Question

Another student investigated the reaction of ammonium nitrate

with water.
The student recorded the temperature every five seconds.
Table 2 shows the student's results.
Time in seconds
0
5
10
15
20
25
30
35
40
Table 2
Temperature in "C
22.0
19.0
16.6
15.0
14.8
13.2
13.0
13.0
13.0
Use the results in Table 2 to plot a graph of temperature in "C
against time in seconds. Draw a line of best fit.
Which point on your graph is anomalous?
Suggest one reason why the anomalous point was obtained.
In the experiment, energy is transferred from the surroundings to
the reaction mixture.
Name this type of reaction.
Give one practical use of this type of reaction.

Answer 1

Summary of Method

In aqueous solution, unionized ammonia exists in equilibrium with ammonium ion and hydroxide ion. The

equilibrium constant for this reaction is a function of temperature and solution pH. Thus, if the equilibrium

constant is known for a particular temperature and the pH of the solution is also known, the fraction of unionized ammonia can be calculated. Then, if the total ammonium concentration is known from laboratory

analysis, the un-ionized ammonia concentration can be calculated.

It must be noted however that the concentration of un-ionized ammonia is also dependent on the ionic

strength of the solution. There is a slight decrease in the un-ionized ammonia fraction of total ammonia as the

ionic strength of the solution increases. A method to estimate un-ionized ammonia in saline waters is provided

in Appendix III.