Investigating Solutions Essay

In this experiment the effect of concentration of different solutions will be investigated against conductivity – The conductivity of an electrolyte solution is a measure of its ability to conduct electricity. The SI unit of conductivity is Siemens per meter (S/m) but can me also measured as microsiemens per meter (S/m). Generally most of Ionic substances conduct electricity whilst molecular substances do not. This is because when ionic substances are dissolved; the electrons are free to move therefore carrying an electric charge.

Research Question – What is the effect of concentration on conductivity in ionic substances?

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Hypothesis – I think that as the concentration increases the conductivity will also increase. I predict this because as there are more ions that can carry a charge and therefore conduct electricity. Also I think that CaCl2 will be the solution that will conduct the most electricity because it is more ionic than NaCl as it is in group 2 (looses more electrons) and AlCl3 shows some covalent properties which reduce the conductivity.

Diagram –

Equipment –

* Lab Quest

* Conductivity probe (set to a range of 0-2000 S.m)

* Utility clamp and stand.

* Dropper bottle of distilled water

* 100cm3 empty beaker

* 400ml of 1M NaCl

* 400ml of 1M CaCl2

* 400ml 1M AlCl3

* 100cm3 measuring cylinder

Risk Assessment –

* Wear goggles at all times, especially when handling the chemicals.

* If your hair is long tie it back.

Variables –

Dependent Variable –

* Conductivity of solution (S/m) Will be measured by a conductivity probe connected to a Data Logger – Lab Quest. The range of the probe will be of 0-2000.

Independent Variable –

* Concentration of solutions (M) This is what I am going to change to have a range of values. The solutions are manually prepared for each concentration. For each solution there will be five concentrations – 1M, 0.8M, 0.6M, 0.4M and 0.2M. These will arrive from the 1M solution by distillation. I will use 3 different with 5 concentrations each, which will act as a repeat as well as another observation – Which of the three is most ionic and therefore conducts the most electricity and why.

Controlled Variables –

* Amounts of solution – The amount of each solution measured should be kept the same (at 100ml) to ensure the trials are kept fair. More of a solution means more electrons available to carry a charge therefore increases conductivity.

Method –

1. Plug the Conductivity Probe into Channel 1 of the Lab Quest. If needed, calibrate the conductivity probe to 0. Set up the apparatus as shown in the previous diagram. Using the measuring cylinder, measure 100ml of NaCl and pour this into the beaker.

2. Carefully raise the beaker and its contents (e.g.100ml of 1M NaCl) up around the Conductivity Probe until the hole near the probe end is completely submerged in the solution being tested. Important: Since the two electrodes are positioned on either side of the hole, this part of the probe should be submerged to get the correct reading.

3. Whilst doing this observe the reading on the data logger. Consider recording the highest but most instant value of each solution on the results table.

4. After recording, with the distilled water bottle rinse the conductivity probe over the empty beaker so that future results will not be affected by the remains of other solutions. Be careful as this process can be quite uncontrolled as water can sometimes project to the sides.

5. Using the measuring cylinder proceed to make other concentrations by distillation. E.g. for 0.8 M measure 80ml of NaCl and 20 ml of distilled water. For 0.6 M measure 60ml of NaCl and 40 ml of distilled water. And so on.

6. Repeat steps 2-5 for the other 2 solutions (CaCl2 and AlCl3)

Results –

Table 1. Conductivity results for the 5 concentrations of each of the 3 salts,

NaCl, CaCl2 and AlCl3

Conductivity/ S.m

Concentration / M

NaCl

CaCl2

AlCl3

0.2

32,930

33,152

33,53

0.4

33,151

32,917

33,782

0.6

33,689

33,287

34,022

0.8

33,893

33,818

34,084

1

33,812

34,203

34,705

Qualitative results: Some unexplained viscosity was observed halfway through calcium chloride and all the way through aluminium tetrachloride.

Conclusion and Evaluation-

Two conclusions can be taken directly from this investigation and its results:

* Generally, as the concentration of the solution increases, so does the conductivity. There is a significant difference in values of conductivity as the concentration increases. Therefore there is a direct link between conductivity and concentration of solution.

* A further side conclusion that can be derived from this experiment relates to how ionic the three salts are. It is obviously seen that Aluminium tetrachloride is the most ionic as its conductivity was highest at all concentrations; yet, there is a strange relationship between sodium chloride and calcium chloride as seen on the graph.

For the first conclusion, I believe I have sufficient data in order to accept that there is a significant difference (Even though there are a few anomalies which will be discussed later). In each case, as seen in the graph, the conductivity mostly increased as the concentration of each solution increased. Some reasons are behind this fact:

For conductivity to occur the substance must possess electrons or ions that are free to move1. As the ion concentration in the solution increases, more ions (electrons) are available to carry a charge as shown below in the diagram2:

When a potential gradient is applied to the solution, the delocalized electrons (which are free to move) are able to travel towards the positive end of the gradient carrying a charge.

Since the graph follows the pattern I believe in this conclusion quite strongly as many quotes from the internet and some books say:

“The conductance of electrolytic solutions depends on the concentration of the ions and also on the nature of the ions present (through their charges and mobilities), and conductance behaviour as a function of concentration is different for strong and weak electrolytes. ” 3

The first conclusion coincides with my hypothesis however the second conclusion does not correlate completely. It was expected that the order of salts that conduct the most was to be in increasing order: NaCl, CaCl2, and AlCl3. However, sodium chloride and calcium chloride are more difficult to decide on as the first and last value of calcium chloride was higher than that of sodium chloride as seen on the graph. The theory should be that as each salt dissociates in water to form a solution, ions are available to conducted electricity:

e.g.

NaCl –> Na+ + Cl-

CaCl2–> Ca2+ + 2Cl-

AlCl3 –> Al3+ + 3Cl-

Aluminium tetrachloride has the biggest number of ions when it dissociates but it has some covalent properties. Yet, my hypothesis was wrong regarding this. It is difficult to come up with a conclusion for the first 2 salts but the general pattern is that the more ions the salt dissociates to the bigger the charge that can be carried. Therefore sodium chloride conducts less electricity than calcium chloride. Aluminium proved to be the best conductor because when aluminium tetrachloride is in water the covalent properties are mostly lost and become ionic. 4

The results from this investigation were wide and had some anomalies. Even though anomalous results were found generally the conductivity of each salt increased proportionally to increasing concentrations. The last result, 1M, of NaCl was lower than that of 0.8M: 33,893 –>33,812. The first result of CaCl2, 0.2M, was higher than the second result, 0,4M: 33,152–>32,917. These are hard to explain why they happened but it can be due to human errors such as measuring the amount of solution or distilled water in the measuring cylinder wrongly. Another reason would be shaking the solution; if the solution when shaken when the probe was reading the value would increase. Therefore some solutions could have been shaken more than others. One way to improve this would be to by putting the probe into the beaker which would be still on the table and let it be for some seconds.

In order to decide the second conclusion with more confidence I believe that at least 2 repeats for each salt should have been made which should prove to decrease uncertainties about the conclusion and increase the reliability of data.

Overall some errors and anomalies can be explained, some not and others we do not know for certain. In all I believe the effect of the anomalous results on the investigation was insignificant for the first conclusion and can be explained with human errors however the repeats for the second conclusion should be made. Still I believe the data is reliable and that this investigation and research was effective and efficient as a whole to show that conductivity increases as concentration of the solutions increases.

References –

1. IB study guides, Chemistry for the IB diploma, Geoffrey Neuss pg.23

2. http://cfbt-us.com/wordpress/?cat=5

3. http://www.tau.ac.il/~phchlab/experiments/Conductivity/conductivity.htm

4. http://www.chemguide.co.uk/inorganic/period3/chlorides.html

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