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# Thermodynamics: Enthalpy of Neutralization/ Calorimetry Essay

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Introduction

The First Law of Thermodynamics states that energy can be neither created nor destroyed. This means that energy, instead of ‘disappearing’, is either transformed, transferred, dispersed, or dissipated. When energy is lost by a system, it will be acquired by the surroundings. Heat can be described as the amount of energy needed to cause the temperature of a substance to rise and it is transferred from warmer areas to cooler ones. In order to be able to measure the change in heat or enthalpy of a reaction, a colorimeter can be used.

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The calorimeter was first introduced in the 18th century and can be used with any procedure that involves the flow heat between a system and it’s surroundings (CACT). It is capable of measuring the heat created or exchanged after a reaction has occurred in a system with a constant pressure.

A calorimeter can be used to find the specific heat of a substance or even the heat of neutralization between a base and an acid. A basic calorimeter is composed of two Styrofoam cups (to provide insulation and prevent heat from entering or exiting the system), a lid covered with aluminum, a thermometer, and a stirrer. Since the calorimeter will still release heat, the first step is to find the heat capacity of it which is the heat absorbed by the calorimeter.

In order to find the heat capacity of a substance, the item must be weighed, heated, dropped into a calorimeter filled with cool water, and allowed to cool for a minute. During this whole process data will be collected including the weight of the item and the water, the temperature of the water and of the item; this information will form the base for the formula through which the specific heat of the item can be measured. In this experiment the specific heat for a unknown metal will be calculated. This specific heat will then be compared to the true value which suggests how accurate the calorimeter is.

Likewise, in order to find the amount of heat released by a chemical reaction that leads to neutralization, solutions of each the acid and the base are formed that have an equal molarity. These two are then mixed inside the calorimeter and stirred for two minutes and then the temperature of the solution taken. All the necessary data is collected and then the heat of neutralization is obtained. In this experiment the base was NaOH and the acid HCl. The experiment goes on to test the specific heat of NaCl in a similar manner as that of the metal cylinder.

Methods

To make the calorimeter, two Styrofoam cups are placed together. The lid is made out of a circular piece of cardboard that is as big as the mouth of the cups and covered with aluminum foil only on the top; the lid should also have a hole in the middle where the stirrer and thermometer can be introduced. See figure 1 for a picture of the set up.

To find the heat capacity the calorimeter is first weighed (note: calorimeter must also be weighed after any new substance is added). Next, 50 mL of distilled water are added inside and have the temperature is taken. Another 50 mL of water is placed on a heating plate until it reaches 65 Cï¿½, taken off, then its temperature is taken and allowed to rest until the temperature stops increasing (this temperature is recorded), and added into the calorimeter along with the cooler water. The lid is placed and the water is stirred for about a minute and afterwards the temperature of the water is taken again (during the one minute, the thermometer should be cooled in a cup filled with room temperature water). With this information the Heat Capacity can be calculated through the formula: (m ï¿½?Tï¿½s)hot = (m ï¿½?T ï¿½ s)cold + (Heat Capacity ï¿½ ?T).

Now with the heat Capacity of the calorimeter, the specific heat of the metal cylinder can be found. The calorimeter is weighed (again, the calorimeter must be weighed after any new substance is added), and 70 mL of distilled water are added (after it’s temperature is taken). The cylinder is placed in a beaker filled with enough water to cover it, placed on a heating plate until the temperature reaches 95Cï¿½, taken off, allowed to rest until the temperature stops rising (this temperature is also recorded), and placed into the calorimeter. The metal and water are stirred for a minute and then the temperature of the water with the metal is taken again and recorded. With this information, the following formula can be used to find the specific heat of the metal cylinder: (m ï¿½?Tï¿½s)metal = (m ï¿½?T ï¿½ s)water + (Heat Capacity ï¿½ ?T)calorimeter .

To find the heat of neutralization between NaOH and NaCl, first two 2 M, 50.0 mL solutions are made for NaOH and NaCl respectively. The temperatures of each of the solutions are taken and recorded. The same steps as before are followed including weighing the calorimeter before and after anything is added. The solutions are then poured into the calorimeter one at a time, covered, and stirred for two minutes. At the end of the minutes the temperature of the solution is taken again. Measure the weight of calorimeter and of the NaCl. In order to find the heat of neutralization of the reaction though, we first need the specific heat of NaCl which is the next part of the experiment.

Using the new solution made by mixing the NaOH and HCl, NaCl the specific heat of NaCl can be found. The calorimeter is emptied rinsed and dried and the NaCl put in a separate beaker. 50.0 mL of the NaCl solution are weighed and placed in the calorimeter and temperature is taken. Another 50 mL of distilled water are measured, put onto a heating plate until it reaches 65 Cï¿½, taken off, allowed to rest until the temperature stops rising (this temperature is taken), and then added to the calorimeter. The solution and the water are stirred for two minutes and then the temperature is taken, and the calorimeter is weighed once again. With this data the specific heat of NaCl can be collected through the formula (m ï¿½?Tï¿½s)solution = (m ï¿½?T ï¿½ s)water + (Heat Capacity ï¿½ ?T)calorimeter .

After obtaining the specific heat, we can go back and calculate the heat of neutralization with the formula q neutralization cal = (m ï¿½?T ï¿½ s)Solution + (Heat Capacity ï¿½ ?T)calorimeter

Results

The data presented below is relevant to Trial 1 of the experiment. These calculations also include uncertainty and the propagation thereof. All the results for trial 1 and the rest of the trials are placed in charts at the end. Note: whenever s is used below, it represents specific heat.

After looking at the charts I can see that there are large discrepancies between trials 1, 2, and 3 for the heat of Neutralization and for the heat capacity of the calorimeter as can be seen in table 3 and table 1.

Some observations made while doing the experiment were that when the items were being measured the scales fluctuated allot so it was difficult to decide what the right weight of the item was. Another problem with the scales that we encountered was that they only measured up 100 g, therefore when we measured the mass of calorimeter and NaCl solution and hot water together, we had to switch scales.

Discussion

In this experiment, the average heat capacity of the calorimeter is rather high. The ideal heat capacity for a calorimeter would be 0 cal/Cï¿½ because it’d mean that it isn’t absorbing any heat from the reaction. Such a calorimeter is impossible to create because all elements absorb a certain amount of heat. Even though the calorimeter is made out of Styrofoam, a good insulator, it still could be better in order to limit the dissipation of heat. Perhaps if this experiment was to be repeated, some more insulation should be added to the calorimeter such as perhaps adding another Styrofoam cup. As for the lid of the calorimeter, since it was not closed tight, perhaps some heat escaped this way as well.

The average heat of capacity for the metal used cylinder was .219cal/gCï¿½ . After looking at a chart with the specific heat for several metals, it was found that the one closest to the results was Aluminum which has a specific heat of .242cal/gCï¿½ . At the end of the experiment it was revealed to us that our metal cylinder was indeed aluminum. Here is the error percentage calculation:

The importance of this step in the experiment is that it allows for the accuracy of the calorimeter to be measured. It shows how accurate the heat of neutralization for NaOH and HCl will be. From this is can be expected that the specific heat for this neutralization will have an error of approximately 9.1%. While this amount of error is not extreme, it still is rather high almost nearing 10 %.

The average heat of neutralization was found to be to -2435.453 cal. This enthalpy is negative which means that heat is being released by the system into the surroundings. This also means that the reaction NaOH + HCl –> H2O + NaCl is exothermic.

Throughout the experiment, when weighing the calorimeter with the different substances inside it, the scales continuously fluctuated making it hard to get a precise reading on the weights of the items. This is noticeable between the first trial and the second trial in which the masses of the hot water and the cold water differ more so than any of the other values respectively leading to the differences between the heat capacity of the first trial and that of the second. During the first trial, it was difficult to become familiar with the procedures as well as the equipment meaning that the steps were executed slowly therefore creating inaccurate results.

This would most likely represent itself through increased heat loss because the slower a step is being carried out, the more time there would be for heat to dissipate. In the data this can be seen through the way that the difference between the temperature of the hot water and the final temperature decreases significantly in the third and final trial by which time the procedures were followed promptly. The use of the stirring rod, even though useful to helping keep the same temperature throughout all the content of the calorimeter could also have added to a source of error because it was inside the system while the neutralization reaction was taking place, and glass like all substances absorbs heat which wasn’t accounted for in the calculations. These are all small sources of error but they impact the results non-the-less.

Additionally, one last source of error is the fact that we changed the calorimeter used in the first two parts of the experiment (heat capacity of the calorimeter and the specific heat of the metal) to the other last two parts (the heat of neutralization and the specific heat of the NaCl solution). Since we used a completely different calorimeter in part two, using the heat capacity of the calorimeter of part one will have given us inaccurate results because the right values weren’t used to complete the calculations.

Were the experiment to be performed again, several steps could be taken in order to improve the process. One is to reinforce the calorimeter in order to improve it’s efficiency by adding more insulation. Another is to become familiar with the procedures beforehand. Instead of using a glass rod for the experiment perhaps an automatic stirrer could be used and it’s heat capacity could be calculated and then added to the equation in order to get a more accurate value.

Calorimetry overall is a very useful process. With this technology we could test several chemical reactions and test to see which ones have higher enthalpies. With this alternative fuels that are equally or more efficient than the gas and oil we use now could be found. It could also be used by dieticians to investigate what foods react with the body to give it more energy.

Bibliography

Computer Assisted Chemistry Tutorial (CACT)

http://www.science.uwaterloo.ca/~cchieh/cact/c120/calorimetry.html

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