Calorimetry and Hess s Law Prelab

Transcription

1 Calorimetry and Hess s Law Prelab Name Total /10 1. What is the purpose of this experiment? 2. Make a graph (using some kind of graphing computer software) of temperature vs. time for the following data: (Hint: Look at the example of the graph on page 8 to see what your graph should look like.) time (seconds) temperature ( o C) Using the information in the introduction of this experiment, calculate the heat (in J) of the reaction if 50.0 ml of HCl is added to 50.0 ml of NaOH in a coffee-cup calorimeter. The initial temperature for both solutions is 23.2 o C. At the end of the reaction after the data is graphed, the final temperature is determined to be 38.5 o C. 4. Using the enthalpy changes given below, calculate the enthalpy change for the following equation: N 2 (g) + 2 O 2 (g) 2 NO 2 (g) using the following equations: 2 NO (g) N 2 (g) + O 2 (g) H = kj NO 2 (g) NO (g) + 1/2 O 2 (g) H = kj 1

2 Calorimetry and Hess s Law You will measure the enthalpy change that occurs when two solutions are mixed together. You will also predict the enthalpy change for a reaction in which the enthalpies of two reactions determined in the laboratory today will be used to determine this third reaction. You will do this using Hess s law. The chemical reactions that will occur during this experiment are 2NaOH (aq) + H 2 SO 4 (aq) Na 2 SO 4 (aq) + 2 H 2 O (l) 2NH 3 (aq) + H 2 SO 4 (aq) (NH 4 ) 2 SO 4 (aq) (a) (b) 2NaOH (aq) + (NH 4 ) 2 SO 4 (aq) Na 2 SO 4 (aq) + 2NH 3 (aq) + 2H 2 O (l) (c) Special mention goes to Brian Stahl, John Witkowski, Leigh Bukowski, Nadia Szymanski, John Curtin and Nevin Gunduz for developing this laboratory experiment to be used here in the Behrend chemistry curriculum. Introduction There is a great interest in studying the change of a thermodynamic property during a physical or chemical change. The substance under study in which a change occurs is called the system. The surroundings are everything in the vicinity of the reaction. Heat is defined as the energy that flows into or out of a reaction because of a difference in temperature between the thermodynamic system and its surroundings. Energy (heat) flows between the reaction and surroundings to establish temperature equality or thermal equilibrium. Heat flows from a region of a region of higher temperature to one of lower temperature. Once the temperature becomes equal, heat flow stops. Heat is denoted by the symbol q. If q is a positive value, then heat is absorbed by the system and has a negative value if heat is evolved. Enthalpy is a property of a substance that can be used to obtain the heat absorbed or evolved in a chemical reaction. The flow of heat between the reaction and the surroundings are described in the following equation q (reaction) = - q (surroundings) (1) This equation states that heat lost or gained by the system is equal to the heat gained or lost by the surroundings. Consider a system in which a chemical reaction occurs. The heat of a reaction at a given temperature is the value of q required to return a system to the given temperature at the completion of the reaction. Chemical reactions are classified as exothermic or endothermic. An exothermic process is a chemical reaction or physical change in which heat is evolved and q is negative. An endothermic process is a chemical reaction or physical change in which heat is absorbed and q is positive. The enthalpy or reaction is the change in enthalpy for a reaction at a given temperature and pressure. This change in enthalpy is denoted as H. The relationship between heat and enthalpy is that the enthalpy of reaction equals the heat of reaction at constant pressure. 2

3 H = q p (2) It requires heat to raise the temperature of a given amount of substance, and the quantity of heat depends on the temperature change. The specific heat capacity of a substance (s) is the quantity of heat required to raise the temperature of one gram of a substance by one degree Celsius at constant pressure. Changing the temperature of the sample from an initial temperature t i to a final temperature t f requires heat equal to q = s x m x T (3) T is the change in temperature and equals T f - T i and the unit is in degrees Celsius. The mass of the substance in grams is m and s is the symbol for specific heat capacity with the units J/ g o C. The device that we use to measure the heat absorbed or evolved during a physical or chemical change is a calorimeter. The simplest apparatus consists of an insulated container, in this case, two polystyrene coffee cups with a thermometer and stirrer in it. The coffee-cup calorimeter is a constant-pressure calorimeter. The heat of the reaction is calculated from the temperature change caused by the reaction, and since this is a constant-pressure process, the heat can be directly related to the enthalpy change, H. Returning to equation 1, we can calculate the heat for our reaction. We must assume that the specific heat and the density of the final solution in the cup are those of water. By using the density of water, you can calculate the mass of the final solution. We will define the calorimeter as the surroundings. With all of this information, including the change in temperature recorded we can determine the heat for our reaction with the following equation q (reaction) = - q (surrounding) = - q (calorimeter) = - q (water) = - (s x m x T) (4) The enthalpy change of the reaction can be calculated by dividing q of the system by the number of moles of the reactant to obtain the units kj/mole. Once you have determined H for your reaction (a or b), you will obtain the enthalpy change from one of you classmates for the other reaction (a or b) which will be performed in the lab and add the two together to determine the enthalpy change of the resulting third reaction (c). This is possible through Hess s law of heat summation which states that if a chemical equation can be written as the sum of other equation, the H of this overall equation equals a similar sum of the H s for the other equations. For example, the reaction of graphite with molecular oxygen to produce carbon monoxide will be used. 2C (graphite) + O 2 (g) 2CO(g) (5) There are two reactions you can use together to determine H for equation 5 they are C (graphite) + O 2 (g) CO 2 (g) H = kj (6) 2CO(g) + O 2 (g) 2CO 2 (g) H = kj (7) 3

4 In order to determine H for equation 5, you must manipulate equations 6 and 7 so when you add them together the result will be equation 5 with the correct stoichiometry. In equation 5, CO is your product so you must reverse the reaction shown in equation 7. You must remember that when you reverse a reaction, the sign of H is changed as well. The result for equation 7 is 2CO 2 (g) 2CO(g) + O 2 (g) H = kj (8) You also have the correct stoichiometry for your product as well. In equation 5, we have two moles of carbon so we must multiply the coefficients in equation 6 by two. This also means that you multiply H by two. The result for equation 6 is 2C (graphite) + 2O 2 (g) 2CO 2 (g) H = 2 x ( kj) (9) You can now take equations 8 and 9 and add their H s together to obtain H for equation 5. You can cancel out the two moles of carbon dioxide and one mole of molecular oxygen just as you would in algebraic equations. 2C (graphite) + 2O 2 (g) 2CO 2 (g) H = 2 x ( kj) (9) 2CO 2 (g) 2CO(g) + O 2 (g) H = kj (8) 2C (graphite) + O 2 (g) 2CO(g) H = kj This is the same procedure you will use to determine H for equation c. Procedure Data that you should have written down for EACH trial are: Molarity of H 2 SO 4 Molarity and identity of base Volume of acid added Volume of base added Initial Temperature ( o C) Final temperature ( o C) (from graph) q(system) (look at eq 4) Moles of limiting reactant H (kj/mole) Mean H (kj/mole) (for each reaction) You should have sample calculations for just ONE trial shown in your laboratory notebook. 4

5 1. Obtain a coffee-cup calorimeter. Clean and dry your graduated cylinders. 2. Obtain exactly 50.0 ml of the 2.0 M solution of H 2 SO 4 into the clean, dry graduated cylinder. Obtain exactly 50.0 ml of the 2.0 M solution of NaOH in another clean, dry graduated cylinder. 3. Measure the temperature of each of these solutions with the thermometers given. If the temperatures are not identical, cool or warm the solution in your graduated cylinder (containing your base) by immersing the graduated cylinder in tap water or warm the cooler solution with your hands. The temperatures should finally agree within o C. Record the mean temperature because this is the initial temperature. 4. Place acid in the calorimeter. Place the temperature probe through the top of the calorimeter and place in the acid solution. 5. Go to the netbook and open up the Chem 111 folder. Select the file labeled Calorimetry and click on that file. You will then see a table and a graph on the screen. Click on the collect icon. 6. Add the base to the calorimeter, secure the top of the calorimeter and begin swirling. 7. You will immediately see points appearing on the graph. Keep swirling the cup gently until Logger Pro stops recording data. This will be after 240 seconds. 8. After 240 seconds you will see a plot that looks similar to the one shown below. 5

6 9. You need to determine the final temperature from this graph. You need to find the y- intercept. You need to left click and hold while moving across the area that has the points that are leveling out. It should look the way it does below. 10. Click on the linear fit icon. You will see a box filled with information appear and you will find the value of the y-intercept (b). This is your final temperature. Record that value. 6

7 11. You need to save this data on your flash drive. On the menu bar, click File, then Export As. Chose CSV (Excel, InspireData, etc). Save on your flash drive. 12. After you have saved the data on your flash drive, click on data located on the menu bar, then clear all data. Now you can start your second trial. 13. Repeat steps 2-12 one more time. 14. Calculate q for the reaction using J/g o C and 1.0 g/ml for the specific heat and density of the solution. 15. Calculate the enthalpy change from q of the reaction and the number of moles of the base. The base is your limiting reactant. 16. Obtain the mean value of the enthalpy change (kj/mole) from your two trials. 17. Repeat steps 2-12 two more times this time using NH 3 instead of NaOH. 18. Determine H for reaction c (question 1). Question 1. Use Hess s law and the measured mean H values for reactions a and b to calculate H for reaction C. 2NaOH (aq) + (NH 4 ) 2 SO 4 (aq) Na 2 SO 4 (aq) + 2NH 3 (aq) + 2H 2 O (l) 7

8 For your graphing assignment, you must construct a graph for one set of data (temperature and time) for each reaction that you studied. Your graphs are not to be drawn by hand. You must use a graphing computer software to construct these graphs. NOTE: Remember these tips when constructing your graph: 1. Decide which data goes on each axis. The independent variable is normally plotted on the x- axis and the dependent variable is plotted on the y-axis. This means that time is plotted on the x-axis and temperature is plotted on the y-axis. 2. Decide on the scales to be used on each axis. This is the most important step for producing a good graph. Use as much of the graph paper as possible to enable you to read the temperatures to the proper number of significant figures. This is the most important step for producing a good graph. NOTE: Your y-axis does not need to start at zero. Your x-axis will start at zero. 3. Label your axes. Your graph gives no information at all unless it is shown what the axes are representing and what units are being used for the data. 4. Give your graph a title. It doesn t have to be unique, just let your instructor know what this graph is representing. 8