6.3.1 (a-b)

Question 1

What are the 2 phases in chromatography?

Answer 1

Stationary Phase: Immobile, usually a solid or a liquid supported on a solid

• In TLC: the thin layer (of silica) [the adsorbent]
• In GC: The packing in the column (high boiling liquid adsorbed onto an inert solid support)

Mobile Phase: Mobile, usually a liquid or a gas

• In TLC: The solvent
• In GC: An inert gas (e.g. He, Ne)

Question 2

In TLC, what is the stationary phase specifically?

In TLC, how are the chromatograms interpreted?

Answer 2

• The TLC plate has an adsorbent surface
  
  • Where the solid silica holds the molecules of gas or liquid
• Separation is dependent on the adsorption of each component to the stationary phase
• TLChromatograms are analysed by comparing R_f (retention factor) values of each component

Question 3

Describe the process of gas chromatography

Answer 3

• Used for separating and identifying volatile organic compounds

1. Inject the mixture at the injection point
2. The mobile phase (gas) will carry the components through the capillary column – where the liquid stationary phase is
3. The components slow down as they interact with the stationary phase
   
   1. The more soluble the component is (in the stationary phase) the slower it moves through the column
4. The components are separated depending on solubility in the stationary phase
5. The compounds in the mixture, reach the detector at different times, the more soluble the component in the stationary phase, the longer the retention time, vice versa

Retention Time: Time taken for each component to travel through the column until detected

Question 4

On a gas chromatogram, what information can be deduced?

Answer 4

• Each peak represents a component
  
  • Known retention times can be compared to the chromatogram to identify the compounds
• Peak integrations (area under the peak) can be used to determine concentrations of components

Question 5

In gas chromatography, how can you calculate the concentrations of components?

Answer 5

1. Prepare standard solution of known concentrations of components being investigated
2. Obtain a gas chromatogram for each
3. Plot a calibration curve or peak integration against concentration – known as external calibration
4. Obtain a gas chromatogram for the investigated compound (same conditions) and use the calibration curve to relate the peak integration to a concentration

Question 6

Describe the chemical test and observations for the identification of:

• 1^o, 2^o alcohol, aldehyde
• alkene/phenol
• haloalkane

Answer 6

1^o, 2^o alcohol, aldehyde​:

• Heat with acidified potassium dichromate (VI)
• Colour change (orange to green)

Alkene:

• Add Br₂ drop-wise
• Decolourises alkenes
• White ppt. for phenol, phenylamine

Haloalkane:

• AgNO₃, ethanol, water bath
  
  • Silver Chloride: white precip
  • Silver Bromide: cream precip
  • Silver Iodide: yellow precip

Question 7

Describe the chemical test and observations for the identification of:

• Carbonyl
• Aldehyde
• Carboxylic Acid

Answer 7

Carbonyl

• Add 2,4DNP
• Orange precipitate

Aldehyde

• Tollen’s Reagent ( Ag⁺(NH₃)₂ ) and warm
• Silver mirror

Carboxylic Acid

• Na₂CO_3(aq)
• Effervescence and CO_2(g)