How to prepare a Schlenk tube

Objective

To prepare a Schlenk line,

Materials

• Schlenk line

• Stopcock

• 2 elastic bands: one is to hold the stopcock to the schlenk line together and the other one is to hold the head of the schlenk flask to its body.

• Vaseline

Method

• Put a small portion of vaseline on the frosty part of the stopcock.
• Rotate the stopcock in your hands so that the vaseline is well spread.
• Grab the elastic band which will be useful to keep the stopcock in its place. To do this, we will turn the stopcock so that it is perpendicular to the Schlenk tube. Then, we will put the elastic band on the lower part of the Schlenk tube, on top of the lip part and pull it far away from the stopcock. Then, pass the elastic band straight over the stopcock, you then twist the elastic band (the cross needs to stay underneath the arms of the Schlenk tube) and pass it over again , twist again (again, the cross needs to stay underneath the arms of the Schlenk line), pass it over again, and finally, put the elastic band in the initial position.

Final stopcock:

• Put a bit of vaseline on the frosty part of the body of the Schlenk tube to keep the head of the Schlenk tube together with the lower part.

• Attach the head of the Schlenk tube to its body.

• Turn twice the elastic band on our fingers, hook it up to one of the two wire supports of the Schlenk tube, pass the elastic band underneath the body of the Schlenk line and hook it up to the other wire support.

Final schlenk tube:

The properties of substances

Objective

To study, evaluate and compare the properties of several substances and relate them to their type of bonding (ionic, covalent or metallic).

Theoretical background

The properties of substances are related to the kind of bonding present in those substances. The type of bonding depends on the atoms present and is related to their position in the periodic table.

Materials

Substances A to D                         Conductivity meter

Test tubes                                          Distilled water

Spatula                                               Acetone

Bunsen burner

Method

Repeat the procedure for each of the substances provided:

1. Take ½ a spatula of the substance in a test tube. Describe the appearance of the substance
2. Gently heat it in the flame of the Bunsen burner and state the approximate melting point. (Low, intermediate or high).
3. Take ½ a spatula of the substance in another test tube
4. Add 10 mL water, stir it and state whether the substance is soluble in water or not.
5. Repeat the steps 3 and 4 using acetone instead of water.
6. Using the conductivity meter, state if the substance is a conductor in solid state.
7. If the substance is soluble in water, test whether the solution is a conductor or not.

Results

1. Sodium Carbonate: It is white and looks like flour or icing sugar.
2. Paraffin: It is round, white, and seems alike frog eggs.
3. Starch: It is white and looks like flour.
4. Iron filling: It is brown and it sparks a little.

Expected results

The only thing where we don't have the correct result is when we tried to see if iron filing conducted electricity which when we did it we obtained that it didn't conduct electricity however, we later looked on the Internet if it was true and we found out that iron filings actually conducted electricity.

Conclusions

After doing this experiment, we can conclude that:

• Ionic compounds are usually a metal and a non metal (sodium = metal, carbonate = non metal).
• Ionic bonded substances have a high melting point because in these molecules ions have a strong electrostatic attraction, therefore, it takes a lot of energy to break those bonds. 
• We already know what polar and non-polar substances are. Polar substances are those where the charge is not evenly distributed, one part of the molecule experiences more negative charge than the other. Polar substances dissolve in polar solvents. Usually, ionic bonded substances are polar, which means that they usually dissolve in water (polar solvent). We have seen this demonstrated in the experiment, sodium carbonate, an ionic bonded substance, dissolved in water. This means that ionic bonded substances won't dissolve in acetone as it is non polar.
• They don't conduct electricity as a solid because, in order for a substance to conduct electricity, it must have charged particles that can move freely, however, in a solid ionic compound, charged ions are held tightly toguether. Furthermore, as a liquid or a gas, ionic compounds conduct electrycity, in other words, they are conductors, because ions can move freely.

• Covalent bonds are usually two non-metals joined that, instead of transfering electrons, they share them.
• Covalent bonded substances have a low melting point because in these molecules, there are weak intermolecular forces (forces between the molecules is weak). Consequently, a low amount of energy is needed to move individual molecules apart from each other. However, because starch is a sugar, it has some different properties like, for example, it doesn't melt, it only burns.
• Non polar substances are those where the charge is evenly distributed. Non polar substances, if they dissolve, they dissolve in non polar solvents. We have to take into account that the rule "lieks dissolve in likes" is a guiding rule of thumb, not a physical chemical law but, usually, covalent bonded substances are non polar, which means that they dissolve in non polar solvents like acetone. In this experiment we have seen that starch, a non polar substance, dissolves in water, a polar solvent, this is due to the fact that the intermolecular forces in starch are weak enough so that they can be broken by water whereas in paraffin, the intermolecular forces are stronger so water cannot break them.
• They don't conduct electricity as a solid nor as a liquid (because they do not dissolve) because there are no free electrons (as electrons are shared).

• Metallic bonds are between two metals in which there are electrons moving around freely shared among the nucleus of the atom
• Metallic bonded substances have a high melting point because in these molecules, ions have a strong electrostatic attraction, therefore, it takes a lot of energy to break those bonds.
• Non polar substances are those where the charge is evenly distributed. Non polar substances, if they dissolve, they dissolve in non polar solvents. We have to take into account that the rule "likes dissolve in likes" is a guiding rule of thumb and not a physical chemical law but, usually, metallic bonded substances are non polar, however, metallic compounds NEVER DISSOLVE in neither polar nor non polar solvents because their intermolecular forces are extremely strong so, neither polar nor non polar solvents can break the forces between the molecules.
• They conduct electricity as a solid because, the electrons moving around produce an electric current. However, because metallic compounds do not dissolve in water, they aren't conductors of electricity.

Evaluation

While performing the experiment, we realised that the method of the experiment had several error:

1. The quantity of each substance was not specified in the method so, we had different amounts of each substance in each of the different tests (solubility, melting point, conductivity...) so the results are not homogeneous from one test to another. For example, we may have used 10 grams of sodium carbonate for the solubility test and 5 grams of iron filing so, if we used the same amount of each substance, the results may have varied. We could avoid this problem by specifying the amount of each substance that we need to use, for example, 5 grams.
2. Another problem that the method had was that everyone had a different Bunsen burner so the flame that each one of us had was different and, therefore, the amount of time it took for the substances to melt or not was different. Maybe our flame was hotter and the substance melted quicker whereas a different group had a cooler flame for the same substance and the time it took for it to melt was bigger so they thought that the melting point was higher (or viceversa). A possible solution for this would be that we had only one Bunsen burner so that the flame remained the same for everyone of us.
3. Also, you may measure more or less water/acetone than the needed so, it may make that one substance does not dissolve when it should (however, this didn't happen). To avoid this from happening, scientists could invent a device in which you type the amount of substance that you want, you put it inside the liquid and it measures the quantity exactly (it would have special sensors that would tell the device when to stop sucking liquid)
4. The last problem that we have seen in the method of this experiment (which has affected our results) was that the way to meausre the conductivity of the substances was not very accurate. We obtained that iron filings did not conduct electricity and later, when we looked it up on the Internet, we found out that actually, iron filings do conduct electricity. Our way to measure the conductivity wasn't very precise because, as we did not have to take a high quantity of the substance, the iron filings probably were not together so, there was a gap, and, consequently, because of the gap, we obtained that iron filings don't conduct electricity. To solve this problem, the solution that we have thought is to get a higher amount of substance when having to do the conductivity test than with the other tests so that there is no gap and, consequently, have the correct results.

Bibliography

Calder, V. (2004). Likes Dissolve Likes. Newton.dep.anl.gov. Retrieved 13 October 2014, from http://www.newton.dep.anl.gov/askasci/chem03/chem03348.htm

Chemwiki.ucdavis.edu,. (2014). Covalent Bonds vs Ionic Bonds - Chemwiki. Retrieved 13 October 2014, from http://chemwiki.ucdavis.edu/Theoretical_Chemistry/Chemical_Bonding/Covalent_Bonds_vs_Ionic_Bonds

Hyperphysics.phy-astr.gsu.edu,. (2014). Chemical Bonds. Retrieved 13 October 2014, from http://hyperphysics.phy-astr.gsu.edu/hbase/chemical/bond.html#c2

Hyperphysics.phy-astr.gsu.edu,. (2014). Chemical Bonds. Retrieved 13 October 2014, from http://hyperphysics.phy-astr.gsu.edu/hbase/chemical/bond2.html