Enthalpy Changes in Simple Processes free essay sample

The primary objective of this lab is to be able to determine the specific heat of a reaction by using a calorimeter. A calorimeter is a device used to determine the specific heat of chemical reaction or a physical change. The specific heat a reactions is used to refer to the amount of heat that is lost or gained when one gram of a particular substance increases or decreases by one degree Celsius. When a chemical reaction occurs in an open container most of the energy gained or lost is in the form of heat. Almost no work is done (i. e. nothing is being moved). Heat flows between the system and surroundings until the two are at the same temperature, when a chemical reaction occurs in which the system absorbs heat, the process is endothermic (it feels cold). When a chemical reaction occurs in which the system produces heat it is exothermic (it feels hot) Under conditions of constant pressure the heat absorbed or released is termed enthalpy (or heat content). We do not measure enthalpy directly, rather we are concerned about the heat added or lost by the system, which is the change in enthalpy (or ? H The quantity of heat gained or lost by a system, ? H, is dependent upon, the mass, m, of the system: the more massive an object the more heat needed to raise its temperature, the change in temperature, (? T): the larger the temperature change in a system the more heat exchanged, and the nature of the substance(s) making up the system. The last quantity is defined by the heat capacity of the system. For a given substance, the specific heat capacity is defined as the quantity of heat needed to raise 1 gram of the substance by 1 degree Celcius. Specific heat capacity has units of joules per degree Celcius per gram, J. g-1.? C-1. The three quantities combine to give the quantity of heat gained, or lost, by a system: ? H = mass x Specific heat capacity x ? T ?T represents the difference between the enthalpy of the system at the beginning of the reaction compared to what it is at the end of the reaction: ? T = Tfinal Tinitial We are considering the enthalpic state of the system. If the temperature has higher enthalpy at the end of the reaction then the initial, then it absorbed heat from the surroundings (endothermic reaction). If the temperature has a lower enthalpy at the end of the reaction then the nitial, then it gave off heat during the reaction (exothermic reaction) I. Determining the specific heat capacity of a metal Fill a 1 L beaker with 900mL of water and place on a hot plate and bring to a boil. Then weigh out 31. 00-34. 00g of Mossy Tin and measure to the nearest 0. 01 g. Put the mossy tin in a large test tube, and insert a thermometer into the center of the tin. When water is boiling, place the t est tube in the boiling water clamping it so that the test tube is not touching the beaker and make the sure water level outside the test tube is higher than the top of the metal inside the test tube. Let the test tube of tin stay in the boiling water for 15 minutes. While waiting for the tin to be heated get the mass of a Styrofoam cup and pour 50 ml of water into it. Then measure the mass of water in the cup by assuming the density of water is 1. 0 g/ml. When the tin has been in the boiling water for 15 minutes, record the temperature of the tin, and using a separate thermometer record the temperature of the water in the cup, the temperature will be the initial temperatures. Quickly pour the hot metal into the water in the styrofoam cup. Stir the mixture, then observe the rise in temperature of the water in the cup. When the temperature stops rising, record the temperature and those are the final temperature of both the water and the tin. Then with the givin infromation determine the specific heat capacity of tin. II. Determining the Enthalpy of solution of a salt. Get the mass of a Styrofoam cup and pour 30 ml of water into it. Then measure the mass of water in the cup by assuming the density of water is 1. 0 g/ml, this will be the calorimeter. The measure out between 1. 5 to 2. 0 grams ammonium nitrate, NH? NO? , to the nearest 0. 01 g. Before adding the ammonium nitrate, measure the temperature of the water in the coffee cup and record the temperature, then add the ammonium nitrate to the water and stir gently with the thermometer. Watch the change in temperature and when the temperature is done change record this temperature as the final temperature. Dispose down sink. III Enthalpy of Neutralization of a Strong Acid with a Strong Base Pour 20 ml of 1. 0 M HCl into a Styrofoam cup and then add 5 drops of phenolphthalein indicator. Pour 20. 0 ml of 1. 1 M NaOH into a small beaker. Make sure the temperature of the two solutions are equal. If they are not, put the beaker of NaOH in hot or cold water to raise or lower its temperature. These are the initial temperatures. Mix the NaOH solution into the HCl solution, pouring the NaOH into the HCl in the cup. Stir gently with the thermometer, and watch the temperature, when is stops changing this is the final temperature, record. Dispose down sink. IV. Enthalpy of Neutralization of a Weak Acid and a Weak Base Take a Styrofoam coffee cup and pour 20 ml of 1. 0 M HC? H? O? into it. Add 5 drops of phenolphthalein indicator allowing visual verification of neutralization of the acid to be done. Then put 20. 0 ml of 1. 1 M NH? OH into a small beaker. Make sure the temperature of the two solutions are equal. If they are not, put the beaker of NH? OH in hot or cold water to raise or lower its temperature. These are the initial temperature of each solution. Mix the NH? OH solution into the HC? H? O? solution, pouring the NH? OH into the HC? H? O? in the cup. Stir gently with the thermometer, and watch the temperature, when is stops changing this is the final temperature, record. Dispose down sink. V. Enthalpy of Solution of a Gas: Hess’s Law Under the fume hood for 100 ml of concentrated ammonia solution into the gas generating bottle. Measure out 100 ml of 1. 00 M hydrochloric acid using a graduated cylinder to, and then pour the hydrochloric acid into a large Styrofoam cup then add 2 – 3 drops of methyl red indicator to the acid. Assemble the apparatus as shown in the lab manual. Make the correct connections to the bottle of ammonia. The compressed air must be connected to the tube that is submerged in the ammonia solution. Record the initial temperatures of the hydrochloric acid, and the ammonia solution. Turning on the air slowly, bubble compressed air at a reasonable rate through the ammonia solution. The compressed air containing ammonia gas is bubbled through the hydrochloric acid. When the methyl red indicator in the acid changes color, turn off the compressed air,and record the temperature of the ammonia solution and the neutralized hydrochloric acid solution. Dispose of solutions properly. Data, Results, and Calculations Data, results, and calculations can be found on the data sheets attached to the lab report, along with a graph of the class data verifying the Law of DuLong and Petit. Discussion Determining the specific heat capacity of a metal- For the first procedure we were assigned mossy 10, the initial mass of the water in the Styrofoam cup was 52. 689g, the mass of the metal 34. 00g, the final temp of the water and metal was 26? C, the initial temp of the water was 23. 5 ? C, The initial temp of the metal was 93. 5? C, ? T Water 2. 5? C, and ? T 67. 5? C, There are a few things that I would say surprised me in this experiment first one being that the initial temp of the metal was 93. 5 ? C. Usually this is a result of a extremely high temperature metal that would have already been heated, however, after checking my equipment I found that the temperature on this metal was correct. After doing the calculations for this experiment to determine the Law of Dulong and Petit, the Specific heat of the metal was 0. 240? C and I found out how to get this by calculating the atomic mass X specific heat + 24. 9. mol^-1, C^-1. Another interesting aspect to this experiment is that it created an endothermic reaction and heated up the water. Determining the Enthalpy of solution of salt- The first step in the process is determining the mass of the water in the cup which was 29. 55g, then finding he final temp of the water +NH? NO? which was 20? C, finding the initial temp of the water which was 24. 5? C then the ? T of the water which was -4. 5? C, once again there were a few things I found interesting about this lab. First was how to calculate the dissolution of ammonium nitrate, and how this process varies from the first one. In the end of the lab the final result was that the ? H Dissolution of NH4NO4 = 555. 84J. This also means that this created an exothermic reaction, which ultimately cooled the water, and this was something that I also was not expecting once mixing water and NH4NO3 together. Enthalpy of Neutralization of a strong acid with a strong base- Starting with the total mass which was 40. 240g, final temp of 30. 0? C and the initial temp of both solutions of 23. 5? C and the final ? T of the water being 6. 5? C, There are a few things that I did not expect from this part of the lab, first was the color change of the phenolphatalein indicator that changed to a bright pink color from a clear once the two chemicals were mixed together. Another thing that interested me was how we determined the neutralization of these chemicals, I would have figured PH paper to determine if the two chemicals equaled out, however, the need for math exists to determine how they balanced regardless of the PH paper. In this case as well it seemed from the paper that the two chemicals should have split the H and O molecules in order to make H2O and leave Cl and Na separate, however, it was difficult to determine if this actually happened just by looking at the mixture of chemicals. Fortunately with the mathematics and the formulas given to us it was rather interesting to find out that they did indeed neutralize resulting in ? H neutralization= -1093. 32 J and ? H neutralization per mole of H? O formed of -54. 66 kJ/mol. Enthalpy of neutralizing of a weak acid and a weak base- In this part of the lab I don’t think that the ending was as surprising as the last few were, or perhaps it is because they were not as exciting as the other ones were. None the less, in this particular lab we observed a few different things that happened which helped us determine that the difference between strong acids and strong bases were not all that different. In this one the total mass of the solution was 38. 13g, final temp was 28. 6? C and the initial temp was 23. 0? C and the ? T of the water was 5. 6? C once again we find that an exothermic reaction occurred cooling the temperature of the solution. Another thing that was added to the solutions which enabled us to see what was happening was the phenolphyhalein indicator, and in this experiment it changed to a light pink color during the change in temperature. Over all it was interesting to see what happened, however, because of how the weak base and weak acid sounded it did not make this experiment seem as interesting as the strong base and strong acid sounded. Enthalpy of solution of a gas: Hess’s Law- Just by looking at this experiment I figured that we would get some kind of off gassing when the two chemicals mixed or merged together. This experiment took several times to do over again because we acquired the incorrect ammonia the first two times, not producing the proper results in the end. Of course with this experiment having a few steps it also made it slightly complicated to get the end result, however, it finally worked out in the end. The mass of the solution was 87. 321g, the final temp of the solution in the bottle was 12. 1? C the initial temp of the solution in the bottle was 18. 2? C and the ? T solution in the bottle was -6. 1? C. Over all the molar ? H was 33kJ/mol which helped us determine the rest of the calculations on down the line. Moreover, it was interesting to see how many steps it took to complete this part of the lab as well as the time involved to see it all the way to the conclusion of the lab. Error analysis In this lab it was rather difficult to miss some of the steps, however, there was one particular lab that we did which gave us trouble, experiment V in which we were to use Ammonia in order to notice change in the cup with the hydrochloric acid can give you different results if the incorrect ammonia is used in the process. I know this first hand because we used the incorrect ammonia in this lab, resulting in us having to do this part over a few times. Other inconsistencies may exist with the lab paperwork resulting in the incorrect calculations, or a few of the numbers being slightly off, however, it is difficult to tell how far I am off on the calculations without having the answers present. After I see the results for the lab, I will better know how much more or little there is in the way of any further error analysis. Another error would be in the graph due to classmate getting incorrect number or not enough for their data. The graphs slope is off due to class data not being close enough to the specific heat of their metal. The slope was suppose to be 24. 9 and I got a slope of 18. 35 from the class data.