C8 Separation techniques

Examples of chemical process separation techniques

Displacement, electrolysis

What kind of separation technique involves metal and metal compound solution

Displacement

How does displacement reaction work to extract element from compound

More reactive metal displaces the other from the metal compound

When a more reactive metal displaces another, does it act as the oxidising or reducing agent

Reducing agent

Is the more reactive metal in a displacement reaction oxidised or reduced

Oxidised

Electrolysis can be used to

Separate ions in ionic compound

What do you need to do to the ionic compound for electrolysis

Dissolve / melt

Why must the ionic compound in electrolysis be in aqueous solution or melted

So ions are mobile

What is applied through the electrolyte using 2 electrodes

Direct current

Where do positive ions go in electrolysis

Cathode

What is the cathode

-ve electrode

What is the anode

+ve electrode

Where are negative ions attracted to in electrolysis

Anode

What happens to cations at the cathode

Reduced - gain e-

What happens to anions at the anode

Oxidised - lose e-

What kind of materials are used to make the electrodes

Inert e.g. platinum

Mnemonic for reactivity series of metals

Please stop calling me a careless zebra. Instead try learning how copper saves gold

Most reactive metal in reactivity series

Potassium

Least reactive in reactivity series

Gold

Which is more reactive, calcium or carbon

Calcium

Which is more reactive, lead or hydrogen

Lead

Could aluminium displace tin

Yes

What are physical processes used for

Separating individual elements/compounds from mixtures

What is decanting

Leave mixture - solid settles at bottom - pour off excess liquid

What is filtration used for

Separate liquid from insoluble solid

What is filtrate

Separated liquid

What is residue

Solid on filter paper

How to separate two immiscible liquids

Separating funnel

What are immiscible liquids

Liquids that don’t mix

How does separating funnel separate immiscible liquids

Denser liquid layer can be poured out the bottom

Why does separating funnel get narrower at the bottom

Easier to close tap when lower layer has passed

How to separate two substances with different boiling points

Distillation

What is typically used to separate solvent from solution

Simple distillation

What temp do u heat mixture to in simple distillation

Above boiling point of solvent

What happens in condenser in distillation

Solvent vapour cools and condenses

What is left in the collection vessel in distillation

Solvent

What remains in the flask in simple distillation (didn’t vaporise)

Solute

Which has higher boiling point in simple distillation, solute or solvent

Solvent

Why can the solute sometimes not be collected once solvent is removed (simple distillation)

May not be heat stable

What kind of condenser is often used (simple distillation)

Active cooling Liebig condenser

How does active cooling Liebig condenser work

Cold water passed through outer jacket

From where is water added to the jacket of Liebig condenser

Bottom

Why is water added at the bottom of jacket of Liebig condenser

Ensure that there is always cold water in jacket / condensation occurs all along condenser

What is fractional distillation used for

Separate miscible liquids (soluble in each other)

What does fractional distillation depend on

Liquids have different boiling points

How are liquids separated through fractional distillation

Component boils; vapour rises towards to condenser; repeat for boiling points of other substances

What is thermometer used for in fractional distillation

Monitor temperature of vapour being condensed

How is separation of two liquids happening all way up fractionating column

Continual evap/condensing on glass beads, so only lower boiling point substance gets to condenser

What is evaporation and crystallisation used for

Separating solvent from solute

When would you want evaporation/crystallisation vs. Distillation

Distillation: want solvent; crystal: want solute

Describe evaporation/crystallisation

Heat gently to crystallising point; leave concentrated solution to evaporate

What is crystallising point

Crystals just begin to appear

Why not heat to dryness (crystallisation)

Some solids may decompose if heated too strongly

What is chromatography used for

Separate mixtures of solutes or liquids

How to calculate Rf value

Distance travelled by sample / distance travelled by solvent

What is the mobile phase (chromatography)

Substance that allows molecules to move over/through stationary phase

What is the stationary phase in chromatography

Substance w affinity to molecules in the mixture being analysed

Relationship between affinity to stationary phase and distance moved

Greater affinity to stationary phase= shorter distance moved with mobile

Relationship between solubility and distance moved with mobile phased

More soluble = moves further with mobile phase

Why does the sample need to be placed above solvent in tank (chromatography)

So it spreads up with solvent instead of dissolving at the bottom

How does solvent move up paper (chromatography)

Capillary action

Can solvent front reach top of paper in chromatography

No

Why is a lid kept on tank (paper chromatography)

Ensure saturated solvent vapour atmosphere; prevent solvent evaporating from paper instead of rising

What is centrifugal ion used for

Separate heterogeneous mixtures based on different particle masses

Why might solid in liquid not settle at bottom

Very small - random motion / collisions - settle very slowly

What is done to mixture in centrifugation

Spin mixture at high speed in horizontal circle

What happens when mixture is spun in centrifugation

Heaviest particles brought to end of tube and settle out first

What is supernatant

Remaining liquid at end of centrifugation

What would you do to separate smaller particles from supernatant

Spin again at higher speed (new tube)

How could gas-gas mixtures be separated

Centrifugation/cryogenic distillation

How to check purity with chromatography

Multiple spots = impurity

Why can u not 100% tell purity of substance with chromatography

Impurity could have same Rf value as substance

How can distillation be used to indicate impurities

Boiling point of liquid should be exact

How can evaporation be used to check for dissolved impurities

evaporate solution to dryness - any solid residue = dissolved impurity

What range of values are Rf value always between

0-1