RP5 - Distillation

• Suggest why sodium carbonate solution was used to wash the distillate

  • Na2CO3 would neutralise/react with/remove (phosphoric) acid/H3PO4/H+
• Explain why it is important to open the tap of the separating funnel periodically.

  • avoid PRESSURE BUILD-UP / release pressure / release CO2/air/gas / prevent stopper blowing out
  • IGNORE EXPLOSION (What I would normally say)
• Give a property of anhydrous calcium chloride, other than its ability to absorb water, that makes it suitable as a drying agent in this preparation.

  • Does not dissolve in/react with the cyclohexene
  • Remains a solid / is inert in cyclohexene
  • MUST refer to cyclohexene
• The cyclohexene separated in Question 02.2 was obtained as a cloudy liquid.

  The student dried this cyclohexene by adding a few lumps of anhydrous calcium chloride and allowing the mixture to stand. Give one observation that the student made to confirm that the cyclohexene was dry

  • Liquid goes clear / not cloudy NOT colourless
• Give two reasons why the apparatus shown in Figure 4 is not safe [2 marks]

  • M1 flask not clamped / ONLY the condenser is clamped
  • M2 sealed system / bung in condenser - allow explanation of effect of bung being there e.g. pressure build up
  • NOTE - This question is not asking about incorrect water direction, it is about safety of the practical

  
• State two more mistakes in the way the apparatus is set up in Figure 4

  • M1 direction of water flow through condenser - should be water in at the bottom and water out at the end (to ensure condenser is completely cool)
  • M2 thermometer not needed

  
• State the purpose of the small glass beads in the flask in Figure 4.

  • to prevent ‘bumping'
  • allow prevent large bubbles / ensure small bubbles
  • (NOT increase rate)
• After correcting the mistakes, the student heats a reaction mixture containing 6.50 g of propan-1-ol with an excess of the oxidising agent. The propanoic acid separated from the reaction mixture has a mass of 3.25 g

  State the name of the technique used to separate the propanoic acid from the reaction mixture.

  • FRACTIONAL DISTILLATION
• Give two ways of maximising the yield of propanal obtained by distillation of the reaction mixture. [2 marks] (2017 AL P2)

  • Keep the temperature of the reaction mixture below the boiling point of propan-1-ol/below 97 °C SO ENSURE ALL PROPANAL EVAPORATES but NOT propan-1-ol
  • Cool the distillate / collecting vessel
• EXPLAIN why the water should enter the condenser at the bottom and not at the top [2 marks], 2023 AS Paper 2

  • M1 idea of ensuring condenser fills with water
  • M2 idea that condenser is cool(er) or ensuring more vapour condenses
• Stage 1 = Anti-bumping granules

  • 1a no anti-bumping granules / add anti-bumping granules
  • 1b to create smaller bubbles / to prevent large bubbles / to prevent mixture jumping into condenser

  
• Stage 2 Open system with no thermometer

  • 2a system should be closed (above flask) to prevent gases escaping
  • 2b should be closed with bung + thermometer
  • 2c to allow collection of propanone only / to prevent distillation of other components // to stay in suitable temperature range
• Stage 3 The water direction in the condenser

  • 3a water flows in wrong direction through condenser / change water direction
  • 3b condenser not cool enough / not full of water
  • 3c product may not condense / comes through as gas