Investigating the effect of ion concentration in electrolytes on the potential difference in a voltaic cell

Variables

Independent

• Electrolyte content
• Electrolyte concentration

Dependent

• Voltage

Controlled

• Type of electrode
• Volume of electrolyte

Method

1. Connect the copper and iron electrode to the voltmeter by using the connecting wire. Attach the electrodes to the wires by alligator clips.
2. By using a measuring cylinder, measure out 50 ml of iron (II) sulfate and pour it into a beaker. Do so for copper (II) sulfate as well. Ensure that both electrolytes are of the same concentration.
3. Connect the two beakers of electrolyte by the salt bridge.
4. Place the two electrodes in their corresponding electrolytes – the iron electrode in the Iron (II) sulfate solution, and the copper electrode in the copper (II) sulfate solution.
5. Repeat steps 1~5 for the following concentrations (mol dm-3): 0.16, 0.12, 0.8 and 0.04. Make the desired concentration by adding appropriate volume of distilled water to the electrolytes. Use the equation,

Mole 1 x Volume 1 = Mole 2 x Volume 2

to calculate the volume of distilled water and electrolyte needed

6. Repeat steps 1~6 for the following combinations:

7. Record the potential difference under the following table format:

Diagram

Data Collection

*Note: the exact uncertainty of the concentration is unknown, but it is assumed to be ± 0.01

Processed Data

In the above table, the percentage change is calculated as follows:

Note: the uncertainty in this table is different for every result, but it is assumed that they all have the value of ±0.01

Note: the average is calculated by adding all three voltage values for a given concentration, and dividing the sum by three.

The overall average was calculated across different metal and metal ion combinations. The rationale behind this is that although each combination for a certain concentration would generate a difference voltage, it should exhibit a similar trend across the different concentrations.

Conclusion

It seems from Graph 1 that as the electrolyte concentration increased, the potential difference decreased. This opposes the hypothesis, which states otherwise. By using data from this experiment alone, it seems that one can conclude as follows: the potential difference and electrolyte concentration have a negative correlation; as one increases, the other decreases. However this opposes the rules known already, such as the collision theory.

To assess the degree of accuracy of this experiment’s results, one can compare it with the Nernst Formula, which is stated as follows:

And the variables are as follows:

E = electrode potential

E° = standard electrode potential

R = universal gas constant

T = temperature

n = number of electrons accepted

F = Faraday (96500 coulombs)

Where [M(s)] is assumed to be 1 because it stays the same.

This equation is used to By using this equation, and assuming that temperature was at 298 K, the following literature values can be generated:

By comparing Graph 2 to Graph 1 it seems that the experimental data did not conform to literature data. A possible explanation for this may be that the distilled water was contaminated with ions. The result of this would be that other ions can be reduced and oxidized as well, and this would render the results inaccurate.

Evaluation

In this experiment the controlled variables were the type of electrode and volume of electrolyte. However, it does not take into account the temperature of the electrolyte and electrode, which can affect the potential difference. To eliminate this limitation, in the future the electrolytes should be warmed to 25℃. Also, the beakers which contain the electrolyte should be placed in water bath of 25℃ in order to maintain the temperature. In addition, a thermometer should be placed in both beakers to monitor the temperature and ensure that it stays within ±1℃ throughout the experiment.

When the voltaic cell was connected, it was noticed that the reading on the voltmeter fluctuated every few seconds or so. This made the recorded data somewhat limited because one had to estimate the most frequently-occurred voltage reading. To improve the data collection, one can connect the voltmeter to a computer or calculator which has a graphing system. Connect the voltaic cell for exactly ten seconds, and by using the graph generated by the computer or calculator, one can calculate the exact average voltage generated. This would make the results more reliable, since it decreases the amount of subjectivity needed to record the data.

A possible source of error as mentioned in the conclusion is in the distilled water. It may have been contaminated, or the filter in the ion exchange resin may need to be changed. However, note that if this were true then all the result should have been influenced in the same way. As a result the general trend would still be the same.

To extend this investigation, one can investigate on the effect of temperature on the potential difference generated in a voltaic cell. This is an area worthy of further investigation because like the electrolyte concentration, temperature is one of the factors determining the electrode potential. Similar to the electrolyte concentration, the temperature and potential difference should have a positive correlation as well.

---