ZiXuan Huang
Purpose
Determine the specific heat by measuring the amount of thermal energy transferred in a chemical reaction.
通过测量化学反应中转移的热能来计算出一个物质的比热容。
2
Materials
3 beakers
12 chromatography paper strips
3 wooden sticks
4 pens (3 knows/ collected from students and 1 unknowns/ left at crime scene)
Diagram
Chemicals
3
Procedure
4
Observation and Data collection
Observation----Each strip of chromatography paper with four different inks is a set of strips of chromatography paper pasted on wooden sticks, and then placed in three different solvents (Mixture, Ethanol, Methanol). As time goes by, Solvent climbs up the chromatography paper at different speeds. In the end, Methanol climbs the highest, Mixture climbs second, and Ethanol climbs the shortest. The dye of Ink 1 and 2 in Methanol separate to transparent， gray, purple. The dye of Ink 2 and 4 in Methanol separate to blue, light purple, dark purple, light pink. The dyes of Ink 1 and 2 in Ethanol separate to transparent and purple. The dyes of Ink 3 and 4 in Ethanol separate to blue, pink, purple. The dye of Ink 1 and 2 in Mixture separate to transparent, black, purple. The dye of Ink 2 and 4 in the Mixture is separate to blue, purple, pink.
Changes within 0 mins ~ 5 mins
Changes within 5mins ~ 10mins
Changes within 10mins ~ 15mins
Data Collection
5
Data Analysis
To compare the position of the band in the sample with the standard position to determine which the band is the same as the unknown pen we have to calculate the Rf value by this equation:
Result: Compare the Rf values of three pens tothe unknowns pen, the pen 3 has most similar Rf values with unknowns pen, therefore pen 3 is the are the items left by the murderer.
6
Discussion
Chromatography paper separates substances on the basis of their differences in solubility in a solvent to achieve the purpose of identifying substances. As the solvent climbs up because of the capillary action, it would also carry the ink up the chromatography paper. Then chromatography paper would appear various pigments based on different chemicals have a different solubility. Different solvents would climb different heights and speeds only affected by the strength attraction to the paper, but the climbing speed and heights of pigments are affected by the strength of their attraction to the paper and the solvent.
In order to understand the principle of Chromatography, we need to start from the reason why the solvent will climb to the paper. As Capillary action occurs at the time that adhesion between the solvent and the object is overcome and cohesion within the solvent, the solvent would ignore the gravity and climb up the paper. According to the concept of polar attract to polar, nonpolar attract to nonpolar, and attraction increase as the IMFs increase, we can infer the Chromatography paper was polar based on all three solvents( Mixture, Ethanol, Methanol) were polar. It is the fundamental requirement for capillary action to occur. All of the three solvents have LDF、hydrogen bonding、dipole dipole, which means chromatography paper must also have hydrogen bonding, dipole dipole, and strongest LDFs to break up the cohesion force between in ethanol molecule, and then form adhesion force with it.
Based on the observation and the data collection, the ranking list for solvents to climb from high to low is Methanol, Mixture, and Ethanol. Under the situation of all of the three solvents having LDF、hydrogen bonding and dipole-dipole, the difference only shows up by the LDF. According to the bigger electrons cloud the stronger LDFs to order strength of LDFs from strong to weak would be Ethanol, Mixture, Methanol. We can infer that the height of solvent increases as the attraction between solvent and paper decreases. The science concept behind it is that as the attraction increases the strength of the intermolecular bonding would be stronger which required more force to separate the molecule so which caused a different climbing height of the solvents.
Generally, the stronger the intermolecular forces between solute(in this case, the chemicals in the ink) and solvent, the more soluble the solute is in the solvent. Because when the solutes are dissolved in the solvent, if solvent and solute have stronger intermolecular forces, it means that they will have more abilities capable of capturing each other and thus combine with each other more quickly and easily. But it doesn't mean the solute will climb higher. In the Chromatography Lab, we cannot ignore a general factor, which is the intermolecular force between Chromatography paper and solute. As we identify the kinds of IMFs that Chromatography paper has in before, it has stronger LDFs than all solvents, as the polarity of the solute increases which means there will be stronger LDFs. Under the same conditions of IMFs, larger LFDs will have more attraction to each other. Therefore, the greater the polarity of Solute, the more attracted to Chromatography paper. In other words, larger molecules which have lower polarity and it would travel longer distances and faster. This is the basic concept supported chromatography can use to separate the different chemicals in a mixture.
After understanding the concept, people can perfectly use Chromatography in many fields. For example, it is used for solving crimes, checking the substances contained in pigments on fruits to infer health, extracting pigments from plants, etc. In conclusion, concentration chromatography is a very effective scientific method to separate organic and inorganic compounds for analysis and research.
7
Error Analysis
The data used to calculate the error percentage Rf value of purple of Ink 3 and Ink 4.
Error 1: The length of the ink 3 strip was longer than ink 4 strip when they both fixed on the stick led to pink in ink3 has longer distance.
Propagation: Because the length of the ink 3 strip was longer than ink 4 strip led to ink3 strip has more time for the chromatography paper to absorb the solvent so the solutes in ink3 would separate more properly than the solute in ink4, therefore the distance of pink in ink3 is further than ink4, which affected the Rf value of pink in ink3 is bigger than the actual Rf value. In order to fix it, after attaching the strips to the wooden sticks, measure their length with a ruler, and repeat the construction with scissors until the length is completely consistent before putting it into the solvent.
Error 2: Because the flame on the Cheeto is too small so it extinguishes too quickly, the temperature has begun to drop before we are aware of it, causing the actual temperature to be a little higher than what we collected.
Propagation: Did not collect data in time after chromatography paper dried led to hard to see where the ethanol stops climbing as the ethanol evaporates over time, which cause finally data of distance of pink in ink 3,4 been unreliable and the Rf value of pink in ink3 is offset. In order to avoid it, people can choose a chemical that is slightly less volatile as the solvent.
Comments