Our main purpose is to familiarise ourselves with a method to find the heat of decomposition of a compound (hydrogen peroxide). Firstly, we will be looking at information about calorimeter and what it meant by heat of decomposition. Also, we will try to understand why we have to subtract the heat absorbed by the calorimeter to find the heat of decomposition of hydrogen peroxide. Therefore, we found the heat capacity of calorimeter. After that, the enthalpy of decomposition of hydrogen peroxide was calculated by finding the amount of heat lost by solution and the heat absorbed by the calorimeter. Then, the heat absorbed by the calorimeter was subtracted from the heat lost by the solution to determine the amount of heat absorbed by hydrogen peroxide. The enthalpy of decomposition of hydrogen peroxide obtained is -57.9kJ/mol. Finally, we would look at how the results differ from the literature value and understand reasons to why the difference in the values occurs and find methods to prevent this.
1A calorimeter is a device that helps to measure the heat of reaction. There are two types of calorimeters which are usually used. The sophisticated and expensive one, while the other can be easily made and available cheap. The one used for this experiment is the simple and cheap one which is basically a Styrofoam cup because its container walls are well insulated to prevent or reduce the heat change to environment.
2However, the calorimeter would also absorb heat as it is a simple calorimeter and thus it is a necessity to take it into the consideration while doing the calculations to find the heat of decomposition of hydrogen peroxide. Also, the reactants are inside the calorimeter with a lid on top before the reaction has been initiated to prevent heat from escaping or it would cause the results to turn inaccurate. Besides this, a thermometer will be placed inside to constantly monitor the temperature as time progresses. It assists in finding the temperature difference before and after the reaction has completed.
* Heat of decomposition
Heat of decomposition is defined as the breaking down of a single compound (Hydrogen peroxide) into 2 simpler compounds or elements upon the application of heat. Enthalpy of decomposition is thus defined as the amount of heat required to chemically break down a compound. In this experiment we will be trying to find the heat of decomposition of hydrogen peroxide. 4Hydrogen peroxide is an unstable compound. Under normal room conditions, it would break down to water and oxygen. However, the process is very slow to be completed in a normal laboratory period. As a solution, catalyst [Iron (III) nitrate] was introduced. It helps to speed up the process without being consumed itself.
There are basically 2 parts to this experiment, thus 2 procedures.
* Heat capacity of calorimeter (Part 1)
A simple calorimeter was firstly made using a thermometer, Styrofoam cup and cover. 30ml of tap water next was poured into the simple calorimeter cup and covered back with the cover and the thermometer. It was left at the room temperature for about 5-10 minutes before recording the temperature to the nearest 0.5oC.
Another 30ml of water was poured into a 250ml beaker and heated using a hot plate until the temperature was about 20oC more than the room temperature which would means about 44-50 oC. Then, the heated water was allowed to stand for 1 minute and its temperature was recorded immediately to the nearest 0.5oC and poured completely into the calorimeter. The cover was covered over it with the thermometer along. The cup was next swirled once. The temperature was observed for 3 minutes and was recorded every 15 seconds.
* Enthalpy of decomposition of hydrogen peroxide solution (Part 2) After the first part of the experiment was completed, the calorimeter and the thermometer were dried completely. 50ml of 1.0M hydrogen peroxide (H2O2) was then cautiously measured and poured into the calorimeter. The cover was then replaced along with the thermometer. The solution was swirled once and temperature was recorded every single minute for 4 minutes. At the 5-minute mark, the cover and the thermometer were removed. 10ml of 0.50M Iron (III) nitrate [Fe(NO3)3] was measured and added into the calorimeter containing the hydrogen peroxide.
Temperature was next measured at 5.5 minute mark and for every succeeding minute till a total of 20 minutes. A temperature vs. time curve was then constructed with the data obtained from the experiment to determine change in temperature. Next, enthalpy of the decomposition of hydrogen peroxide was calculated. A starting temperature can be found by extrapolating the 5 points prior to adding the catalyst to the point of mixing. The final temperature can also be found by extrapolating the linear portion of the graph to the point of mixing.
As the literature value stated in the data sheet, the heat of decomposition of the hydrogen peroxide is -94.6kJ/mol. However the value obtained was only -57.9kJ/mol. This was because the heat has been lost to the surroundings around outside the calorimeter. The calorimeter is not perfect and simple and thus its cover is not proper and does not fully cover the top of the Styrofoam cup, leaving a small opening. The heat could have escaped from the calorimeter through here. Besides this, the experiment was done for a long period of 20 minutes. The longer the period, there is more time for the heat to escape from the imperfect calorimeter. This leads to a lower temperature than expected to be recorded as time progresses. This leads to lower temperature difference leading to a lower heat of decomposition of hydrogen peroxide to be obtained from the reaction.
Another possibility of losing heat occurred during the addition of the Iron (III) nitrate as catalyst inside. Once it was added, the reaction was increased rapidly producing a lot of heat. However, the lid was not covered on time and was slow which led to a significant amount of heat to be lost to the surroundings. Also, the temperature shown was not accurate due to the poor calibration of the thermometer. Often estimation to the nearest 0.1oC has to be done when recording the results. Due to the usage of an imperfect calorimeter, the results obtained were inaccurate. Thus we can improve this experiment by using a perfect calorimeter which will help to reduce heat from escaping from the calorimeter drastically and also reduce the heat from being absorbed by the calorimeter itself. 3One perfect example would be the usage of the adiabatic calorimeter.
The calorimeter’s vessel is basically surrounded by a jacket containing water. Therefore, the temperature of which will automatically be same as the temperature inside the vessel. Since the temperature of the reacting system and the surroundings is the same, there is no heat passed in either direction. This would lead to much more accurate results. Besides this, we can also try to reduce the time taken to 15 minutes. I would recommend this because the longer the time taken for the experiment, the higher the possibilities for the calorimeter to lose heat to the surroundings. Hence, by reducing the time, the possibilities of losing heat is minimised. Apart from this, we can also use a thermometer with a finer calibration to note the slightest difference in the temperature which could lead to the significant changes in the results obtained.
From the results obtained, I can conclude that the usage of the simple calorimeter is not an ideal choice due to possibilities of losing heat to the surroundings. A sophisticated and better calorimeter has to be used. 5Styrofoam cup calorimeter isn’t recommended for this decomposition reaction as it involves production of a gas, Oxygen which can escape from the cup easily.
Besides this, we were better familiarised with a method to find the heat of decomposition of compounds with the use of calorimeter. It can also be concluded that hydrogen peroxide is not a stable compound and breaks down slowly in room temperature and pressure and can be speed up by the usage of catalyst. In overall, the values obtained from the experiment are not useful because of the severe errors which occurred during the experiment mainly due to the apparatus used.
1CaCT, Calorimetry: Measuring heats of reaction. http://www.science.uwaterloo.ca/~cchieh/cact/c120/calorimetry.html [Accessed 19 July 2012] 2Solomon, S., Rutkowsky, S and Boritz, C (2009) Everyday investigations for General Chemistry John Wiley & Sons 4Cool sciences. Heat of decomposition of hydrogen peroxide. http://www.coolscience.org/CoolScience/KidScientists/h2o2.htm [Accessed 22 July 2012] 5About Chemistry. Measurement of Heat flow and Enthalpy Change; Calorimetry – Coffee Cup Calorimetry and Bomb Calorimetry. http://chemistry.about.com/od/thermodynamics/a/coffee-cup-bomb-calorimetry.htm [Accessed 25 July 2012] 3Silcocks, C.G. Physical Chemistry: Thermochemistry and thermodynamics,3rd Ed,: Macdonald & Evans Ltd, 1982.