Question

data i got from the experment in the lab.

"the NH₃ + HCl reaction " & "ammonium chloride into water"

Time	Temperature		Time	Temperature
0.00	21.7		0.00	21.7
0.25	21.7		0.25	21.7
0.50	21.6		0.50	21.7
0.75	21.6		0.75	21.7
1.00	21.6		1.00	21.7
1.25	21.6		1.25	21.7
1.50	21.6		1.50	21.7
1.75	21.6		1.75	21.7
2.00	21.5		2.00	21.7
2.25	21.4		2.25	21.7
2.50	21.4		2.50	21.6
2.75	21.4		2.75	21.6
3.00	21.4		3.00	21.6
3.25	27.6		3.25	21.3
3.50	27.9		3.50	20.5
3.75	27.9		3.75	20.4
4.00	27.9		4.00	20.4
4.25	27.8		4.25	20.4
4.50	27.8		4.50	20.4
4.75	27.7		4.75	20.4
5.00	27.6		5.00	20.4
5.25	27.6		5.25	20.4
5.50	27.5		5.50	20.4
5.75	27.5		5.75	20.4
6.00	27.4		6.00	20.4
6.25	27.4		6.25	20.4
6.50	27.3		6.50	20.5
6.75	27.3		6.75	20.4
7.00	27.2		7.00	20.4
7.25	27.2		7.25	20.4
7.50	27.2		7.50	20.4
7.75	27.2		7.75	20.4
8.00	27.1		8.00	20.4

questions:

1) Use your data and the plots you made to calculate ΔH for the two reactions you studied in this experiment.

2) Use the two ΔHs you just calculated and the information in the background to calculate the enthalpy of formation of solid ammonium chloride.

3) Lookup the enthalpy for the formation of solid NH₄Cl. Cite your source.

4) How close is your calculated enthalpy to the value you looked up? What might have happened to cause them to be different (think about the different parts of your procedure and describe some specific things).

5) The procedure you used in this experiment was not idea because you did not check your accuracy or precision. Explain what you would have need to do to check these things.

Answer 1

4)

The main experimental problem in any calorimetric measurement is obtaining an accurate value of ΔT. The initial temperature, Ti, of the reactants can be determined directly using a thermometer. However, it is difficult to obtain a precise value for the final temperature, Tf (the instantaneous temperature when the reactants are mixed together and react), because (1) reactions do not occur instantaneously, and (2) calorimeters are not perfectly insulating, but actually allow some heat energy to slowly enter or escape from the calorimeter over time. This occurs both during the reaction and after its completion. Figure 1 illustrates the situation, and suggests a solution. If an exothermic reaction occurs in a hypothetical calorimeter that is perfectly insulated, all of the heat produced by the reaction will remain in the calorimeter, resulting in a constant final temperature. This would yield the same ΔT whether or not the reaction is instantaneous. Now consider a hypothetical exothermic reaction that occurs instantaneously, but in a realistic calorimeter that is not perfectly insulated. In this case, the temperature of the calorimeter would diminish over time due to the gradual escape of heat energy to the surroundings. The “final” temperature to be used in determining ΔT in this case is actually the maximum temperature reached immediately after reaction occurs, since this temperature change is due exclusively to the heat produced in the reaction, and no escaping of heat to the surroundings has occurred yet.