Enthalpy of Formation of Ammonium Chloride Version 6.2.5 Michael J. Vitarelli Jr. Department of Chemistry and Chemical Biology Rutgers University, 60 Taylor Road, Piscataway, NJ 08854 I. INTRODUCTION Enthalpy of formation, also known as heat of formation, Why? Why do? Define heat. The change in internal energy of a system is equal to the sum of the heat gained or lost by the system and the work done on or by the system: U = q + w () Even though heat, q and work, w depend on the path the system takes to get from its initial point to its final point the change in internal energy does not depend on this path. The change in internal energy of a system depends on the final and initial states of the system, not on how the system arrives there. If we consider only PV work (maybe remove), then the work associated with a gas expanding against a constant external pressure is: Thus the change in internal energy at constant pressure is: w = P V (2) U = q p P V (3) where q p is the heat transferred at constant external pressure, usually atmospheric pressure, on the system. Let s rearrange Equation 3 and solve for q p : q p = U + P V (4) Many chemical reactions take place in containers open to the atmosphere with a constant external atmospheric pressure. Next, we define Enthalpy as sum of the internal energy and the pressure volume product: H = U + PV (5) This seems somewhat abstract. However, consider the change in enthalpy of a system at constant pressure: H = U + P V (6) Notice the right side of equation 6 is equivalent to the right side of equation 4. Thus the change in enthalpy is the heat transfer to or from a system at constant external pressure: H = q p (7) Like internal energy, the enthalpy of a system depends on the current state of the system, and not how it got there. Since the enthalpy of two states of a system have fixed values, the change in enthalpy of a system has a fixed value. Hess s Law states that the individual changes in enthalpy of steps in a reaction can be added together to find the overall change in enthalpy of the reaction. Thus, the net heat transferred in a reaction (at constant external pressure) is the same no matter if the reaction is performed in one or multiple steps. Since there exists a standard set of reactions we can use the additive property of H to generate other reactions. These standard set of reactions are known as formation reactions. The enthalpies associated with these reactions are known as standard enthalpies of formation, this is when a product is formed from its constituent elements in their standard state. An example is the formation of nitrogen dioxide: 2 N 2(g) + O 2 (g) NO 2 (g) H f = 33.8 kj/mol (8)
If we reverse the direction of a reaction we reverse the sign of change in enthalpy. Keeping this and Hess s law in mind we find: H = H f H f (9) Products Reactants If this is confusing, consider climbing a mountain. The change in elevation as your hike up the mountain is equal to the difference between the elevation of where you stopped and where you began. Lastly, consider the change in enthalpy of converting graphite to diamond. This could end up being particularly profitable. Separately, the combustion of graphite: and the combustion of diamond: C(s, graphite) + O 2 (g) CO 2 (g) H = 393.5 kj/mol (0) C(s, diamond) + O 2 (g) CO 2 (g) H = 395.4 kj/mol () each have their own change in enthalpies. However, by reversing Equation and adding it to Equation 0 we find: C(s, graphite) C(s, diamond) H = 393.5 kj/mol ( 395.4 kj/mol) =.9 kj/mol (2) Which is our desired result. Graphite is the reference form of carbon and thus has a H f =0, this is analogous to the elevation above sea level. If we are at sea level then our elevation is zero. There is no absolute elevation, thus we have to pick a reference point. When graphite is converted to diamond, we find an increased in enthalpy of.9 kj/mol. If this is true, then what is the enthalpy of formation of diamond? (See pre-lab question.) In this experiment the student will be finding the enthalpy change of a chemical reaction, Equation 3, that cannot be easily measured. We will perform two simpler reactions and find the H of those reactions. In conjunction with this data, along with known standard enthalpies of formations we will able to find the H of the our reaction by using Hess s law. The reaction presented in Equation 3 is our primary reaction of interest. 2 N 2(g) + 2H 2 (g) + 2 Cl 2(g) NH 4 Cl(s) H f =? (3) During the lab you will experimentally determine the H for the reactions presented in Equations 4 and 5. NH 3 (aq) + HCl(aq) NH 4 Cl(aq) H =? (4) 2 NH 4 Cl(aq) NH 4 Cl(s) H =? (5) You will also use known changes in enthalpies (at 25 C and atm) for Equations 3 and 4. 2 H 2(g) + 2 Cl 2(g) HCl(aq) H f = 67.2 kj/mol (6) 2 N 2(g) + 3 2 H 2(g) NH 3 (aq) H f = 80.29 kj/mol (7) Notice if we add Equations 4,5,6 and 7 together we arrive at Equation 3. However, in the second half of your experiment you will be finding the enthalpy change when converting solid ammonium chloride to aqueous ammonium chloride. Thus for your calculations you will use the negative of this experimentally determined change in enthalpy. To determine these changes in enthalpy we first need to measure the heat q released or absorbed by each reaction. The heat exchanged in a process can be calculated by multiplying the mass m [grams] of the substance, by the specific heat c [J/(g C)] of a substance, by the change in temperature T [ C]: q = mc T (8) Equation 8 shows the heat gained or lost by a medium in the ideal case. Unfortunately, some of this heat will be lost though the walls and lid of our cup. To correct for this we introduce a calorimeter constant: C c =5 J/K. Thus our corrected equation becomes: q = (mc + C c ) T (9)
The magnitude of the heat gained, or lost, by the solution is equal to but opposite in sign to the magnitude of the heat lost by the reaction: q rxn = q sln (20) Lastly, the change in enthalpy of the reaction, H rxn, can be calculated by dividing the heat lost, or gained, by the reaction by the number of moles of NH 4 Cl formed: 3 H rxn = q rxn moles NH 4 Cl (2) II. PRE-LAB QUESTIONS. (2 Points) Given the following reaction: H 2 (g)+i 2 (s) 2HI(g) with a H of 52.9 kj. What is the change in enthalpy for the following reaction: HI(g) 2 H 2(g)+ 2 I 2(s)? Express your answer in kj. 2. (2 Points) How much heat is released during the formation of 3.8 mol HCl(g) in the following reaction: H 2 (g)+cl 2 (g) 2HCl(g) with a H of -84.6 kj. Express your answer in kj. 3. (2 Points) What will be the final temperature if a 4.00 g silver ring at 4.0 C if it gives off 8.0 J of heat to the surroundings. The specific heat capacity of silver is 0.235 J/(g C) Express your answer in C. 4. (2 Points) Let s say you hold some ice in your hand. The ice melts and your hand feels cold. Is the reaction H 2 O(s) H 2 O(l) endothermic or exothermic? Is the change in enthalpy of the reaction positive or negative. A) (endothermic, positive) B) (endothermic, negative) C) (exothermic, positive) D) (exothermic, negative) 5. (2 Points) B 5 H 9 (l) is a colorless liquid that will explode when exposed to oxygen. How much heat is released when 0.2 mol of B 5 H 9 reacts with excess oxygen where the products are B 2 O 3 (s) and H 2 O(l). The standard enthalpy of formation of B 5 H 9 (l) is 73.2 kj/mol, the standard enthalpy of formation of B 2 O 3 (s) is -272 kj/mol and that of H 2 O(l) is -285.4 kj/mol. Express your answer in kj. 6.(2 points) When 0.05 mol propane, C 3 H 8 is burned in an excess of oxygen, how many moles of oxygen are consumed? The products are carbon dioxide and water. You have to properly balance the equation before you can answer this question. A) 0.420 mol O 2 B) 0.525 mol O 2 C) 0.875 mol O 2 D) 0.905 mol O 2 E) 0.975 mol O 2 7.(2 points) The enthalpy of solution is defined as H soln = H solute + H solvent + H mix. Each of the terms on the right side of the equation are either endothermic or exothermic. Which answer properly depicts this. A) (Endothermic, Endothermic, Endothermic) B) (Endothermic, Endothermic, Exothermic) C) (Endothermic, Exothermic, Endothermic) D) (Exothermic, Endothermic, Endothermic) E) (Exothermic, Exothermic, Endothermic) 8. (2 points) Find the H rxn for the reaction: 3C(s)+4H 2 (g) C 3 H 8 (g)
4 Using these reactions with known H s: C 3 H 8 (g) + 5O 2 (g) 3CO 2 (g) + 4H 2 O(g) H = 2043 kj C(s) + O 2 (g) CO 2 (g) H = 393.5 kj 2H 2 (g) + O 2 (g) 2H 2 O(g) H = 483.6 kj Express your answer in kj. 9. (2 points) A 2.0 g sample of a metal is heated to 90.0 C. It is then dropped into 25.0 g of water. The temperature of the water rises from 22.5 to 25.0 C. The specific heat of water is 4.8 Jg C. Calculated the specific heat of the metal. Express your answer in Jg C. 0. (2 points) What is the process by which a solute forms a solution in a solvent. A) calisthenics B) bumping C) dissolution D) neutralization E) delineation. (2 points) The enthalpy of combustion of ethane gas, C 2 H 6 (g), is about -.5*0 3 kj/mol. When ethane reacts with O 2 (g), the products are carbon dioxide CO 2 (g) and water H 2 O(l). How much heat is released during the combustion of ethane gas when 4 mols of O 2 (g) are consumed. Express your answer in kj. 2. (2 Points) A metal initially at 450. C and mass 2.00 g is dropped into a liquid with specific heat capacity 4.00 Jg C, mass 0.0 g and initial temperature 40.0 C. The final temperature of the liquid is 50.0 C. What is the specific heat capacity of the metal. Express your answer in Jg C. III. PROCEDURE Safety P recautions: Always wear goggles in this lab and in every lab. Do NOT clamp the mercury thermometer to the ring stand. The thermometer is fragile and can break if stressed. If you do manage to break the thermometer tell your TA immediately. You will lose points, but will not fail the lab. Note: you will be given two nested Polystyrene cups. They are glued together for added insulation; don t separate them. There are two halves of this experiment. During the first half (Part A), you will be finding the change in enthalpy for Equation 4. While in the second half (Part B) of the experiment you will find the change in enthalpy for the reverse of Equation 5..) Insert your thermometer into the lid of the cup. The rubber ring will support the thermometer. Place it such that it does not touch the bottom of the cup. Again, do not clamp the thermometer to the ring stand. 2.) Using a graduated cylinder, add 00.0 ml of distilled water to your cup. Then add 50.0 ml of 2.00 M HCl. 3.) Measure the temperature of your solution every minute for five minutes. Record these values in your data table. 4.) Using a graduated cylinder, add 50.0 ml of 2.00 M NH 3 to your cup. Swirl the solution gently to mix the solutions. 5.) Measure the temperature of your solution every minute for 0 minutes. Record these values in your data table.
5 Part A TABLE I: Data Table Part B Time [min] Temp [ C] Time [min] Temp [ C] 2 2 3 3 4 4 5 5 6 6 7 7 8 8 9 9 0 0 2 2 3 3 4 4 5 5 6.) Calculate the concentration of ammonium chloride formed in step 4. Then calculate the mass of ammonium chloride required for the same concentration however, in a 00.0 ml distilled water. 7.) Rinse and dry your Polystyrene cup. Using a graduated cylinder add 00 ml distilled water to the cup. Record the temperature of the water every minute for five minutes. Then add your mass of ammonium chloride. Swirl the solution gently to mix the solution. Record the temperature of your solution every minute for 0 minutes. IV. POST-LAB QUESTIONS.) Using your experimental data, what is the change in enthalpy for part A of this experiment. That is the enthalpy of neutralization for equation 4. Express your answer in kj/mol. 2.) Using your experimental data, what is the change in enthalpy for part B of this experiment. That is the negative of the change in enthalpy found in Equation 5. Express your answer in kj/mol. 3.) You now have all you need to find the enthalpy of formation of ammonium chloride! Using the above results along with the values given in equations 6 and 7, what is your experimentally determined enthalpy of formation of ammonium chloride? Express your answer in kj/mol. 4.) Look up the accepted valued for the enthalpy of formation of ammonium chloride. Calculated the percent different between your experimentally determined value and the accepted value. 5.) For a fixed number of moles of the reactants found in Equation 4, if you increase the amount of the surrounding water would the change in temperature increase or decrease? Also, how would q rxn be affected? Explain your answer. 6.) While on the international space station, the space shuttle, or any other space going vehicle, human waste (urine) is often expelled to the void of space. If you look out the window during this time you would see a cloud of solid crystals, not the liquid as you would expect. The liquid has rapidly changed to a solid form. Explain what is happening during this process. Include in your answer if this is an endothermic or exothermic process, the sign of H, and the direction of heat flow for this process. Any other comments would be encouraged.
6 V. CHEMICAL HAZARD AWARENESS FORM This form is to be turned in prior to beginning your experiment Name: Section: RUID: Experiment: List below any chemicals used in this experiment. Also list their hazards and any handling precautions for these chemicals.