Period 3 Chlorides. Aim: To study the chlorides of period 3 elements and classify them according to structural type and bonding.

International Baccalaureate Chemistry

Teacher: Minati Sahu

Chemistry Lab Report – Period 3 Chlorides

Kelwin Joanes

Chemistry Lab Report – Investigating the properties of period 3 Chlorides

Introduction:

Aim: To study the chlorides of period 3 elements and classify them according to structural type and bonding.

Hypothesis:

If the compound dissolves in the water there may be a temperature change. If it’s a small temperature change, then this will indicate that a physical process has taken place. While a large temperature change will indicate that a chemical process has taken place. This will help distinguish between the physical process of dissolving and chemical one of hydrolysis when you add these substances to water. And if there are any pH changes that take place when the chlorides are reacted with water. A decrease in pH will indicate that hydrolysis has taken place. Finally, consider a physical data for each compound and reach a conclusion its bonding and structure.

General Background:

As the number of valence electrons increases, there is a steady increase in the number of electrons available for bond formation and hence in the number of chlorine atoms that each element bonds to – NaCl, MgCl2, AlCl3 (more correctly written as Al2Cl6 in its gaseous state), SiCl4, PCl5 (though PCl3 also exists).

The chlorides of metals, such as sodium chloride and magnesium chloride, are ionically bonded crystalline solids with high melting points. When added to water these chlorides dissolve without chemical reaction, to give solutions in which the component ions can behave independently. Because of the presence of these mobile ions, these compounds conduct electricity when molten or in aqueous solution.

NaCl(s) Na+ (aq) + Cl– (aq)

The chlorides of non-metals, such as phosphorus trichloride, have molecular covalent structures. As a result of the weak forces between the molecules these compounds have low melting and boiling points. When added to water a hydrolysis (i.e. splitting by action of water) reaction occurs in which the bonds between the element and oxygen. The result is an acidic solution containing hydrogen ions, chloride ions and oxide, or an oxyacid of the element. This oxyacid may then also dissociate. In the case of phosphorus trichloride, for example, the reaction is:

PCl3 (l) + 3H2O (l) ➔ P(OH)3 (aq) + 3 H + (aq) + 3 Cl – (aq)

P(OH)3 or H3PO3 is phosphoric (III) acid which is a weak acid and partially dissociates as shown below.

H3PO3 (aq) H+ (aq) + H2PO3 – (aq)

These chlorides in the molten state do not conduct electricity, as would be expected for molecular covalent compounds. In aqueous solution however they do not conduct electricity because of the ions formed in the chemical reactions above.

Chlorine itself (Cl2), which may be regarded as chlorine chloride, fits in with this pattern of behaviour, being a ...

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PCl3 (l) + 3H2O (l) ➔ P(OH)3 (aq) + 3 H + (aq) + 3 Cl – (aq)

P(OH)3 or H3PO3 is phosphoric (III) acid which is a weak acid and partially dissociates as shown below.

H3PO3 (aq) H+ (aq) + H2PO3 – (aq)

Chlorine itself (Cl2), which may be regarded as chlorine chloride, fits in with this pattern of behaviour, being a molecular covalent substance that reacts with water in an analogous hydrolysis reaction.

Even though aluminium is a metal the behaviour of many of its compounds, especially when anhydrous, is more typical of non-metals. This is a result of the small size and high charge of the ion that aluminium forms. Aluminium chloride for example, although a solid, sublimes at the surprisingly low temperature of 178°C to give a vapour consisting mainly of Al2Cl6 molecules. Anhydrous aluminium chloride undergoes vigorous hydrolysis when added to water. Even the hydrated chloride produces quite acidic solutions owing to the dissociation of the water molecules associated with the small, highly charged, Al3+ ion.

Source: John Green and Sadru Damji. "Chemistry, International Baccalaureate Diploma Programme." 2nd Edition. Victoria: IBID Press, 2001.

Variables:

Independent variable:

There was no independent variable present.

Dependent variable:

The dependent variables of this experiment were the measurements of temperature and pH level.

Controlled variable:

The controlled variable of this experiment was the volume of distilled water used in each experiment.

Materials and Methods:

Apparatus:

8 test tubes
2 measuring cylinders
Distilled water
Thermometer
Spatula
Universal indicator solution and colour chart
pH paper
Teat pipette

Chemicals:

2M of NH3 Solution
NaCl
MgCl2
Anhydrous AlCl3
Acetone

Method:

First of all, I examined the provided chloride samples and noted their physical state (solid, liquid, or gas) and their colour (if they had any).

Next, I took 4 test tubes, washed them and filled them each with 5 cm3 of distilled water, and placed them side by side in a test tube rack.

Then I took the first test tube, measured the temperature of the distilled water in it, and took half a spatula tip of NaCl and added it to the test tube.

After 1 minute, I noted the temperature of the solution, whether the chloride dissolved and any other observation, e.g. has any gas evolved at any time? If so, is the gas acidic? Can it be identified using a simple test?

Using either universal indicator solution (2 – 4 drops) or a piece of pH paper, I compared the colour with the chart provided, and noted the pH indicated.

All the above steps were repeated for the rest of the chloride samples (MgCl2 and AlCl3) provided.

For comparison with the above, measure the pH of the distilled water in a test tube by using either universal indicator or pH paper.

On mixing with acetone, repeat the steps of that of water, only now using acetone. This time, there is no need to examine the pH.

Results:

Data collection:

On mixing the given chlorides with water:

Note: pH of water = 7

On mixing the given chlorides with cyclohexane (acetone):

Physical and chemical properties for the given chlorides:

Sources:

Wikipedia, the free encyclopaedia. Magnesium Chloride. 31 October 2007. 17 November 2007 <http://en.wikipedia.org/wiki/Magnesium_chloride>.

Wikipedia, the free online encyclopaedia. Aluminium Chloride. 4 November 2007. 17 November 2007 <http://en.wikipedia.org/wiki/Aluminium_chloride>.

Wikipedia, the free online encyclopaedia. Magnesium Chloride. 15 November 2007. 17 November 2007 <http://en.wikipedia.org/wiki/Magnesium_chloride>.

AUS-e-TUTE. Trends Across Period 3 of the Periodic Table. 18 November 2007. 17 November 2007 <http://www.ausetute.com.au/trendpd3.html>.

Conclusion:

After following the necessary procedure, taking the correct measurements and recording all the results, I can say the following about each of the three period 3 chlorides tested: firstly, sodium chloride and magnesium chloride both consist of giant ionic lattice structures at room temperature, and both are ionic compounds. The ionic bonds that hold their structures together are formed as a result of the metal ions (of sodium and magnesium) reacting with the non-metal ions (of chlorine). Ionic bonds are very hard to break because of the strong forces of attraction between the ions, and so that’s why sodium chloride and magnesium chloride both have such high melting and boiling points. Also since in the solid state the ions are not free to move (lack of mobility), then this is why ionic compounds do not conduct electricity in such a state. But when the bonds are broken (in the liquid/molten state), the ions are free to move and so that’s why conductivity improves.

Aluminium chloride on the other hand is different from the other two chlorides, even though it is an ionic compound. From the table on the previous page, we can see that one of these differences is that it has a low melting and boiling point when compared to the other two ionic compounds. When aluminium chloride is in the solid state it is an ionic compound, but when it melts into a liquid, it changes to a covalent structure. Covalent bonds are formed when two or more atoms share a pair of electrons. And so because of this, covalent structures have no free electrons and thus cannot conduct electricity. This is why aluminium chloride in the liquid state is a poor conductor of electricity.

If we take a look at the temperature change between each chloride, we can see that sodium chloride and magnesium chloride both have a small change (showing the physical process of dissolving), while aluminium chloride has a large temperature change (showing the chemical process of hydrolysis). And if we take a look at the pH change for each of them, we can see that when aluminium chloride was added to water, the pH decreased from 7 to 2. This decrease in pH indicates that hydrolysis has taken place. So therefore, I can conclude that both sodium chloride and magnesium chloride dissolve when added to water, while aluminium chloride undergoes hydrolysis when added to water.

Evaluation:

During the experiment there were a couple of errors in the measurement of the temperatures and pH of the solutions, and in the volume of distilled water used.

Weaknesses in my method (Sources of error):

While measuring the pH of the solutions it was a bit hard determining the colour change to compare with the pH colour chart.

A measuring cylinder was used to measure the volume of distilled water used.

When the thermometer was placed in the solution of aluminium chloride and water/ acetone, the temperature increased rapidly and so it was hard to follow the meniscus of mercury and note the highest reading.

Prevention of those errors:

When measuring the pH of the solutions, a white tile or even white piece of paper should be kept behind the test tube so as to find out the correct colour change to be compared with that on the pH colour chart. Usage of a digital pH meter would make the results much more accurate.

A pipette should be used instead of a measuring cylinder in order to accurately measure the volume of the distilled water.

The error due to the thermometer could be improved by repeating the measuring of the temperature of aluminium chloride with water/acetone at least five times, and finding the average value. Otherwise, a digital thermometer could be used to improve the accuracy of the results.

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