Lab Session 1

Lab Session 2- 07/10/2014

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.

TABLE OF RESULTS

1. Data table:

	Iron(II)sulfate	Paraffin	Starch	Iron
Appearance	Tiny yellow and green rocks	White transparent tiny balls	White pouder	Brown Nail, solid
Melting Point	high	Very low	Intermediate-high	high
Solubility in water	soluble	insoluble	Almost soluble	insoluble
Solubility in acetone	soluble	insoluble	soluble	insoluble
Conductivity	yes	no	no	yes

2. Type of bonding

Iron (II) Sulfate:  This compound is clearly ionic, as it is formed between a metal and a non-metal and it has all the characteristics of this type of bonding.
Ionic compounds are formed because of a large difference in electronegativity between the atoms. As we can see in the periodic table below, iron´s value is of 1,8, Sodium´s 2,5 and Oxygen 3,5.

What happens between the atoms of this molecule is that the electronegativity of metals is much smaller than of non-metals, and therefore non-metals will remove valence electrons from metals. The result of it is a positively charged metal cation and a negatively charged non-metal anion. The positively charged ion attracts the negatively charged ion and they form an ionic bond.

Most ionic compounds are polar and that is the reason why they dissolve in water. Like dissolves in like.However, ionic compounds have a high melting point because the forces between the atoms are quite strong.

Paraffin (CnH2n+2):This compound is covalent, as it is formed between two non-metals. These bonds are bond where two atoms share valence electrons and the difference between their electronegativity values is low, less than 1,8. As the atoms within the molecule have similar electronegativity they will share the electrons in the bond.
Covalent compounds are most of the time polar, but this can change if the molecule is very large.

Starch (C6 H10 O5): This forms a covalent bonding as it is formed, like paraffin, between two non-metals. A starch molecule is a polysaccharide and it contains glucose molecules joined into a single structure by covalent bonds. (Chemistryexplained.com, 2014). These molecules are held in a lattice and have weak forces between them. The properties of starch prove that is is a covalent compound, as you can see in the next point.

Iron (Fe): Iron is obviously a metallic bond, as it is formed by a unique metal. The bond between two metal atoms of the same element is strong and that is why they do not dissolve in water in spite the theory.

When iron atoms come together, a molecular orbital is formed in much the same sort of way that a covalent bond is formed. The difference however is, that the electrons can move freely within these molecular orbitals, and so the electrons are said to be delocalised. The metal is held together by the strong forces of attraction between the positive nucleus and the delocalised electrons.

3. Expected results

	Iron(II)sulfate	Paraffin	Starch	Iron
Type of bonding	Ionic	Covalent	Covalent	Metallic
Melting Point	high	Very low	Low	high
Solubility in water	soluble	insoluble- slightly soluble	insoluble- slightly soluble	insoluble
Solubility in acetone	insoluble	soluble	soluble	insoluble
Conductivity	Solid: no Liquid: yes	no	no	yes

4. Conclusion

Comparison of both tables:

	Iron(II)sulfate	Paraffin	Starch	Iron
Type of bonding	Ionic--polar	Covalent---non polar	Covalent---polar	Metallic---polar
Melting Point	High *high	Very low *very low	Low *intermediate	High *high
Solubility in water	Soluble *soluble	insoluble- slightly soluble *insoluble	insoluble- slightly soluble *almost soluble	Insoluble *insoluble
Solubility in acetone	soluble *soluble	insoluble *insoluble	Soluble *insoluble	Insoluble *insoluble
Conductivity	Solid: no Liquid: yes *yes	No *no	No *no	Yes *yes

*red=obtained results

*back=expected results

*highlighted yellow=problematic 

To start with, the table shows that most of the results coincide; however there are some, such as the solubility in acetone, that do not. From the three different tasks carried with the substances, it is clear that the more problematic one was the solubility of the solutes in the solvent acetone; whilst the solubility in water and melting points were the mostly the same. At the same time, it is quite obvious that the iron nail, due to its strong bonding followed the expected behavior and resulted being insoluble. However, Starch did not behave as expected; this could be due to the bonding in acetone.

Acetone is a polar molecule; however it has properties of both, polar and non-polar molecules.  Therefore, if we apply the “like dissolves like” theory of intermolecular forces it should have dissolved iron (II) sulfate and starch; whilst paraffin and iron should have been insoluble.

“The molecule C3H6O, also known as Acetone, is Triangular Planar. Its Lewis Structure is:”



*(Maccomber, 2011)

ANALYSIS OF ACETONE C3H6O:

 Acetone is said to have characteristics of both, polars and non-polars; although it is a polar molecule. It is a made up of three non-metals; hydrogen, carbon and oxygen. Concurrently, if we analyze the electronegativity of its components, we find out that hydrogen and carbon, both have similar electronegativity values (2.5 and 2.1) whilst oxygen is slightly higher with 3.5; and hence all electrons would be attracted towards its nucleus. The similarity between the electronegativity values means that they form a covalent bond where oxygen is slightly more electronegative and therefore more negative than the rest of the molecule. This distribution of charge within the different parts of the molecule leans to form a polar bond. Therefore acetone is Polar and it will dissolve polar solutes “like dissolves like”.

According to this, most of the results are correct, which means that we performed the experiment quite well.  On the other hand, starch, which is polar, did not dissolve as expected; this could be due to the magnitude of the molecule. Furthermore, according to the “like dissolves like”, acetone should have also dissolved Iron. Nevertheless, Iron is a metallic bond; leaning to a very strong bonding between the iron molecules (see background) which makes the substance insoluble (this could be an exception to the common rule).

To sum up, I would say that the experiment resulted being quite successful as the results obtained mostly coincided with the expected data and therefore there was little deviation. The only task which was clearly more problematic was the use of Acetone as solvent because of the contrast between the covalent relationship between carbon and hydrogen and the double bond between oxygen and carbon.

5. Evaluation 

During the performance of this experiment, we have come up with some problems such as how to measure the solubility and the melting point of a substance. It was difficult to determine whether a substance had a low, intermediate, or high melting point due to the lack of measuring units. I would say that the determination of solubility and melting point were mostly subjective and hence inaccurate. Therefore; the lack of equipment to measure in units was the main problem. Especially when dissolving substances like starch; which at first seemed to dissolve quite easily and some minutes later the solution resulted being separated into water and starch (insoluble). To solve this problem I would recommend, for the boiling point the use of a thermometer so that we could get some number results to compare and draw in a graph. On the other had; I would suggest to time with a stop watch the time taken for a substance to dissolve, being time= infinite the result for insoluble solutes (in seconds).

At the same time, the fact that the substances were measured randomly (half a spatula) suggests that the quantities were once more inaccurate, and this may have lean to inappropriate results. From my point of view, to get correct data, the substances should have been first weighed in the electric balances.

In addition, referring again to the accuracy of results, I would say that the data obtained is not reliable as each task was only experimented once, rather than three times as it should be in theory. Hence, the solution to this problem would be as easy as carrying out each experiment three times, recording the data in a table and coming up at the end with an average.

This is a picture of the test tubes after the experiment was done.

6. References 

-Elementalmatter.info,. (2014). Non-Metals - Hydrogen, Nitrogen, Oxygen, Phosphorous, Sulfer and Selenium. Retrieved 13 October 2014, from http://www.elementalmatter.info/non-metals.htm

-Macomber, L. (2011). Chemistry Molecule: Acetone. Chemistrymolecule.blogspot.com.es. Retrieved 13 October 2014, from http://chemistrymolecule.blogspot.com.es/2011/02/moleculechcl3.html
 

- Bbc.co.uk, (2014). BBC - Higher Bitesize Chemistry - Bonding, structures and properties : Revision. [online] Available at: http://www.bbc.co.uk/bitesize/higher/chemistry/energy/bsp/revision/1/ [Accessed 10 Oct. 2014].


-Chemistryexplained.com, (2014). Starch - Chemistry Encyclopedia - structure, reaction, molecule. [online] Available at: http://www.chemistryexplained.com/St-Te/Starch.html#ixzz3G19aVRqT [Accessed 13 Oct. 2014].

- Google.es, (2014). electronegative periodic table - Buscar con Google. [online] Available at: https://www.google.es/search?q=electronegative+periodic+table&client=firefox-a&hs=8T1&rls=org.mozilla:es-ES:official&channel=fflb&source=lnms&tbm=isch&sa=X&ei=x5I6VJaOOpbxaJe1gOgM&ved=0CAgQ_AUoAQ&biw=1366&bih=608#facrc=_&imgdii=_&imgrc=Il8EANPqHwLrtM%253A%3BMJ7oP_SmLjUDMM%3Bhttp%253A%252F%252Fwww.green-planet-solar-energy.com%252Fimages%252FPT-small-electroneg.gif%3Bhttp%253A%252F%252Fwww.green-planet-solar-energy.com%252Felectronegativity-values.html%3B500%3B353 [Accessed 13 Oct. 2014].