May 2016

Question and answers from the past

J. Devenport, SAIT, Ingle Farm South Australia

In the 1970s and ’80s Chem 13 News had a long-running question and answer series. The questions were numbered and printed in the magazine and readers were asked to help answer questions. This is a reprint of one question, which was answered by several readers. If you decide to use the advice given, make sure you carefully evaluate the current safety considerations — as you would for any demonstration. For example, thionyl chloride (suggested below) is now banned by many school boards.

Question 65 (asked in October 1978, page 12)

Can readers suggest endothermic reactions [or processes], other than the one between Ba(OH)₂ and NH₄NCS, that would be good for demonstrations?

Answer 65 (December 1978, page 14)

A visual endothermic demonstration (more fun if the students do it) is:

Dissolve cupric chloride in water until the solution becomes greenish — do not use a lot of water as this takes quite a bit of cupric chloride. Parts a-g (below) of this student procedure will then be suitable.

Obtain a clean, large test-tube, also a lab burner.
Place approximately 2 mL of cupric chloride solution in the test-tub.
Add to this an equal quantity of water.
Heat the resulting mixture almost to boiling.
Cool under running tap water.
Heat the test-tube again.
Cool.
Add, drop-by-drop with mixing, 6 M HCl (hydrochloric acid) until the green color is obtained again.
Add water until the green colour disappears.

Paul Wodchyc
Prince George Senior Secondary School
Prince George BC

Answer 65 (December 1978, page 14)

There are several spontaneous endothermic processes in the literature. The simplest is Alyea’s NH₄NO₃⁺ water reaction as given in section 7-4 on page 17 of Tested Demonstrations in Chemistry, 1965, by Hubert Alyea and Frederic Dutton.

Two others, requiring more expensive chemicals are the familiar:

Co(H₂O)6Cl₂(s) + 6 SOCl₂(l) →
CoCl₂(s) + 12 HCl(g) + 6 SO₂(g)

This proceeds readily at room temperature and the liberation of the gases also serves to demonstrate the entropy factors which help reactions “go”. [This probably would not be done in a high school lab and definitely should be done in a fume hood.]

The second is the addition of propanoic (propionic) acid to ammonium carbonate;

2 CH₃CH₂(COOH)(l) + (NH₄)₂CO₃(s) →
2 CH₃CH₂COONH₄(aq) + CO₂(g) + H₂O(l)

which shows a similar cooling effect.

Rob Dowling
Holderness School
Plymouth NH

Answer 65 (January 1979, page 16)

Mix equal volumes of urea and ammonium nitrate (about four spoons of each) in a plastic bottle. Add a small quantity of water. Cap the bottle and shake. The bottle becomes very cold to touch.

Glen Loveridge
Silver Heights Collegiate
Winnipeg MB

Answer 65 (April 1979, page 8)

The reaction between sodium hydrogen carbonate and citric acid; 10.5 g of citric acid is dissolved in 50 cm3 of water. To this solution add 12.6 g of sodium hydrogen carbonate. As before a fall in temperature of approximately 20 °C is noted. This experiment has the added advantage of being safe with no noxious fumes being involved.