Chemistry 3202 Lab 6 Hess s Law 1

Transcription

1 Chemistry 3202 Lab 6 Hess s Law 1 Lab 6 Hess's Law Introduction Chemical and physical changes are always accompanied by a change in energy. Energy changes may be observed by detecting heat flow between a system and its surroundings. In thermochemistry labs, this is achieved using a calorimeter. In this activity, you will build another simple calorimeter using nested styrofoam cups and hydrochloric acid solution as the calorimeter water. You will assume that only the solution in the cups exchanges heat with the system. As in Lab 5, you will record temperature change in the calorimeter using the Vernier LabPro and temperature sensor. The basis of calorimetry is the first law of thermodynamics, the energy of the universe is constant: energy is neither created nor destroyed, it is simply converted from one form to another. In this activity, the universe consists of the calorimeter water (surroundings) and the chemical species (system) added to and formed in the water. The heat given off by the system is assumed to be entirely absorbed by the calorimeter water. In this lab activity, you will calculate the molar enthalpy change for the decomposition of sodium bicarbonate: by manipulating and adding these equations: NaHCO3 (s) + HCl(aq) NaCl(aq) + CO2 (g) + H2 O(l) Δ H =? Na2CO3 (s) + 2 HCl (aq) 2 NaCl (aq) + CO2 (g) + H2 O (l) Δ H =? IMPORTANT ASSUMPTIONS There are five important assumptions associated with the calculations carried out in this activity. 1. Only the calorimeter water (i.e. HCl solution) absorbs or releases energy. 2. The density of the solution is the same as the density of water (i.e. 1 g/ml). 3. The mass of the calorimeter water is equal to the mass of solution plus the mass of the soluble reactant. 4. The mass of CO 2 gas that escapes is negligible. 5. The specific heat capacity of the reaction mixture is the same as the specific heat capacity of water. c soln = c water = J ( gc ) ( )

2 Chemistry 3202 Lab 6 Hess s Law 2 IMPORTANT NOTES 1. In this activity, you will be using the Vernier LabPro and a temperature sensor to measure temperature. This device and associated peripherals are stored in a large toolbox at your school. The LoggerPro software must be installed on the computer being used with the Vernier LabPro. 2. The temperature sensor is should be handled with care. It should be cleaned with distilled water and stored in its original packaging upon completion of the activity. Problem What is the molar enthalpy of reaction for the thermal decomposition of sodium hydrogen carbonate at standard conditions? Materials The materials used in this activity per pair of students are: 1 centigram balance 2 50 ml beakers or weighing boats (to obtain mass of solids) 1 graduated cylinder ml beaker 2 styrofoam cups 2.00 g of solid sodium carbonate 2.00 g of sodium hydrogen carbonate (baking soda) 100 ml of 1.0 M hydrochloric acid distilled water safety eye-glasses stirring rods Vernier LabPro and LoggerPro software (see your m-teacher) Vernier temperature sensor for use with Vernier LabPro (see your m- teacher) Safety 1. You will be handling liquids around sensitive electronic equipment throughout the duration of this activity. It would be prudent to keep your work space free of unnecessary materials and to organize the materials carefully before you begin. Keep plenty of paper towels and rinse water on hand to clean up minor spills. 2. Hydrochloric acid is a corrosive substance than may cause burns and severe eye injuries. You must wear eye protection at all times during this activity. Hydrochloric acid can discolour and damage clothing, so you are advised to wear a lab coat or an apron. 3. A paste of sodium hydrogen carbonate (baking soda) can be applied to skin to neutralize hydrochloric acid. Should you come in contact with any of the

3 Chemistry 3202 Lab 6 Hess s Law 3 solutions used in this activity, you must rinse the affected area under running water for at least 15 minutes. 4. Advise your supervising teacher of any incidents as soon as they occur. 5. You may dispose of the solutions used in this activity by rinsing them down a sink with plenty of water. Procedure 1. Obtain a lab coat and a pair of safety glasses and wear them throughout the activity. 2. Create a data table to record quantitative and qualitative observations. For each reaction, you will need to record: o the initial temperature of the water (t i ) o the highest or lowest temperature reached after addition of the solute (t f ) o the volume of acid in the calorimeter (this is the calorimeter water) o o the mass of the solid reactant dissolved any qualitative observations like sounds, odours, colour changes, gas formation, etc. 3. Setup the calorimeter. Leave the Vernier temperature sensor out of the calorimeter for now. To make the calorimeter more stable, you should place it in a 250 ml (or larger) beaker. 4. Double click on the LoggerPro 3.3 icon on your desktop. 5. Prepare your computer for data collection by: connecting the Vernier LabPro to a USB port on your computer connecting the temperature sensor to Channel 1 on the Vernier LabPro unit, and connecting the power cord to the Vernier LabPro unit. Part 1: Sodium Hydrogen Carbonate (NaHCO 3 (s) ) 1. Using a centigram balance, measure 2.00 g of sodium hydrogen carbonate ( NaHCO 3(s) ) in a 50.0 ml beaker or small clean, thoroughly dry container/weighing boat. 2. Using a 50.0 ml graduated cylinder, obtain exactly 50.0 ml of hydrochloric acid. Add it to the clean, dry calorimeter. 3. Under the file menu in the LoggerPro 3.3 software, click File --> Open ---> Chemistry with Computers ---> Experiment 18 Hess's Law 4. Carefully place the tip of the Vernier temperature sensor into the calorimeter. It is a good idea for one person hold the sensor in place while another manipulates the computer keyboard and mouse. 5. When the calorimeter water (i.e. HCl) temperature seems relatively stable, click Collect.

4 Chemistry 3202 Lab 6 Hess s Law 4 6. After 5-10 seconds, add the 2.00 g sample of sodium hydrogen carbonate. 7. Stir using the stirring rod. Do not use the sensor as a stirring rod. 8. Click Stop after the temperature of the calorimeter has reached a minimum and begins to increase again. 9. Click File --> Print --> Data Table. Enter your name(s) in the appropriate field, and click OK. Make one copy per student. 10. Click File --> Print Graph. Enter your name(s) in the appropriate field, and click OK. Make one copy per student. 11. Save the experiment by clicking File --> Save As. Name the file c3_lab06_trial01.xmbl 12. Remove the temperature sensor and clean it thoroughly with distilled water. Pat it dry with paper towel. Do not rub the tip. 13. Empty the calorimeter contents into a sink and rinse with plenty of water. 14. Rinse and dry the calorimeter cup thoroughly. Part 2: Sodium Carbonate (Na 2 CO 3 (s) ) 1. Repeat steps in Part 1 this time using 2.00 g of sodium carbonate (Na 2 CO 3(s) ).

5 Chemistry 3202 Lab 6 Hess s Law 5 Name(s): School: Results 1. Include the data table you constructed in Step 2 and the printed graphs and data tables. Analysis Part A. Sodium Hydrogen Carbonate (NaHCO 3 ) Reaction 1. Calculate Δt for the calorimeter water: 2. Calculate the mass of calorimeter water (mass of solid reactant and mass of calorimeter water; assume density of HCl = density of water). 3. Calculate the heat change of the calorimeter. Show workings here: 4. Calculate the number of moles of NaHCO 3, reacted. Show workings here: 5. Use the first law of thermodynamics to determine the heat change for NaHCO Calculate the molar enthalpy of reaction for NaHCO Write the thermochemical equation for the reaction between hydrochloric acid and sodium hydrogen carbonate.

6 Chemistry 3202 Lab 6 Hess s Law 6 Name(s): School: Part B. Sodium Carbonate Reaction 1. Calculate Δt for the calorimeter water. 2. Calculate the mass of calorimeter water (mass of solid reactant and mass of calorimeter water; assume density of HCl = density of water). 3. Calculate the heat change of the calorimeter. 4. Calculate the number of moles of Na 2 CO 3, reacted. Show workings here: 5. Use the first law of thermodynamics to determine the heat change for Na 2 CO Calculate the molar enthalpy of reaction for Na 2 CO Write the thermochemical equation for the reaction between hydrochloric acid and Na 2 CO 3.

7 Chemistry 3202 Lab 6 Hess s Law 7 Name(s): School: Part C. Application of Hess's Law 1. Write the thermochemical equations you produced in Parts A and B in the spaces provided, and then using the Hess s Law approach, manipulate the thermochemical equations to produce the overall equation below. A) B) 2 NaHCO Na CO + CO + H O Δ H = 3 (s) 2 3 (s) 2 (g) 2 (g) 2. Calculate the experimental molar enthalpy value (in kj/mol) for the production of one mole of NaHCO 3 and express it in ΔHnotation. Part D. Application of the Summation of Heats Formula 1. Calculate the theoretical molar enthalpy for the reaction (in kj/mol) using these standard molar enthalpy of formation data. 2 NaHCO Na CO + CO + H O Δ H = 3 (s) 2 3 (s) 2 (g) 2 (g)

8 Chemistry 3202 Lab 6 Hess s Law 8 Name(s): School: Conclusion 1. Calculate the percent discrepancy for your experimental value using this formula: 2. Compare your experimental value to the theoretical value and suggest at least two specific reasons for the observed discrepancy.