EXPERIMENTAL APPLICIATIONS OF HESSS LAW

EXPERIMENTAL APPLICATION OF HESS LAW

INTRODUCTION:

There are many reactions for which the heat of reaction cannot be easily measured experimentally. Some of these are dangerous to perform in the lab, while others generate so much heat that simple calorimeters cannot be used. In such cases, it is practical to examine a series of reactions whose net effect is the desired reaction, but whose heats of reaction are more easily measured. From these multi-step reactions, the required heat of reaction can be calculated using Hess Law.In this lab, you will design and conduct an investigation in order to determine the enthalpy for the decomposition of solid ammonium chloride, given by the following equation:

NH4Cl(s) ( NH3(g) + HCl(g)

The equation above can be obtained by manipulating four other equations:

(1) NH3(aq) + HCl(aq) ( NH4Cl(aq)

H = ?

(2) NH4Cl(s) ( NH4Cl(aq)

H = ?

(3) NH3(g) ( NH3(aq)

H = ?

(4) HCl(g) ( HCl(aq)

H = ?

You will experimentally determine H for reactions (1) and (2) and use theoretical values for (3) and (4) to determine the enthalpy change that accompanies the decomposition of ammonium chloride.

PRE-LAB /19 I /5C (table of results + general communication)

1. Using the thermochemical data table in your textbook (page A19), determine theoretical values for:a. H for reactions 3 and 4 b. H for the decomposition of ammonium chloride (2I)2. Show, using Hess law, how the above four equations can be manipulated to obtain the overall equation and enthalpy change for the decomposition of solid ammonium chloride. (2I)3. You will be using 1.5 M solutions of NH3 and HCl. When performing reaction (1), what is the concentration of NH4Cl that will be produced? (2I)4. Using the information you calculated in 3, what mass of NH4Cl should be used in order to generate an NH4Cl solution of the same concentration when performing reaction (2I)?

5. Design an experiment which will allow you to calculate the heat of reaction for the decomposition of ammonium chloride using the following:i. 1.5 M solutions of NH3(aq) and HCl(aq) in 50.0 mL aliquots

ii. solid NH4Cl

iii. distilled water

iv. the information you calculated in questions 1-4

v. Any other lab materials you deem necessary to carry out the investigation

In your lab notebook, write the purpose, an itemized materials list and a numbered procedure, all using proper headings and formatting. (8 I)6. Suppose you used 25.0 mL of reactants as opposed to 50.0 mL. What would be the effect on the (T value? What would be the change in q for the reaction? Explain your answers. (3 I)

7. Create a table of results in your lab notebook for this experiment. (3C) CALCULATIONS:

1. Show your calculations for the molar enthalpy for all equations from your experimentation. (4A)2. Calculate the heat of reaction for the equation NH4Cl(s) ( NH3(g) + HCl(g) using Hesss Law. Show all your work. (3A)

3. Calculate your experimental error. (1A)

4. Does your data support Hess Law? Why or why not? (2 A)

5. List three ways you can improve your experimental protocol. Provide the errors that could be avoided or minimized by your improvement. (3A)CONCLUSION: 2ALAB PERFORMANCE: 3A

Lab write-up

/15A

/5C