Worksheet: Reactions under Reflux

In this worksheet, we will practice describing the procedure for performing reactions under reflux, assessing hazards and common errors.

Q1:

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

  • ARisks of leakage and flooding are avoided.
  • BWater consumption is reduced.
  • CThe apparatus can be assembled more easily.
  • DAir conducts heat away more effectively.
  • EUse of the condenser requires less space.

Q2:

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 faucet and sink?

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

Q3:

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

Q4:

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

  • AHotplate
  • BRetort stand
  • CStirring bar
  • DErlenmeyer flask
  • ECondenser

Q5:

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

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

Q6:

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

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

Q7:

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

  • ASteam bath
  • BWater bath
  • CBunsen burner
  • DOil bath
  • ESealed oven

Q8:

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?

  • A1 and 3
  • B3 and 4
  • C2 and 4
  • D2 and 5
  • E1 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?

Q9:

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

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.
  • BA boiling stick should be added to the reaction mixture.
  • CThe condenser should be stoppered.
  • DA coil condenser should be used instead of a Liebig condenser.
  • EThe flask should be connected to a nitrogen supply.

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

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

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.
  • CMetallic sodium could be added to the reaction mixture.
  • DOxygen could be used instead of nitrogen.
  • EActivated charcoal could be added to the reaction mixture.

Q10:

A reaction produces hydrogen sulfide, a hazardous gas. To minimize risk, a student performs the reaction in a fume hood using an oil bath, dropping funnel, and loosely clamped reflux condenser, as shown. In addition, evolved gases are bubbled through a suitable liquid or suspension before being vented into the fume hood.

H 2 O H 2 O N 2

In what way is the apparatus incorrectly assembled?

  • AThe nitrogen line is not connected to the flask via the condenser.
  • BWater is flowing through the condenser in the wrong direction.
  • CThe pressure-equalized dropping funnel is not clamped.
  • DThe round-bottom flask is not clamped.
  • EThe pressure-equalized dropping funnel is upside down.

Which of the following is not a hazard of hydrogen sulfide?

  • AOxidizing
  • BFlammable
  • CCorrosive
  • DPungent
  • EToxic

Through which item of glassware should the hydrogen sulfide be bubbled, and in which direction?

  • A
    H 2 S
  • B
    H 2 S
  • C
    H 2 S
  • D
    H 2 S
  • E
    H 2 S

Through which liquid or suspension should the hydrogen sulfide be bubbled?

  • AAqueous sodium hypochlorite
  • BDry ice in acetone
  • CActivated charcoal in water
  • DHydrochloric acid
  • EConcentrated aqueous ammonia

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