organic analysis (updated)

• What are the steps involved in simple distillation?

  1. Heat the mixture of liquid in a flask.
  2. Collect the vapor produced.
  3. Cool the vapor to condense it back into liquid form.
  4. Collect the distillate in a separate container.
• What are the steps involved in fractional distillation?

  1. Heat the mixture until vaporization occurs.
  2. Vapor passes through a fractionating column filled with glass beads and metal mesh.
  3. Components condense and run back down if they do not reach the top.
  4. Heat is gradually increased until all components are vaporized and collected.
• What are the steps to prepare a standard solution?

  1. Weigh pure solid on an electronic balance.
  2. Transfer to a volumetric flask using a clean, dry funnel.
  3. Rinse remaining solid into the flask with deionized water.
  4. Half-fill flask, stopper it, and swirl to dissolve.
  5. Add deionized water up to the calibration line.
  6. Stopper and shake the flask for even concentration.
• What are the steps involved in conducting a redox titration?

  1. Add reducing agent with unknown concentration to a conical flask.
  2. Add a few drops of indicator if used.
  3. Add oxidizing agent with known concentration to the burette and record initial volume.
  4. Add drops of oxidizing agent to the conical flask until the indicator changes color.
  5. Record the final volume to determine the titre.
  6. Repeat titration for average of 3 concordant titres.
• What is the definition of concordant titres?

  Concordant titres are those that are within 0.1 mL of each other.
• What is the importance of rinsing glassware in titration?

  • Rinse burette and pipette with reactant solution to avoid dilution.
  • Rinse volumetric and conical flasks with distilled water to prevent contamination.
• What are the types of errors that can occur in titrations?

  • Systematic errors: parallax reading, standard solution reacts with atmosphere, unsuitable indicator, poor calibration.
  • Random errors: over-titration (overshooting endpoint).
• What is the effect of rinsing a pipette with water instead of solution?

  It decreases the concentration of the sample.

  ↓v (cr2o7 2-) = ↓ n (cr2o7 2-) = ↓ n (ethanoic acid) 20 (sample) = ↓ n (ethanoic acid) 250 (dilution) = ↓ n (ethanoic acid) 25 (original) = ↓ [ethanoic acid] 25 (original)
• What is the consequence of washing the burette with water?

  The solution added will be diluted slightly, leading to a higher mean titre.
• How is a sample processed in a mass spectrometer?

  1. Sample injected into ionization chamber
  2. High voltages or chemical reactions ionize sample.
  3. beam of electrons dislodges electrons to form positive ions
  4. Neutral molecules vacuumed out
  5. Ions separated in magnetic field based on mass-to-charge ratio (m/z).
  6. number of ions with different m/z values measured + recorded as mass spectrum
• What are the advantages and disadvantages of mass spectrometry?

  Advantages:

  • Only small sample required
  • Unique spectrum formed

  
  

  Disadvantages:

  • Very expensive (trained technician required)
  • Sample destroyed in the process
• Explain how the concentration of a substance in mixture can be determined using HPLC


  1. Create standard solutions of substance in questions → atleast 3 diff conc
  2. Run sample under identical conditions to original sample
  3. Identify peak that matches substance using Rt + bank of known values
  4. Record absorbance (peak area) of each standard
  5. Create calibration curve: conc vs absorbance (or peak area)
  6. Determine conc of sample
• How can chromatography be used to separate components of a mixture?


  • Diff substance in mixture → diff polarities, FGs, and molecular sizes
  • Substances will hv diff relative attractions to SP + MP
  • Components more attracted to SP spend more time adsorbed to SP = higher Rt
  • Molecules with larger molecular size → form more dispersion forces with SP = higher Rt
• what are the 2 quantitative techniques to analyse the purity of a substance?

  1. melting point determination
  2. boiling point determination → simple distillation, fractional distillation
• how does melting point determination work?

  Melting point: temp range from solid → liquid

  • pure solid mpt: within v narrow range (within 0.5 - 2 °C)
  • impure solid mpt: wider + lower range → impurity disrupts crystallised structure, weakening intermolecular forces b/w crystals

  Technique:

  1. Solid sample dried + forced into capillary tube
  2. Placed into specialised electronic mpt device → raises temp until sample melts → see magnified image of sample (starts to finish melting)
  3. lower mpt, but narrow, unknown substance → mixture of pure compounds
• how does simple distillation work?

  
  

  • separates liquid from mixture with sig diff bpts (more than 50°C apart)
  • mixture heated → component with lower bpt evaporates → vapor condensed back into a liquid (distillate) + collected
  • less effective for closely boiling mixtures → can't completely separate components with similar boiling points
  • IMPURE = higher BPT + wider range
• how does fractional distillation work

  
  

  • separates components of mixture with closer boiling points (less than 50 °C apart)
  • mixture heated + vapors pass through fractionating column before condensation → column contains temperature gradient which allows multiple condensation + evaporation cycles → more refined separation of components
  • more effective for mixtures with similar boiling points → allows for gradual separation of components based on small differences in volatility
• what the qualitative techniques to analyse the purity of a sample?

  1. Bromine test → C=C
  2. Baeyer’s test → C=C
  3. Lucas test → -OH
  4. Silver Mirror test (Tollen’s test) → -OH in primary
  5. Oxidation → -OH in primary or secondary
  6. Esterification → -OH and -COOH
  7. Reaction with metal carbonate → -COOH
  8. Acid-base indicator test → -COOH
• what is iodine number and how do you find it?

  Iodine number: mass of I2 that reacts with 100g of the fatty acid

  1. find no. C=C in one molecule of fatty acid
  2. n(fatty acid) = 100/M(fatty acid)
  3. n(I2) = n(fatty acid) x no. C=C
  4. m(I2) = n(I2)x M(I2) → 126.9 x 2
  5. Iodine number = m(I2) → no units
• Bromine test →C=C

  • Use: tests for unsaturation (C=C)

  • Reaction: addition reaction

  • Positive Test: brown colour of bromine solution fades to colourless

  • Negative Test: solution remains brown

  • Cons: no information about type or location of unsaturated bonds; can be slow reaction
• Baeyer’s test → C=C


  • Use: tests for unsaturation C=C)

  • Reaction: alkaline KMnO4 reacts with C=C

  • Positive Test: solution changes from purple to colourless and brown precipitate forms

  • Negative Test: solution remains purple

  • Cons: only helpful if you know it is a HC (as alcohols and aldehydes would also react); no information on C=C location
• Lucas test → -OH


  • Use: tests for -OH

  • Reaction: mix Lucas reagent (concentrated HCl and ZnCl2) with alcohol

  • Positive Test: cloudy liquid forms

  • Negative Test: no change in colour

  • Cons: does not differentiate between primary, secondary or tertiary alcohols
• Silver Mirror test (Tollen’s test) → -OH in primary


  • Use: tests for -OH in primary alcohols (use to differentiate primary and secondary)

  • Reaction: react a small amount of acidified Cr207 to oxidise alcohol into aldehyde, then mix with Tollen's reagent

  • Positive Test: silver mirror-like substance

  • Negative Test: no change in colour
• Oxidation → -OH in primary or secondary


  • Use: tests for -OH in primary or secondary

  • Reaction: react acidified MnO4 or Cr2072-with suspected alcohol in oxidation reaction

  • Positive Test: solution changes colour from purple to pink (then colourless); or orange to green

  • Negative Test: remains purple or orange

  • Cons: positive test will occur for aldehydes
• Esterification → -OH and -COOH


  • Use: tests for -OH and -COOH

  • Reaction: react suspected alcohol with carboxylic acid (or vice versa) and H2O4(1); esterification (condensation) reaction will occur to produce ester and water

  • Positive Test: sweet-smelling odour formed

  • Negative Test: no sweet-smelling odour

  • Cons: does not indicate type of alcohol
• Reaction with metal carbonate → -COOH


  • Use: tests for -COOH (weak acid)

  • Reaction: acid reacts with metal carbonate in neutralisation reaction to produce salt + H2O + CO2

  • Positive Test: bubbling

  • Negative Test: no change observed

  • Confirmation: bubble gas into limewater → forms a cloudy white precipitate if CO2
• Acid-base indicator test → -COOH 


  • Use: tests for -COOH (weak acid)

  • Phenolphthalein: pink to colourless

  • Red Litmus Paper: remains red in colour

  • Blue Litmus Paper: changes to red in colour

  • Universal Indicator (mixture of indicators: turns green indicator to yellow-orange
• what is a standard solution?

  solution with an accurately known concentration
• what is a primary solution and the properties?

  substance so pure that amount (mol) can be calculated accurately from mass

  Properties of primary standard:

  • Known chem formula
  • Higher molar mass - minimise effect of errors in weighing
  • Easy to store w/o deteriorating or reacting w atmosphere (e.g. CO2)
  • Soluble in solvent (usually water)
  • Inexpensive
  • Readily obtainable in pure form
• How to prepare a standard solution?

  1. Weigh pure solid on electronic balance
  2. Transfer to a volumetric flask using clean, dry funnel
  3. Rinse remaining solid particles into flask w deionised water
  4. Half-fill flask + stopper it ; swirl to ensure solid particles dissolve
  5. Add deionised water up to calibration line on the neck of the flask
  6. Stopper the flask + shake the solution to ensure an even concentration throughout
• end point vs equivalence point

  End point: when indicator changes colour permanently

  Equivalence point: when reactants react in the correct mole ratio according to the balanced chem reaction
• what are the possible indicators used in titration?

  • Potassium permanganate (KMnO4 ): strong oxidising agent that changes colour from purple → pink → colourless
  • Potassium dichromate (KCr2O7 ): strong oxidising agent that changes from orange → green
  • Starch: use when iodine is the titrant, as when I2 reduced to I ion (I-), it reacts w starch to form a blue- purple colour
  • MnO4- and Cr2O72- ions, end point = equivalence point
• why isn't an indicator always required?

  some compounds used as primary or secondary standards in redox titrations undergo a colour change of their own when reaction occurs (KMnO4, KCr2O7)
• volumetric analysis calculations

  1. half equations
  2. overall balance redox equation
  3. average titre
  4. n (standard) = cv → mole ratio
  5. n (unknown) = cv
• what are some possible errors in a titration?

  Systematic:

  • Parallax (reading)
  • Standard solution reacts w atmosphere
  • Unsuitable indicator selected
  • Poor calibration of equipment
  • Random
  • Over titration (overshooting endpoint)
• what is the aliquot?

  comes out of the pipette
• what is the titre?

  comes out of burette
• volumetric analysis with dilution

  Dilution done to obtain conc. that will result in titres within the range of the burette

  Dilution factor = volume of diluted /volume of undiluted

  Steps:

  1. Draw diagram to represent
  2. n (standard) = cv
  3. n(unknown) = n(standard) x UOK
  4. [unknown] diluted (sample)= n/v (mol/L)
  5. Calc [unknown] undiluted
  6. [unknown] diluted = [unknown] diluted (sample)
  7. [unknown] undiluted = [unknown] diluted x (v(diluted)/v(undiluted))
• converting between mol/L and g/L

  mol/L → g/L

  Concentration x molar mass = g/L

  g/L → mol/L

  Conc / molar mass = mol/L
• how does rinsing the pipette with water instead of the solution affect the concentration of the solution?

  becomes diluted + ↓[sample]

  
  

  ↓v (cr2o7 2-) = ↓ n (cr2o7 2-) = ↓ n (ethanoic acid) 20 (sample) = ↓ n (ethanoic acid) 250 (dilution) = ↓ n (ethanoic acid) 25 (original) = ↓ [ethanoic acid] 25 (original)