IR

  • MS

• Chromotography

  • TLC

how much does a small impurity (1-2%) depress the mp?

1-2°C

how does a 10% impurity change the mp?

typically depresses the mp by 5-10°C, often also broadening the range

You suspect that an unknown is acetanilide (mp 113.5°C-114.0°C). Give a qualitative estimation of the melting point when the acetanilide is mixed with 10% by weight of naphthalene.

105-110°C

You have an unknown with an observed melting point of 90.1°C–93.5°C. Is your unknown compound A with a reported melting point of 95.5°C–96.0°C, or compound B with a reported melting point of 90.5°C–91.0°C? Explain.

Compound A because the temperature at which the compound is 95% melted is higher than the final melting point temperature of compound B

An unknown compound is suspected to be acetanilide (mp 113.5°C–114°C). What would happen to the melting point if this unknown were mixed with (a) an equal quantity of pure acetanilide? (b) an equal quantity of benzoic acid.

(a) If the sample were mixed with pure acetanilide, the melting point would remain the same

(b) If the sample were mixed with benzoic acid (mp 122°C), the melting point of the mixture would be below that of acetanilide and the mixture would melt over a broadened, large, depressed range, perhaps 100-105°C or lower