Chromatography and intermolecular forces

Vapour pressure

the pressure exerted by a gas or vapour (from liquid) when it sits above the liquid in a container, and the collisions it has with the container wall.

What is vapour pressure with molecules with strong IM forces:

low because molecules are held tightly together meaning less number of particles escape from liquid

What is vapour pressure with molecules with weak IM forces:

high, particles are held less strongly

What happens to vapour pressure when substances are being heated

: molecules tend to have a higher VP because it closer to their melting or boiling point so weaker IM forces and there is overall more kinetic energy so there are more collisions with container walls so higher VP.

 

Melting point

the temperature at which a substance changes from a solid to a liquid

Boiling point

the temperature at which a liquid change into a gas

Solubility

the ability of a substance to dissolve in a particular solvent

Electronegativity:

the ability of an atom in a covalent bond to attract electrons

What does electronegativity have to do with polarity:

more electronegative the more polar the molecule will be

 

In a polar diatomic molecule where will the electrons go:

towards the more electronegative atom

What is dipole

the uneven distribution of charge in a bond/molecule cause by one atom attracting the shared electrons more strongly.

What is a symmetrical molecule:

non-polar

What is an asymmetrical molecule

polar

 

What are polar substances soluble in

other polar substances

What are non-polar substances soluble in:

other non-polar substances

 

What does the shape of a molecule have to do with polarity:

if the shape of the molecule is symmetrical it means all forces are pulling on the most electrostatic atom equally, so they balance each other out and they become non-polar

What are dipole-dipole forces:

occurs only in polar molecules, attraction between the positive and negative ends of the molecules, the more polar the molecule is the stronger the d-d forces are. The overall strength of the d-d forces are, relate to the MP and BP of the molecule (stronger the d-d forces are: higher the MP and BP).

How do dipole-dipole forces occur:

A molecule has polar covalent bonds, meaning the electrons are shared unequally.

This creates a partial positive (δ⁺) end and a partial negative (δ⁻) end on the molecule (a dipole).

The δ⁺ end of one molecule is attracted to the δ⁻ end of a nearby molecule.

This attraction between polar molecules is called a dipole-dipole force

 

What is hydrogen bonding

a special form of dipole-dipole forces that only occur between molecules that have a hydrogen covalently bonded to an oxygen, nitrogen or fluorine atom, it’s the strongest type of intermolecular force

 

How does hydrogen bonding occur:

A hydrogen atom is covalently bonded to a highly electronegative atom (nitrogen, oxygen, or fluorine).

The electronegative atom pulls the shared electrons towards itself, making:

 

the hydrogen atom partially positive (δ⁺),

the nitrogen, oxygen, or fluorine atom partially negative (δ⁻).

The δ⁺ hydrogen on one molecule is strongly attracted to the δ⁻ nitrogen, oxygen, or fluorine on a nearby molecule.

 

What are the 2 key requirements for hydrogen bonding:

1. Hydrogen atom is covalently bonded to oxygen, nitrogen or fluorine. 2. A lone pair of electrons of the nitrogen, oxygen or fluorine neighbouring atom is available.

What are dispersion forces:

forces of attraction between non-polar and polar molecules (all atoms and molecules)  

How do dispersion forces occur:

·  Electrons move randomly around an atom or molecule.

·  At one moment, more electrons may be on one side, creating a temporary dipole:

• one end becomes slightly negative (δ⁻),

• the other end becomes slightly positive (δ⁺).

·  This temporary dipole induces a dipole in a nearby atom or molecule.

·  The opposite charges attract, creating a weak force called a dispersion force.

What is chromatography:

a separation technique used to separate and analyse substances present in a mixture.

 

What is the stationary phase:

this phase doesn’t move, compounds in the mixture are attracted or adsorbed (stuck) and slowed down.

What is the mobile phase:

the moving phase that carries the components of the mixture, the more soluble compounds are carried faster as they dissolve quickly.

What is adsorption:

the adhesion or stickiness of molecules to the surface of a solid/liquid (to stationary phase)

What is paper chromatography:

Paper chromatography is a technique used to separate and identify the different substances in a mixture, such as inks or plant pigments.

It works because different substances dissolve in the solvent to different amounts, so they travel different distances up the paper.

How does paper chromatography work:

·  Draw a pencil line near the bottom of the chromatography paper.

·  Place a small spot of the mixture on the pencil line.

·  Put the paper into a container with a small amount of solvent, making sure the spot is above the solvent level.

·  The solvent moves up the paper by capillary action.

·  As the solvent moves, the different substances travel at different speeds depending on how soluble they are and how adhesive they are.

·  Remove the paper before the solvent reaches the top.

·  Mark the solvent front with a pencil and let the paper dry.

·  The separated substances appear as different spots on the paper.

 

What is the mobile phase in paper chromatography:

the solvent that’s physically moving up the paper

What is the stationary phase in paper chromatography

paper

What is thin layer chromatography:

Thin-layer chromatography (TLC) is a technique used to separate and identify the substances in a mixture.

It works the same way as paper chromatography, but instead of paper, it uses a plate coated with a thin layer of silica or alumina.

Different substances travel different distances because they have different solubilities in the solvent and different attractions to the silica plate.

 

What are the steps of thin layer chromatography:

·  Draw a pencil line near the bottom of the TLC plate.

·  Place a small spot of the mixture on the pencil line using a capillary tube.

·  Put the TLC plate into a container with a small amount of solvent, making sure the spot is above the solvent level.

·  The solvent moves up the plate by capillary action.

·  The substances separate because they travel at different speeds.

·  Remove the plate before the solvent reaches the top.

·  Mark the solvent front with a pencil and allow the plate to dry.

·  If the substances are colourless, view the plate under UV light or spray it with a locating agent to make the spots visible.

What is the mobile phase in TLC:

solvent

What is the stationary phase in TLC:

silica gel

In chromatography does more soluble substances travel further or not, why:

yes because they can dissolve in solvent them move across chromatography area

 

In chromatography does more soluble substances travel further or not, why

no, because they don’t dissolve in solvent as well meaning they cant move far across chromatography area

 

What is the retardation factor

used in TLC to help compare and identify compounds, distance travelled by sample divided by distance travelled by solvent. The higher the Rf factor is, the faster the sample has progresses through the solvent (less adhesion).

How does separation work in TLC

all different compounds have different solubility, polarity and adhesion these all affect how far and fast something goes

 

What is the detection for TLC:

visible light or UV

What is molecular volatility

how easily a substance evaporates

What is gas chromatography:

chromatography technique that works because different substances travel through the column at different speeds, so they come out at different times, because of different BP, polarity, molecular size, and adhesion.

How does gas chromatography work:

·  Inject a small sample into the gas chromatograph.

·  The sample is heated, turning it into a gas.

·  An inert carrier gas (such as helium or nitrogen) carries the sample through a long column.

·  As the substances travel through the column, they move at different speeds depending on how strongly they interact with the column e.g adhesion, solubility ect.

·  The substances leave the column one at a time.

·  A detector records each substance as it leaves the column.

·  The results are shown as a chromatogram, with each substance producing a peak.

What is the mobile phase is GC:

the inert carrier gas e.g nitrogen

What is the stationary phase in GC:

thin layer of liquid or sticky coating e,g silica gel

When analysing a gas chromatograph what does the number or peaks mean:

number or different compounds detected in the mixture

When analysing a gas chromatograph what is the concentration of the compound shown by:

the vertical height (more compound= higher)

How does separation occur in GC:

based on different attraction and boiling points of substances (higher BP means longer to enter GC so longer overall retention time).

 

How is GC detected:

a detector identifies each compound as it exits the column, producing a signal which goes to the chromatograph.

 

What is high performance gas chromatography:

chromatography technique that works because different substances move through the column at different speeds, depending on how strongly they are attracted to the stationary phase, as well as polarity ect.

What are steps for HPLC:

1. Dissolve the sample in a liquid solvent.

2. Inject the sample into the HPLC machine.

3. A liquid solvent (mobile phase) is pumped through the column at high pressure.

4. The substances travel through the column and separate because they interact differently with the stationary phase.

5. The substances leave the column one at a time.

6. A detector records each substance as it leaves the column.

7. The results are shown as a chromatogram, with each substance producing a peak.

What is the mobile phase in HPLC:

liquid solvent

What is the stationary phase in HPLC:

some sort of material made of tiny packed particles inside the column and sticks to them  e.g silica particles

How does separation occur in HPLC:

different substances react differently with mobile and stationary phase e.g adhesion/sticking more, therefore different components come out at different times

In HPLC what would happen to more polar components:

more polar analytes will adsorb strongly to the stationary phase because they interact more strongly, overall making their retention time longer.

In HPLC what would happen to non-polar components

less polar analytes will not adsorb strongly, overall having a shorter retention time.

In HPLC will larger molecules move slower or faster through the column:

faster because they are too big to interact with small spaces in the stationary phase (opposite to other chromatography techniques).