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?

  • AThe apparatus can be assembled more easily.
  • BAir conducts heat away more effectively.
  • CUse of the condenser requires less space.
  • DRisks of leakage and flooding are avoided.
  • EWater consumption is reduced.

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 tubes cannot be easily trimmed if the ends become cracked or contaminated.
  • BIt is harder to connect the tubes to the condenser without excessive tension.
  • CWater flowing through shorter tubes cools the condenser less effectively.
  • 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.

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.
  • BLonger tubes are more likely to become entangled with each other.
  • CLong tubes may block access to other apparatus.
  • DLonger tubes are more likely to become internally contaminated.
  • EThe use of longer tubes is less cost-effective.

Q4:

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

  • ACondenser
  • BHotplate
  • CErlenmeyer flask
  • DStirring bar
  • ERetort stand

Q5:

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

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

Q6:

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

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

Q7:

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

  • ASealed oven
  • BSteam bath
  • CWater bath
  • DBunsen burner
  • EOil bath

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?

  • A3 and 4
  • B1 and 3
  • 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?

  • AA reaction under reflux should be performed in a pear-shaped flask.
  • BThe water is flowing through the condenser in the wrong direction.
  • CAn egg-shaped stir bar should have be used.
  • DThe condenser is not connected to a nitrogen supply.
  • 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 boiling stick should be added to the reaction mixture.
  • DThe condenser should be stoppered.
  • EA coil condenser should be used instead of a Liebig condenser.

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

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

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?

  • AOxygen could be used instead of nitrogen.
  • BMetallic sodium could be added to the reaction mixture.
  • CNitrogen could be bubbled through the reaction mixture.
  • DThe reaction mixture could be cooled.
  • 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.

H2OH2ON2

In what way is the apparatus incorrectly assembled?

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

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

  • AToxic
  • BFlammable
  • CCorrosive
  • DPungent
  • EOxidizing

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?

  • AActivated charcoal in water
  • BDry ice in acetone
  • CHydrochloric acid
  • DAqueous sodium hypochlorite
  • EConcentrated aqueous ammonia

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