Chemistry Lab I: ThermochemistryPrepared by: Dr. Dharshi BopegederaIntroduction to Natural Science (2006/07) Spring 2007 Quarter Chemistry Lab I: Thermochemistry Prepared by: Dr. Dharshi Bopegedera2Experiment I: Determination of the enthalpy of neutralization of and acid-base reaction We will use constant pressure calorimetry to determine the enthalpy change for the reaction between hydrochloric acid and sodium hydroxide. The calorimeter we will use is a homemade device made of Styrofoam coffee cups. We will compare the experimental value for the enthalpy of neutralization obtained in the lab to theoretical values calculated using standard tables. Pre-Lab Assignment: 1. In an acid-base neutralization reaction 50.00 ml of 2.00M HCl was mixed with 50.00 ml of 2.00M NaOH. Calculate the number of moles of HCl (aq) and NaOH (aq) in the above samples. Also calculate the number of moles of each of the products formed by the above reaction. 2. Is there a limiting reagent for the above reaction? If so, which one? Which one is the excess reagent? Show all work. 3. Using ∆Hf° values, calculate the enthalpy of neutralization for the reaction between 1 mole of HCl and 1 mol of NaOH. Refer to standard tables in your chemistry text to obtain ∆Hf° values. Show all work. 4. Is this reaction endothermic or exothermic? Why? 5. Draw a diagram of a “coffee cup calorimeter”. Refer to your chemistry text if you need help. Lab Assignment: Do this lab in pairs. Obtain following supplies. • Three Styrofoam coffee cups • Magnetic stir bar • Magnetic stirrer • Thermometer set up connected to Vernier software 1. To a clean beaker, obtain 50.00 ml of 2.00M HCl. Weigh the beaker (analytical balance) after the HCl is placed in it and record the weight. Label the beaker. 2. To a second clean beaker obtain 50.00 ml of 2.00M NaOH. Weigh the beaker (analytical balance) after the NaOH is placed in it and record the weight. Label the beaker. 3. Nest one Styrofoam cup in another and pour the HCl into a Styrofoam coffee cup. Do not clean or discard the beaker that contained HCl. Use the third Styrofoam cup to make a cover for the cups containing the acid (this will be shown in lab). You also need to make a hole in the cover to insert the thermometer (as shown in lab). Now you have a “coffee cup calorimeter”. Put the bar magnet into this calorimeter. Start recording the temperature of the contents in the coffee cup calorimeter using the data recording system as shown in lab. Record the temperature every 15 seconds.34. After about 2 minutes, while continuing stirring and recording the temperature, pour all of the NaOH into the coffee cup calorimeter quickly and close the cover. Do not discard or clean the beaker containing NaOH. 5. Continue to record the temperature every 15 seconds. You will notice that upon the addition of NaOH the temperature rises quickly and then begins to slowly taper off. When the temperature tends to go down again (a slow process), record for another 2 minutes and stop. 6. Weigh the empty HCl and NaOH beakers using the analytical balance. Record this data. Post lab Assignment: 1. Calculate the weights of HCl and NaOH used in this experiment. 2. Without doing calculations, determine if this is an exothermic reaction or an endothermic reaction. State the reasons for your decision. 3. Using the temperature curve, determine ∆T for this reaction (instructions will be given in class). Report in units of Kelvin. 4. The products of the reaction are NaCl and water. Since there is very little NaCl, we can assume that the entire product is “water”. Calculate the heat absorbed by “water” when the reaction was completed. In order to do this, you need to know the mass of “water” [take this as the mass of HCl (aq) + the mass of NaOH (aq)] and the specific heat of water (look up in standard tables or your chemistry text book). 5. Use the above information to calculate the enthalpy change for the reaction between HCl and NaOH. 6. Because the base neutralizes the acid, the enthalpy change for this reaction is called the “enthalpy of neutralization”. It is commonly given the symbol ∆Hneutralization. Now calculate the enthalpy of neutralization for one mole of HCl reacting with one mole of NaOH. 7. Compare the value you obtained above, to the theoretical value you calculated in the pre-lab assignment. Calculate the percentage error. Show all work. 100% valueltheoretica valuecal theoreti- valuealexperiment error Percentage⎥⎦⎤⎢⎣⎡= 8. Give reasons for the above error.4Experiment II: Determination of the calorie content in nuts using bomb calorimetry Bomb calorimetry (constant volume calorimetry) is often used to determine the calorie content in food products. In this experiment, we will use this technique to determine the calorie content of nuts. Pre-Lab Assignment: Draw a block diagram of a bomb calorimeter. Refer to your chemistry text for help. Label the important components clearly. Lab Assignment: Do this lab in pairs. You will be given instructions on how to operate the bomb calorimeter. 1. Select a nut (use forceps only to handle it) and weigh it using the analytical balance. 2. Measure out about 10 cm of the wire provided for the bomb calorimeter. Record the exact length of the wire. 3. Place the nut in the bomb calorimeter as directed in lab. 4. Place 1.00 ml of distilled water into the bomb as directed in lab. 5. Place 2000.0g of distilled water into the outer chamber of the bomb calorimeter as directed in lab. 6. Record the initial temperature of water as directed in lab. Continue recording the temperature for about 2 minutes. 7. Ignite the nut inside the calorimeter while continuing to record the temperature. 8. You will note that the temperature will increase and then begin to taper off. Continue recording the temperature for another 2 minutes. Save and print the temperature data (raw data as well as the temperature vs. time graph). 9. Disassemble the bomb calorimeter. Remove any left over wire and measure the unburned length of the wire carefully. Record this in your lab notebook. 10. Read the nutrition label on the nut package carefully. Write down the calorie content of nuts as reported on the label. Write down the brand name on the package. 11. Collect data from students who did this experiment with other nuts. Be sure to read the nutrition label on the