To check the properties of chosen elements we decided to dissolve all of them in water, acid and basic solutions. We wanted to examine the solubility and pH of the oxides and chlorides and check the volatility by burning all the substances. If the bonding is strong, the substance is not volatile; similarly, if the bonding is weak, the substance is volatile.
Hypothesis: On the basis of our experiments, we analyse the connections of different metals.
Planning (b)
Requirements:
Method:
Dissolving in water or acid solutions, using pH meter and pH indicator test strips to measure the pH.
Procedure:
We tested the solubility of each oxide in water. To do this, we added a very small measure of each solid to about 3 cm3 of lime water and shook thoroughly. Subsequently we tested the pH of each solution obtained from each oxide using pH indicator test strips and pH meter.
After this we tested the solubility of oxides in dilute hydrochloric acid and in dilute sodium hydroxide solution.
We gathered the results in table 2.
Later we investigated the effect of water on each chloride by adding a small quantity to about 3 cm3 of lime water in the test tube. We tested the pH of the mixture obtained using pH indicator test strips and pH meter.
We also investigated the volatility of the chlorides by warming small samples of each chloride gently with a Bunsen.
We gathered the results in table 3.
Data Collection
Patterns in the properties of oxides.
Table 2.
Patterns in the properties of chlorides.
Table 3.
Data Processing & Presentation
- The oxides
The oxides of the elements chosen are all poorly volatile solids. They change their pH from basic to acidic, with amphoteric at the border.
Ionic solids such as Na and Mg react with water to form an alkaline solution of the hydroxide.
Na2O(s) + H2O(l) → 2Na+(aq) + 2OH-(aq)
MgO(s) + 2H+(l) → Mg2+(aq) + H2O(l)
Aluminium oxide, which is amphoteric, dissolves in both alkalis and acids.
Al2O3(s) + 6H+(aq) → 2Al3+(aq) + 3H2O(l)
Al2O3(s) + 2OH-(aq) + 3H2O(l) → 2Al(OH)4-(aq)
Silicon dioxide is capable of forming silicates in hot concentrated alkalis.
SiO2(s) + 2OH-(aq) → SiO32-(aq) + H2O(l)
Phosphorus oxide is a solid with molecular covalent bonding and low melting point resulting from it. It react with water to form acid that later may become dissociated.
P4O10(s) + 6H2O(l) → 4H2PO4-(aq) + 4H+(aq)
Chosen reactions of oxides with water, hydrochloric acid and sodium chloride are presented below (to simplify states of matter were omitted):
Table 4.
- The chlorides
The chlorides of sodium and magnesium dissolve in water without any chemical reaction. They give solutions in which their component ions behave independently. The bonds that occur in those chlorides are ionic.
NaCl(s) + H2O(l) → Na+(aq) + Cl-(aq)
MgCl2(s) + H2O(l) → Mg2+(aq) + Cl-(aq)
Calcium chloride, which is strongly hygroscopic, reacts with water to produce hydrated calcium chloride.
CaCl2 + 2 H2O → CaCl2·2H2O
Aluminium chloride partially hydrolyses with H2O. Aqueous solutions of AlCl3 are fully ionized, that is why they conduct electricity well. Such solutions are found to be acidic. Aluminium chloride has a covalent bonding.
The chlorides of non-metals have molecular covalent structure. The process of hydrolysis (splitting by the action of water) occurs and results in acidic solution containing hydrogen ions, chloride ions and the oxide.
SiCl4(aq) + 4H2O(l) → Si(OH)4(aq) + 4H+(aq) + 4Cl-(aq)
PCl3(aq) + 3H2O(l) → P(OH)3(aq) + 3H+(aq) + 3Cl-(aq)
PCl5(aq) + 5H2O(l) → P(OH)5(aq) + 5H+(aq) + 5Cl-(aq)
Chosen reactions of chlorides with water, hydrochloric acid and sodium chloride are presented below (to simplify states of matter were omitted):
Table 5.
Conclusion & Evaluation
After carrying out a series of experiments we can define some simpler physical properties, and knowing physical properties, we also can work out the chemical ones. In this lab we gathered only qualitative data instead of quantitive data, so it was impossible to analyse it as such. We can draw some conclusions that are based on our experiment.
As far as volatility and solubility are concerned, we can observe the crucial differences in those properties between oxides and chlorides. Apart from sodium, which is in both cases either poorly volatile or not volatile, chlorides tend to be much more volatile than oxides. Solubility in water for sodium, magnesium and calcium is the same for oxides and chlorides, while aluminium, silicon and phosphorus do not dissolve in water as oxides, but do dissolve as chlorides. Silicon and phosphorus chlorides react violently when added to water.
What influences volatility and solubility is type of bonding between given atoms. Ionic bonds are very strong, linking elements with different electronegativities (metal and non-metal, as in NaO, MgCl). This decreases the potential energy of the molecule and hence makes the system more stable. Ionic bonding, because of its strength, is difficult to be broken, thus the volatility of a given oxide is rather poor. Covalent bonding that is found between atoms with quite high electronegativities (non-metals) and is generally weaker than ionic bonding, that’s why covalently bonded atoms are more volatile than ionically bonded and quite soluble. Giant covalent bonding (found in SiO4) is very strong, so the structure cannot dissolve in solvents.
Another property easy to measure and evaluate is pH. As we can see in Table 1., oxides tend to change in periods from basic (on the left-hand side), through amphoteric to acidic (on the right-hand side), while all of the chlorides are of acidic pH. Sodium, magnesium and calcium are acidic as chlorides and basic as oxides and aluminium oxide is amphoteric (can dissolve in both bases and acids).
To evaluate this experiment I would suggest using pH meter for each solution instead of pH indicator test strips. Those, although new, did not give us an exact value of pH, varying up to 2 in comparison to pH meter, which is a serious mistake. Furthermore, the pH meter was checked by the teacher whether it did work properly, but there might still have been deviations while measuring pH.
Another mistake was done by adding too large amounts of the substances resulting in insolubility of some of them, although theoretically they are soluble. Greater attention should be paid to use an amount of substance that would be proportional to the amount of lime water so that the results will not be changed.
Sources:
- Green J, Damji S. 2001. Chemistry. Second edition. IBID Press, Victioria, Australia.
- http://en.wikipedia.org/wiki/Periodic_table
- http://www.chemicool.com/
- http://en.wikipedia.org/wiki/Periodic_table
- http://en.wikipedia.org/wiki/Physical_properties
- http://dict.die.net/chemical%20property/
- http://en.wikipedia.org/wiki/Ph
This definition comes from http://en.wikipedia.org/wiki/Periodic_table
This definition comes from http://en.wikipedia.org/wiki/Physical_properties
This definition comes from http://dict.die.net/chemical%20property/
This definition comes from http://en.wikipedia.org/wiki/Ph