DETERMINING AND USING H INTRODUCTION CHANGES IN CHEMISTRY Chemistry is the science that studies matter and the changes it undergoes. Changes are divided into two categories: physical and chemical. During a physical change, some physical property of the substance changes, although the identity of the substance remains the same. For example, the melting of ice to make liquid water is a physical change: H 2 O(s) H 2 O(l) However, when a piece of sodium metal is placed in water, a chemical change occurs. The element sodium reacts with water to make the compound sodium hydroxide and the element hydrogen: 2 Na(s) + 2 H 2 O(l) 2 NaOH (aq) + H 2 (g) When the process is complete, the original substances no longer exist. New substances, with new properties have been made. INTERNAL ENERGY Every change, whether physical or chemical, is accompanied by a change in the internal energy (E) of the system. In the case of melting ice, the molecules absorb heat from the surroundings. Their internal energy increases as they liquefy. In the reaction between sodium and water, the internal energy of these two substances decreases as sodium hydroxide and hydrogen are produced. The change in internal energy is symbolized by E. The first law of thermodynamics defines the change in internal energy as E = q + w, where q refers to the heat exchanged between reaction system and surroundings, and w refers to the work done by or on the reaction system. A sign convention here is important. With the reaction system as the point of reference, positive signs are used for q and w if the system absorbs heat or is worked upon. Negative signs are used if the reaction system loses heat or does work. MEASURING E AND H To measure E values, a sealed bomb calorimeter is required. In such a calorimeter, no work can be done. Therefore, w = 0 and E = q. This q is often labelled q V since the reaction is carried out at constant volume. In lab, simple calorimeters are usually used. They are open to the atmosphere. The reaction system can exchange both heat and work with the surroundings. However, the work component is difficult to measure. Furthermore, it is usually small compared to the heat component. For that reason, we ignore the work component and measure just the heat component. Since the atmospheric pressure is essentially constant for the duration of the reaction, we label the heat as q P. This value, q p, is not equal to E (since E = q + w, and w is not 0 in this case). The q P term has been named H and symbolizes the change in enthalpy. Enthalpy is similar to, but not the same as, internal energy. It is often called the "heat content" of a substance.
MEASURING HEAT Heat is an energy transfer that results because of a temperature difference between two things. Since it is a form of energy, heat has energy units, either Joules (J) or calories (cal). Heat flows from hot to cold. The energy transfer stops when the objects have reached the same temperature. The amount of heat lost equals the amount of heat gained. To calculate heat, we use the q equation: q = (m)(c s )( T) where m = mass, C s = specific heat (a unique constant for each substance) and T = change in temperature, defined as T FINAL T INITIAL. If T FINAL is higher than T INITIAL, then T is positive and q is positive. An object with a positive q has gained heat during the energy transfer. If T FINAL is lower than T INITIAL for an object, then its T is negative, causing its q to be negative. A negative q value for an object signifies that it has lost heat during the energy transfer. Heat exchanges between system and surroundings are measured in special containers called calorimeters. They may be as sophisticated as a bomb calorimeter, or simple as a Styrofoam coffee cup or thermos. The system is whatever substances undergo the change. The surroundings are all the other contents of the container as well as the container itself. If the system loses heat, all of it goes to the surroundings. Therefore, the ability of the container to absorb heat must first be established. Often, as in this experiment, a calorimeter constant, C CAL is determined. The value of C CAL equals the m(c s.) portion of the heat equation. Therefore, the q equation can be written as follows, when it is applied to a calorimeter: q = C CAL T H INFORMATION AND EQUATIONS Once H data has been determined for a reaction, it is often written at the end of the balanced equation. Its value corresponds to the balanced equation when interpreted in terms of moles. For example: H 2 O(s) H 2 O(l) H = +6.0 kj 2 Na(s) + 2 H 2 O(l) 2 NaOH(aq) + H 2 (g) H = 367 kj In the first case, when one mole of solid water (ice) undergoes melting, 6.0 kj of heat are absorbed by the water molecules. In the second case, when two moles of solid sodium react with two moles of liquid water, two moles of aqueous sodium hydroxide and one mole of gaseous hydrogen are produced, and 367 kj of heat are given off to the surroundings. Note that there is no such thing as positive or negative heat. The positive and negative signs simply signify the direction of heat flow, into or out of the system. HESS'S LAW Sometimes a chemical product is made in one step; in other cases, several steps may be required to make the desired product. For example, substance "E" can be made as follows: (Step 1) A + B C (Step 2) C + D E The net reaction can be found by adding the two steps: (Net) A + B + D E The C's in steps 1 and 2 cancel. The value of H for the net reaction can be found by adding the
H values for each step. If the values are: (Step 1) H = +100. kj (Step 2) H = 300. kj then the value of H for the net reaction is 200. kj. PURPOSE OF THIS LAB In this lab, we will determine the value of H for two processes. One is a chemical change, the other is a physical change. Then, we will use this lab data as well as additional data for two other reactions to determine H for a fifth reaction by means of Hess's law. PROCEDURE A. Determination of calorimeter constant (C CAL ) 1. Place 50.0 ml of water at room temperature into a calorimeter, and record its temperature. For this and every other measurement, assume that the temperature of the calorimeter equals the temperature of its contents. 2. In a beaker on a hot plate, heat approximately 125 ml of water until its temperature is 70-80 o C. Do not let the thermometer touch the bottom of the beaker as you heat. 3. Carefully measure 50.0 ml of the hot water in a graduated cylinder and record its temperature. 4. Immediately add the hot water to the water in the calorimeter. Stir the water and measure its final temperature. 5. Empty the calorimeter and dry it with a paper towel. Then repeat steps 1, 3 and 4 for a second trial. When finished, dry the calorimeter before beginning part B. B. Determination of H for the reaction HCl(aq) + NH 3 (aq) NH 4 Cl(aq) 1. Measure out 50.0 ml of 2.00 M NH 3. Pour it into the calorimeter. Measure and record the temperature of the solution. 2. Measure out 50.0 ml of 2.00 M HCl in a clean, dry graduated cylinder. Measure and record its temperature. NOTE: The temperatures of the two solutions are probably almost the same. If they are not, use the average of the two as the initial temperature of the chemical system. 3. Add the HCl solution to the NH 3 solution in the calorimeter. Stir, and record the highest temperature reached. 4. Pour the contents of the calorimeter down the drain. Dry the calorimeter with a paper towel. When finished, dry the calorimeter before beginning part C.
C. Determination of H for the process NH 4 Cl(s) NH 4 Cl(aq) 5. On the balance, weigh out a 4.500-5.000 gram sample of solid ammonium chloride, NH 4 Cl. Record the actual mass you use. 6. Place 50.0 ml of room temperature water in the calorimeter. Measure and record its temperature. 7. Add the NH 4 Cl to the water. Stir and record the lowest temperature reached. 8. Pour the contents of the calorimeter down the drain. Dry the calorimeter with a paper towel.
MEASURING AND USING H Data Sheet Name Partner Be sure to use the correct number of significant figures for the volumes and temperatures you record below! A. Determination of calorimeter constant (C CAL ) BEFORE MIXING Trial 1 Trial 2 temperature of cold water (T INITIAL for cold) temperature of hot water (T INITIAL for hot) AFTER MIXING temperature of the water (T FINAL for water and calorimeter) B. Determination of H for the reaction HCl(aq) + NH 3 (aq) NH 4 Cl(aq) BEFORE MIXING volume of 2.00 M NH 3 temperature of 2.00 M NH 3 volume of 2.00 M HCl temperature of 2.00 M HCl average temperature (T INITIAL for solution and calorimeter) AFTER MIXING temperature of mixture (T FINAL for solution and calorimeter) ml ml C. Determination of H for the process NH 4 Cl(s) NH 4 Cl(aq) BEFORE MIXING Volume of H 2 O mass of NH 4 Cl(s) temperature of water (T INITIAL for solution and calorimeter) AFTER MIXING temperature of mixture (T FINAL for solution and calorimeter) ml g
MEASURING AND USING H Calculations Sheets Name Partner Show your work clearly, in the space provided, for each of the following calculations. Enter the answer on the line provided. The specific heat of water is 4.184 J/gC o. Assume this is also the specific heat of the dilute aqueous solutions we use here. Also, watch signs (+ or ) carefully and think about their meanings. A. Determination of calorimeter constant (C CAL ) Trial 1 Trial 2 1. mass of the cold water used g g 2. mass of the hot water used g g 3. T COLD C o C o 4. T HOT C o C o 5. q for the hot water J J 6. q for the cold water J J 7. q for the calorimeter J J Recall that heat lost = heat gained. You know how much heat was lost by the hot water. Did every Joule go to the cold water? If not, assume the rest went to the calorimeter. 8. T CAL C o C o 9. C CAL J/C o J/C o 10. Average C CAL J/C o Use this average in any calculation below which requires C CAL.
B. Determination of H for the reaction HCl(aq) + NH 3 (aq) NH 4 Cl(aq) 11. T SOLUTION C o 12. mass of solution g Its density is 1.03 g/ml 13. q for solution J q sol n = (m)(sp.ht.)( T) 14. q for calorimeter J q cal = C CAL x T CAL 15. q for surroundings J The q for the surroundings equals the q solution plus the q calorimeter. 16. q for reactant system J Watch your signs! 17. moles of NH 3 used mol Remember, you know the volume and the molarity. 18. moles of HCl used mol 19. Is there a limiting reactant? Explain: 20. H for one mole of limiting reactant J/mol 21. H (HINT: Look at units to do this conversion.) kj/mol
C. Determination of H for the process NH 4 Cl(s) NH 4 Cl(aq) 22. T SOLUTION C o 23. mass of solution (NH 4 Cl and water) g 24. q for the solution J q sol n = (m)(sp.ht.)( T) 25. q for the calorimeter J q cal = C CAL x T CAL 26. q for the surroundings J 27. q for the reactant system J 28. moles of NH 4 Cl used mol 29. H for one mole of NH 4 Cl J/mol 30. H kj/mol
31. Application of Hess's law to the lab data You have now determined the value of H for a chemical change and a physical change involving ammonium chloride. Use your lab data as well as the data given below to determine the value of H for the decomposition reaction: NH 4 Cl(s) NH 3 (g) + HCl(g) Consider the following as you make your determination. (1) HCl(aq) + NH 3 (aq) NH 4 Cl(aq) H = kj (2) NH 4 Cl(s) NH 4 Cl(aq) H = kj (3) NH 3 (g) NH 3 (aq) H = 34.6 kj (4) HCl(g) HCl(aq) H = 75.1 kj Show all manipulated equations as well as your calculations here.
DETERMINING AND USING H Pre-Lab Assignment Name 1. How much heat is lost when 75.0 g. copper (sp. ht. 0.385 J/g o C) at 57.2 o C are cooled to 11.7 o C? 2. In an experiment a student obtained 450 J for the q calorimeter and 1150 J for the q solution. a. Calculate the q surroundings. (Watch your significant figures.) b. Calculate the q reactant system. 3. Manganese metal can be obtained by reaction of manganese dioxide with aluminum. 4Al (s) + 3MnO 2(s) 2Al 2 O 3(s) + 3Mn (s) Use Hess s Law to calculate H for this reaction using the following data. Show all calculations. H (kj) a. 2Al (s) + 3 2 O 2(g) Al 2 O 3(s) -1676 b. Mn (s) + O 2(g) MnO 2(s) -521