Lab 5 Enthalpy of Solution Formation

Transcription

1 Chemistry 3202 Lab 5 Enthalpy of Solution Formation Page 1 of 9 Lab 5 Enthalpy of Solution Formation Introduction This lab activity will introduce you to the measurement of energy change associated with the process of solution formation. You will use temperature change data for a constant pressure calorimeter to determine the molar enthalpy of solution formation for two ionic solids. Important Assumptions of Calorimetry: There are four important assumptions associated with the calculations carried out in this activity. 1. Only the calorimeter water absorbs or releases energy. 2. The density of the solution formed 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 water plus the mass of the dissolved solute. 4. The specific heat capacity of the solution is the same as the specific heat capacity of water. IMPORTANT NOTES 1. In this activity, you will be using the Vernier Lab Pro and a temperature sensor to measure temperature. This device and associated peripherals are stored in a large toolbox at your school. Consult with your m-teacher before attempting to use this device. If this device was used for Lab 4, then it should be ready for use in this and subsequent activities. If not, then a special firmware upgrade procedure must be performed on the unit before can be used in this activity. The LoggerPro 3.1 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 are the molar enthalpies of solution formation for ammonium chloride and sodium hydroxide?

2 Chemistry 3202 Lab 5 Enthalpy of Solution Formation Page 2 of 9 Materials The materials used in this activity per pair of students are: 1 centigram balance 2.68 g of ammonium chloride 2.00 g of sodium hydroxide or exactly 10 pellets massed* distilled water 2 50 ml beakers ml beaker 2 styrofoam cups 2 graduated cylinders safety eye-glasses stirring rods Vernier LabPro and LoggerPro 3.1 software (see your m-teacher) Vernier temperature sensor for use with Vernier LabPro (see your m-teacher) * If NaOH is in pellet form, find the mass of 10 pellets to a precision of two decimal places using a centigram balance just before you are ready to use it in the experiment (see step 5 in Part 1 below). ** NaOH is a deliquescent solid meaning that it absorbs moisture from the air which alters the mole amount acquired by massing. 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. Sodium hydroxide is a caustic substance than may cause burns and severe eye injuries. You must wear eye protection at all times during this activity. Sodium hydroxide 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 sodium hydroxide. Should you come in contact with any of the 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.

3 Chemistry 3202 Lab 5 Enthalpy of Solution Formation Page 3 of 9 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 solute dissolved, 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 water in the calorimeter o the mass of the solute dissolved 3. Setup the calorimeter as illustrated, leave the Vernier temperature sensor out of the water for now. To make the cups more stable, you should place them in a 250 ml (or larger) beaker. 4. Double click on the LoggerPro 3.1 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. (If this is the first use of the Vernier LabPro, then you may be prompted to perform a firmware upgrade. Simply click yes and accept all defaults. This may take several minutes so be patient. Wait until you hear a short musical piece from the device before proceeding any further.) Part 1: Sodium Hydroxide 1. Under the file menu in the LoggerPro 3.1 software: click File Open Chemistry with Computers Experiment 01 Endo-Exothermic.xmbl. 2. Double click in the graph window and edit the title so that it reads: Heat of Solution Formation of Sodium Hydroxide 3. Using a 50.0 ml graduated cylinder, obtain exactly 50.0 ml of 20 C distilled water. Add water to the clean, dry calorimeter. 4. Carefully place the Vernier temperature sensor into the calorimeter. It may be a good idea to have one person hold the sensor in place while another manipulates the computer keyboard and mouse.

4 Chemistry 3202 Lab 5 Enthalpy of Solution Formation Page 4 of 9 5. Measure out 2.00 g or the mass of exactly ten pellets of NaOH in a 50.0 ml beaker or small clean, thoroughly dry container. 6. When the calorimeter water temperature seems relatively stable, click Collect. 7. After several seconds (10-15), add the weighed mass of sodium hydroxide pellets. Remove stubborn pellets using the stirring rod. 8. Stir using the stirring rod. Do not use the sensor as a stirring rod. 9. Click Stop after the temperature of the calorimeter has reached a maximum and begins to decline again. 10. Click File --> Print Data Table. Enter your name(s) in the appropriate field, and click Ok. Make one copy per student. 11. Click File --> Print Graph. Enter your name(s) in the appropriate field, and click Ok. Make one copy per student. 12. Save the experiment by clicking File Save As. Name the file c3_lab05_trial01.xmbl 13. Remove the temperature sensor and clean it thoroughly with distilled water. Pat it dry with paper towel. Do not rub the tip. 14. Empty the calorimeter contents into a sink and rinse with plenty of water. 15. Rinse and dry the calorimeter cup thoroughly. Part 2: Ammonium Chloride 1. Repeat the procedure above using 2.68 g of ammonium chloride. 2. Be sure to change the name of the files when you save in Logger Pro.

5 Chemistry 3202 Lab 5 Enthalpy of Solution Formation Page 5 of 9 Results: Record your data in the tables below. Also include the printed Lab Pro tables/graphs. Data Table: Part 1 Sodium Hydroxide Calorimeter Water NaOHl Mass Initial Temperature -- Final Temperature -- Specific Heat Capacity J/(g C) -- Data Table: Part 2 Ammonium Chloride Calorimeter Water NH 4 Cl Mass Initial Temperature -- Final Temperature -- Specific Heat Capacity J/(g C) -- Analysis: Part 1: Sodium Hydroxide 1. What is the system and what is the surroundings in Part 1? System: Surroundings: 2. Calculate Δ T for the calorimeter water.

6 Chemistry 3202 Lab 5 Enthalpy of Solution Formation Page 6 of 9 3. The mass of the calorimeter water is equal to the mass of the water plus the mass of any dissolved ions. Calculate the mass of calorimeter water used 4. Calculate the heat change of the calorimeter water. 5. Apply the first law of thermodynamics to determine the heat change for the dissociation of sodium hydroxide. 6. Calculate the number of moles of sodium hydroxide, NaOH 7. Calculate the molar enthalpy of solution formation ( Δ H soln ), for sodium hydroxide. 8. Write a dissociation equation for NaOH (aq) (Hint: see pp MHR) that includes the enthalpy term and classify it as endothermic of exothermic.

7 Chemistry 3202 Lab 5 Enthalpy of Solution Formation Page 7 of 9 Analysis: Part 2: Ammonium Chloride 1. What is the system and what is the surroundings in Part 1? System: Surroundings: 2. Calculate Δ T for the calorimeter water. 3. The mass of the calorimeter water is equal to the mass of the water plus the mass of any dissolved ions. Calculate the mass of calorimeter water used 4. Calculate the heat change of the calorimeter water. 5. Apply the first law of thermodynamics to determine the heat change for the dissociation of ammonium chloride. 6. Calculate the number of moles of ammonium chloride, NH 4 Cl

8 Chemistry 3202 Lab 5 Enthalpy of Solution Formation Page 8 of 9 7. Calculate the molar enthalpy of solution formation ( Δ H soln ), for Ammonium Chloride. 8. Write a dissociation equation for NH 4 Cl (aq) (Hint: see pp MHR) that includes the enthalpy term and classify it as endothermic of exothermic. 9. List the assumptions associated with simple calorimetry. 1) 2) 3) 4) Conclusion The accepted ΔHvalues for NH 4 Cl is kj/mol and for NaOH is kj/mol. Compare your experimental values to these and account for differences be referring to specific systematic or random errors that may have been committed.

9 Chemistry 3202 Lab 5 Enthalpy of Solution Formation Page 9 of 9 Extension 1. Using this data, calculate the molar enthalpy of solution formation for sodium perchlorate. Calorimeter Water NaClO 4 Mass g 2.00 g Initial Temperature C Final Temperature C Specific Heat Capacity J/(g C)