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Lesson: Reactions under Reflux

Worksheet • 9 Questions

Q1:

A student sets up a reaction under reflux using the glassware shown, in addition to other apparatus. A solution is prepared in dry solvent extracted from a Schlenk flask and added dropwise to a second solution under a nitrogen atmosphere.

1 2 3 4 5

To which position should the reflux condenser be attached?

To which position should the dropping funnel be attached?

Which positions should be secured with a clamp?

  • A2 and 4
  • B1 and 3
  • C3 and 4
  • D1 and 5
  • E2 and 5

Nitrogen is supplied via a Schlenk line. How many outlets of the Schlenk line must be used when assembling the apparatus and reaction mixture?

Q2:

When a reaction is performed under reflux, cooling water flows into the reflux condenser via rubber tubes. Which of the following is not a disadvantage of using very long tubes to connect the condenser to the tap and sink?

  • ALonger tubes are more likely to become internally contaminated.
  • BLonger tubes are more likely to become entangled with each other.
  • CLong tubes may block access to other apparatus.
  • DThe use of longer tubes is less cost-effective.
  • ELonger tubes may come into contact with the hotplate more easily.

Q3:

Which of the following pieces of apparatus is not used when heating a reaction mixture under reflux?

  • AErlenmeyer flask
  • BCondenser
  • CRetort stand
  • DStirring bar
  • EHotplate

Q4:

A researcher wishes to heat a reaction mixture in a nitrogen atmosphere overnight. Which heating method is the most appropriate?

  • AOil bath
  • BSealed oven
  • CWater bath
  • DBunsen burner
  • ESteam bath

Q5:

Which of the following is not a reason for the slow addition of a reagent via a dropping funnel?

  • AThe quantity of reagent must be accurately measured.
  • BThe high viscosity of the reaction solution makes it difficult to mix.
  • CThe reaction is highly exothermic.
  • DThe product must be distilled as it forms to avoid further reaction.
  • EMultiple reaction pathways are possible.

Q6:

When a reaction is performed under reflux, cooling water flows into the reflux condenser via rubber tubes. What is the main disadvantage of using very short tubes to connect the condenser to the tap and sink?

  • AIt is harder to connect the tubes to the condenser without excessive tension.
  • BWater flowing through shorter tubes cools the condenser less effectively.
  • CThe tubes cannot be easily trimmed if the ends become cracked or contaminated.
  • DShorter tubes are more likely to be ruptured by the water pressure.
  • EThe use of shorter tubes results in a lower maximum water flow rate.

Q7:

Which of the following is not an advantage of an air-cooled reflux condenser as compared to a condenser using a water flow?

  • AAir conducts heat away more effectively.
  • BUse of the condenser requires less space.
  • CWater consumption is reduced.
  • DThe apparatus can be assembled more easily.
  • ERisks of leakage and flooding are avoided.

Q8:

A student performs a reaction using a water bath and a loosely clamped reflux condenser, as shown.

H 2 O H 2 O

In what way has the apparatus been incorrectly assembled?

  • AThe water is flowing through the condenser in the wrong direction.
  • BAn egg-shaped stir bar should have be used.
  • CThe condenser is not connected to a nitrogen supply.
  • DA reaction under reflux should be performed in a pear-shaped flask.
  • EThe second neck of the round-bottom flask is not in use.

What change should be made to the apparatus if the reaction is to be run overnight?

  • AAn oil bath should be used instead of a water bath.
  • BThe flask should be connected to a nitrogen supply.
  • CA coil condenser should be used instead of a Liebig condenser.
  • DA boiling stick should be added to the reaction mixture.
  • EThe condenser should be stoppered.

Which of the following is not an advantage of using a low flow rate in the water condenser?

  • AThe temperature of the condenser is prevented from becoming too low.
  • BFlooding is less likely if the sink becomes blocked.
  • CWater consumption is reduced.
  • DThe water pressure is less likely to force the inlet tube off the tap.
  • EThe water pressure is less likely to force the inlet tube off the condenser.

The reaction is performed under nitrogen to lower the concentration of air in the reaction solvent. What change could be made to the apparatus to further protect reactants from unwanted reactions with air?

  • ANitrogen could be bubbled through the reaction mixture.
  • BThe reaction mixture could be cooled.
  • COxygen could be used instead of nitrogen.
  • DMetallic sodium could be added to the reaction mixture.
  • EActivated charcoal could be added to the reaction mixture.

Q9:

When heating a reaction mixture under reflux, which of the following steps is not required?

  • AThe condenser is secured with a Keck clip or clamp.
  • BThe flask is maintained at a suitable temperature.
  • CThe flask is securely clamped.
  • DA stirring bar or anti-bumping granules are added.
  • EPressure in the heated flask is released via a tap, bubbler or open condenser.
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