Undertake calorimetry to study cooling curves. Introduction:First of all, calorimetry is theprocess of measurement of the heat transferred in a science investigation. Ituses a set up called a calorimeter, which is a basic container such as a glassbeaker with a thermometer placed inside used to measure the changes of state,phase and chemical reactions. The general uses of this include monitoringendothermic and exothermic reactions or measuring a specific heat capacity. Afterundertaking this process, a cooling curve can be created.
A cooling curve is arepresentation of the changes of phase of matter which is usually displayed ina line graph. The independent variable is time and the dependent variable istemperature. During the experiment, the task I was given was to find the bestmethod to measure the cooling curve of 10g of stearic acid and 10g of paraffin.This experiment took a considerable length of time to complete, and a keyfactor in this was the room temperature which held at 25 degrees Celsius. Hypothesis:The temperature should get decreaseas the stearic acid cools down, and it should continue to decrease after thestate has changed. Method:1 )Put on a lab coat, and put on apair of safety googles to complete PPEto ensure safety during the experiment. Once prepared, collect a boiling tube from the heat and a stand from thecupboard so everything is ready.
2) Ensure the thermometers werecorrectly calibrated. This can this by placing the first thermometer in a bathof boiling water to make sure that the boiling temperature on the thermometershowed to be 100 degrees Celsius. Then place the thermometer in a tub of iceand cold water to make sure that the freezing temperature was 0 degreesCelsius. This was repeated four times tomake sure accurate results were obtained.3) Draw a table on a piece of paperso the results can be recorded. After this is done, proceed to set up theequipment.
4) As soon as the apparatus is setup, begin the stop watch and after everyminute, record the temperature on the thermometer into the table5) Continue this until thesubstance was completely frozen, and a completely white solid. Equipment:1. Filterfunnel stand2. Testtube3.
Thermometer4. Hotbath5. Safetygoggles6. Stopwatch7. Labcoats8.
Stearicacid and paraffin wax9. Paperpen10. Graphpaper Resultstables:1) Stearicacid 10g Time (minutes) Temperature (degrees Celsius) 0 65 1 59 2 55 3 54 4 53 5 53 6 53 7 53 8 53 9 52.
5 10 52.5 11 52 12 52 13 52 14 52 15 51 16 51 17 51 18 51 19 50.5 20 50 21 50 22 49 23 49 24 48 25 47.5 2) Paraffin10g Time ( Minutes) Temperature ( degrees Celsius) 0 59 1 55 2 55 3 54 4 53 5 53 6 52 7 52 8 52 9 52 10 52 11 52 12 52 13 51 14 51 15 50.5 16 50 17 49 18 48 19 46 20 45 21 42 22 41 23 40 24 39 25 38 3) Calibration ofthermometer CALABRATION BOILING ( Degrees Celsius) FREEZING( Degrees Celsius) 1 100 5 2 100 5 3 100 5 4 100 5 1( Graph of Stearic Acid A B C D 2) Graph of Paraffin wax A B C D Graph: As my graphs show, the melting point canclearly be seen as 52 degrees Celsius. I know this because there is a straightline along that temperature.
This shows that energy has been removed from thesubstance with no change to the temperature. This is when it changes from aliquid to a solid. This is because it had reached its melting, or freezingpoint. A phase change occurs when amatter in one phase turns into a matter in a different phase. This is due totemperature, or pressure. As the temperature decreased as it was cooling down.
It can be said that the substance lost energy. This resulted in the moleculesmoving a lot slower, and the cohesion, which is the attractive force betweenthem, ends up pulling the moleculescloser together leaving them in a fixed position. The melting point I had gotcompares very closely to theoretical melting point which ranges from 67 degreesCelsius.
A number of errors could have been the reason for the slightinaccuracy, such as human error, and the environment it Is done in.The percentagedifference in my melting point in comparison to theoretical results can beworked out by doing the following: % difference = differencein melting points x 100 literature meltingpointTherefore, thepercentage difference would be 22.38%.
The cooling curve isn’t very smooth for the stearic acid,as it seems to have more sudden drops in comparison to paraffin, which appearsto be a nice curve. Perhaps this was due to the rate at which the substancesmelts, as the particles in each substance tend to act differently and unpredictably,or simply human or apparatus error. For stearic acid, it seems to be in aconstant decrease with no signs of any rapid decrease, this means that itremained approximately constant. This is different to paraffin where it can beobserved that up until the 19th minute, it decreases constantly andsteadily, however when it passes this point it begins to rapidly decrease. Evaluation One key factor inthis experiment was the calibration of the thermometer. This is importantbecause if the thermometers were not as accurate as they could have been, ourresults wouldn’t be accurate therefore it would throw off our entireexperiment. Even though I did calibrate them beforehand, I still came across afew errors. As you can see in thecalibration table of results, the iced tub of water didn’t seem to reach 0degrees Celsius.
This could be due to a variety of factors. For example, theiced water could have began to melt before I could calibrate the thermometer,or because the thermometer was faulty, or the environment in which the icedwater was kept could have heavily affected its temperature. Another possible reason could be that theiced water was contaminated, as the water did have a brownish appearance. Thisisn’t good because whatever else was in that water was preventing it fromreaching the freezing point of 0 degrees Celsius, which affected ourcalibration. To avoid this next time, we should used an insulated tub and use acleaner water source. Furthermore, since the iced water was kept out for aconsiderable amount of time before I could reach it, I believe this is thereason the temperature didn’t reach 0 degrees Celsius. In order to avoid thisnext time, I should keep the iced water in a controlled environment like aninsulated tub until I am able to calibrate it straight away.
The reason why theboiling water was so successful, was because I had calibrated it straight froma kettle, hence the reasons why It continually read 100 degrees Celsius. Another importantfactor when it comes to thermometers is the type of thermometers used. In thecalibration, Mercury was used to calibrate the boiling water and an alcoholthermometer was used to calibrate the freezing water. Mercury thermometers aregenerally used clinically in hospitals, or for liquid temperature measurementsin schools and scientific investigations. The advantages of a mercurythermometer is that it responds quickly to the temperature change. This meansthat you wouldn’t have to wait very long for a reading, which would be apositive factor for my investigation as we were measuring the temperature perminute. Mercury also has a colour to it, which makes the line of temperatureeasy to spot which is an advantage as you don’t have to waste time looking forthe line to read. However, adisadvantage is that the displays are generally difficult to read.
This is ahindering factor because it could be easy to misread the temperature, leadingto inaccurate results. On the other hand,alcohol thermometers are generally used in hospitals, industry and schools andcolleges, and for domestic uses. The advantage of alcohol thermometers is thatit can be used to measure low temperatures more accurately since its freezingpoint is -115 degrees Celsius, and it is quite accurate due to its ability toexpand. This is good because it will be able to more accurately read thechanges in temperature when the substance is frozen, however, this wasn’tneeded in our experiment as the lowest temperature that was reached was 28degrees Celsius.
Alcohol is also lessharmful than Mercury, as Mercury is poisonous .However a disadvantage ofAlcohol thermometers are that they are bad for measuring high temperatures dueto its really low boiling point. Taking all of thisinto account, it was decided that alcohol thermometers were the best ones touse for the experiment.
All in all, theydid work well to an extent. The lines were really hard to read off of, and itcould have been a lot more accurate. Perhaps a better thermometer choice wouldhave been a data logger.
A data logger is a portable measurement instrumentthat is able to autonomously record temperature over a defined period of time.An advantage of a data logger is that the measurements are automatically taken atthe correct time. This is good because unlike a person, it will not forget totake the temperature at the right time and it will take it accurately. Anotherbenefit of data loggers is that the measurements are always taken correctly, asthe computers will never make mistakes unlike a computer. Lastly, when using adata logger, a graph and table can be automatically made using a data loggersoftware. This is good because it will save us the time and potentially errorsof making our own tables.
However, there is a disadvantage of data loggerswhich is that they are quite expensive, and perhaps quite unrealistic forcollege use.There are otherfactors which come into consideration when evaluating the accuracy of anexperiment. One factor to take into account is the room temperature. The roomtemperature throughout the experiment varied. For example, at the beginning thewindows were left opened, then about halfway through they were closed. The doorwas opened and closed throughout the experiment as well.
This is importantbecause if the room temperature wasn’t kept at the average of 25 degrees, itmeans that it wasn’t constantly monitored therefore we can’t know what affectthis had on the cooling of the substances. Next time, the windows and doorsshould be kept shut and have a thermometer in the room to measure the averagetemperature a lot more accurately. Another factor totake into consideration when assessing accuracy is the surface area of thestearic acid and paraffin wax containers used. Surface area affects the meltingpoint by a considerable amount, therefore if the boiling tube used for paraffinwax was different to the boiling tube used for stearic acid it is possible thetwo had different surface areas, therefore the experiment wasn’t completelyfair or accurate. In the experiment,a lid was not used to cover up the top of the boiling tube. It is possible thatthis could have caused slight inaccuracies in the experiment due to heatescaping from the top, or other substances in the air which could have easilycontaminated the experiment since the air isn’t clean. If a lid was to be used,it would have ensured that nothing contaminated got in, and nothing useful gotout therefore next time the experiment is conducted, for optimum results, a lidwould be beneficial.
Analysis:As shown by mygraphs and my results table, there a various trends and patterns to bediscussed. For instance, they both appear to begin to drop at around the 52degrees Celsius mark. This is because this is the melting point of both thesubstances.
When a substance melts, it goes through a change of state. Whenmatter changes state, energy is either lost or gained. When the stearic acidand paraffin wax cooled down, energy was lost. Liquids solids and gases aredetermined by their balance of intermolecular forces which keep them together,and the kinetic energy of the molecules which drive them apart. We can measurethe amount of energy lost by using a thermometer- because temperature isessentially a measure of the average kinetic energy. To understand the kinetictheory, the basics of solids and liquids must be understood. In a solid, youwill find closely packed particles which vibrate in a fixed position in aregular arrangement, and in a liquid you will find closely packed particleswhich can move freely, in a random arrangement.
So when something freezes orcools down, so much energy is lost that the particles slow down so much due tothe lack of energy, that the forces of attraction are strong enough to keep theparticles in a solid arrangement. When the new bonds are forming, the heatenergy begins to evolve. When the stearic acid and the paraffin wax were cooleddown, the solidification releases latent heat into the air. Latent heat isdefined as the amount of heat required to turn a solid into a liquid or gas.The cooling curveisn’t very smooth for the stearic acid, as it seems to have more sudden dropsin comparison to paraffin, which appears to be a nice curve. Perhaps this wasdue to the rate at which the substances melts, as the particles in eachsubstance tend to act differently and unpredictably, or simply human orapparatus error.
For stearic acid, it seems to be in a constant decrease withno signs of any rapid decrease, this means that it remained approximately constant.This is different to paraffin where it can be observed that up until the 19thminute, it decreases constantly and steadily, however when it passes this pointit begins to rapidly decrease.