Deck 15: Carrying Out Typical Reactions

Question 1

Suggest reasons for the following procedures when you are setting up a reaction assembly.
(a) The ground-glass joints are lightly greased.
(b) Only a small amount of indicator desiccant is placed in a drying tube, instead of the tube being filled completely with this material.
(c) Powdered drying agents are not used in drying tubes.

Answer 1

(a) The grease provides an air-tight seal and prevents the joints from becoming frozen together.
(b) The indicating desiccant is expensive and just a little of it in the bulk ofanother desiccant will tell you if the bulk desiccant is active or not.
(c) A powder will prevent the free flow of gas through the drying tube. This effectively closes the reaction vessel to the atmosphere.

Question 2

List three ways of preventing a "runaway" reaction.

Answer 2

Three ways to prevent a runaway reaction are: (1) cool the reaciton vessel; (2) add the reactants slowly, and (3) stir the reaction mixture so that the heat is dissipated more efficiently. Using a larger than normal reaciton vessel also helps. The large vessel can contain more of a vigorously boiling solution.

Question 3

Why are the following procedures invalid?
(a) A boiling chip is not added to a reaction mixture that is to be boiled (but not stirred).
(b) Boiling chips are saved for use in a future experiment.
(c) After use, boiling chips are washed down the drain.

Answer 3

(a) A boiling chip must be used to prevent bumping of the solution. Product can be lost if the solution bumps and splashes out of the flask.
(b) Boiling chips cannot be re-used because the pores fill with iwuid and lose their ability to release bubbles.
(c) Used boiling chips are contaminated with chemicals and thus must be disposed of accord- ing to safe environmental practices. Also, the chips can clog the drains.

Question 4

Why should a reaction never be carried out in a closed system?

Question 5

Explain why the following are incorrect practices while heating a macroscale reaction under reflux.
(a) The water to the condenser jacket is not turned on.
(b) A gentle reflux is used rather than a vigorous reflux.
(c) The heat is adjusted so that the reflux level is at the very top of the condenser.
(d) The heat is adjusted so that the solvent in the reaction flask is not boiling.

Question 6

When HCl is emitted from a reaction mixture, why do we not simply bubble the effluent gas through a solution of NaOH to trap it?

Question 7

The experimental procedure in a chemical journal describes the exothermic reaction of 10.0 g of compound A in 25 mL of solvent with 75 mL of solution B in a 250-mL Erlenmeyer flask.
(a) What size flask would you choose for the reaction of 500 mg of compound A? Explain.
(b) What size flask would you choose for the reaction of 15.0 g of A? Explain.

Question 8

Suggest a reason why nitrogen, rather than argon, is more commonly used as an inert atmo- sphere.