The amount of current in a circuit depends on the amount of voltage available to move the electrons, and also the amount of resistance in the circuit to oppose electron flow. Here are the standard units of measurement for electrical current, voltage, and resistance in Figure 2:
Hypothesis 1
In reference to the Nernst Equation and Le Châtelier principle, our group believes that by having increased concentration in the cathode and a reduced concentration in the anode in each electrolyte will cause the voltage and current readings to increase and improve the Daniell Cell.
The general concentration of a Daniell Cell is presumed to be of 1 M (molar) for the theoretical E0 values to abide by the Standard Reduction Potentials for Half-Reduction table (Redox Table). Research and equations were used to identify the hypothesis, some of which were the Nernst Equation and Le Chatelier’s Principle both of which are involving the equilibrium between concentrations.
Electrochemistry deals with cell potential as well as energy of chemical reactions. The energy aspect is also related to the chemical equilibrium. All these relationships are tied together in the concept of Nernst equation. The maximum potential difference is called the electromotive force (EMF). The Nernst equation also indicates that you can build a battery simply by using the same material for both cells, but by using different concentrations. Cells of this type are called concentration cells. < All about Electrochemistry, by Stephen Lower, 2005>
Suppose, for example, that by reducing the concentration of Zn2+ in the Zn/Cu cell from its initial value of 1M to a much smaller value
Zn(s) | Zn2+ (aq, 0.1M) || Cu2+ (aq) | Cu(s)
This will then increase the E value for the cell reaction
Zn(s) + Cu2+ → Zn2+ + Cu(s)
The Le Chatelier’s Principle can explain why reducing the concentration in the anode and increasing it in the cathode will improve the overall voltage of the cell. If a chemical system at equilibrium experiences a change in concentration, then the equilibrium shifts to counter-act the imposed change. Changing the concentration of the electrolyte will shift the equilibrium to the side that would reduce that change in concentration. <Chemistry Guide, 2003>. In the case of the Daniell Cell when the Copper sulfate concentration is increased, the equilibrium positioned is moved and more of the solution is used, and thus generates higher voltage. Likewise, increasing the concentration of the solution of the Zinc sulphate results in the reverse effect, where the equilibrium moves in the opposite way and less of the solution is used, thus generating lower voltage. In its most fundamental form the Nernst equation is written as: See Figure 3
Where RT is the room temperature, z is the charge number of the electrode reaction (which is the number of moles of electrons involved in the reaction), and F is the Faraday constant (96,500 C mole-1). <Science Waterloo, 2008> The notation ared represents the half cell which appears on the reduced side of the cell and the notation aox represents the half cell which appear on the oxidized side of the cell.
The concentration in the cathode should be increased since it speeds up the reaction of the anode forcing more electrons to flow through a resistor. By completing and testing this hypothesis our group believes that it will factor in the aim of increasing the voltage and current of our Daniell Cell.
Hypothesis 2
According to the Standard Reduction Potentials for Half-Reactions table, our group hypothesises that by changing the half cells so that their E0 values are a greater distance apart that the voltage and current will show an increase and improve the Daniell Cell.
The general make up of a Daniell Cell is two half cells, one with copper metal (Cu) and the other cell using zinc metal (Zn). The electrolyte then is both sulfates of each metal, CuSO4 and ZnSO4 making up the Cell. According to the Standard Reduction Potentials for Half-Reductions (SRPHR), Copper and Zinc is a good pair since Copper is a fairly strong oxidizing agent and Zinc is a very good reducing agent which gives a high voltage between these two half cells. <Corrosion Doctors, 2006>
The group has decided in changing one of these half cells to a better oxidizing agent or a reducing agent the voltage of our cell will increase and therefore support our hypothesis. Since this is a school experiment, it is very hard to access the best oxidizing agents, which has left us with Potassium Permanganate (KMnO4) as our best oxidizer and magnesium (Mg2+) as our best reducing agent.
When both ionic concentrations (the electrolyte) are at 1M and the room temperature is at 25oC you can calculate the voltage of what two half cells would be by using the given E0 value. Copper has an E0 of +0.34 volt and Zinc has an E0 of -0.76 volt and you subtract the oxidizer from the reducing agent which gives 1.10 V (volts). The calculations for the magnesium and potassium permanganate are as follows, potassium permanganate E0 value is 1.52 and the magnesium E0 value is -2.37 which gives 3.89 volts. Under these conditions the voltage should increase from 1.10 volts to 3.89 volts, in which will support our second hypothesis.
Hypothesis 3
After extensive research into different forms of salt bridges, our group believes that in using the porous pot salt bridge the decrease in resistance provided by the pot will increase the voltage and current readings of the original Daniell Cell.
A salt bridge, in chemistry, is a device used to connect the oxidation and reduction half-cells of an electrochemical cell. It is also apart of the internal circuit of the cell with the external circuit being the wire that connects the anode to the cathode. Salt bridge’s usually comes in two types: glass tube and filter paper with other portable options still available.
One type of salt bridges consists of U-shaped glass tubes filled with a relatively inert electrolyte, usually potassium nitrate. The conductivity of the glass tube bridges depends mostly on the concentration of the electrolyte solution. The other type of salt bridges consists of a filter paper, also soaked with a relatively inert electrolyte, usually potassium nitrate because it is chemically inert. Conductivity of this kind of salt bridges depends on a number of factors: the concentration of the electrolyte solution, the texture of the filter paper and the absorbing ability of the filter paper. Generally smoother texture and higher absorbency equates to higher conductivity. A porous pot or other porous barrier between the two half-cells may be used instead of a salt bridge; they give a much higher current flow since the resistance is much less which is ideal for this investigation. <Chemistry Virtual Textbook, 2006>
In the Daniell Cell the porous pot cell consists of a central zinc anode dipping into a porous pot containing the zinc sulfate solution. The porous pot is, in turn, immersed in a solution of copper sulfate contained in a large beaker, which acts as the cell's cathode. The use of a porous barrier prevents the copper ions in the copper sulfate solution from reaching the zinc anode and undergoing reduction. This would render the cell ineffective by bringing the battery to equilibrium. Our group has hypothesised that using the Porous Pot as the salt bridge will bring upon an increase in voltage and current therefore making a “Better Battery”.
Balanced Chemical Equations and E0 values
Zn2+ (aq) + 2e- → Zn(s) E0 = -0.76 V
Cu2+ (aq) + 2e- → Cu(s) E0 = 0.34 V
Zn(s) → Zn2+ (aq) + 2e-
Cu2+ (aq) + 2e- → Cu(s)
Zn(s) + Cu2+ (aq) → Zn2+ (aq) + Cu(s)
E0C = E0ox + E0red
= 0.76 + 0.34
E0C = 1.10 V
Mg2+ (aq) + 2e- → Mg(s) E0 = -2.37 V
Cu2+ (aq) + 2e- → Cu(s) E0 = 0.34 V
Mg(s) → Mg2+ (aq) + 2e-
Cu2+ (aq) + 2e- → Cu(s)
Mg(s) + Cu2+ (aq) → Mg2+ (aq) + Cu(s)
E0C = E0ox + E0red
= 2.37 + 0.34
E0C = 2.71 V
5 Mg2+ (aq) + 10e- → 5 Mg(s) E0 = -2.37 V
2 MnO4- (aq) + 16 H+ + 10e- → 2 Mn2+ + 8 H20 (l) E0 = 1.52
5 Mg(s) → 5 Mg2+ (aq) + 10e-
2 MnO4- (aq) + 16 H+ + 10e- → 2 Mn2+ + 8 H20 (l)
5 Mg(s) + 2 MnO4-(aq) + 16 H+ → 5 Mg2+ (aq) + 2 Mn2+ + 8H2O (l)
E0C = E0ox + E0red
= 2.37 + 1.52
E0C = 3.89
Method
Daniell Cell
1. Using two beakers place them close together and fill the beakers half way with the chosen electrolytes.
2. Place the Copper and Zinc metals in their respective salt solutions which are both 0.1 M in concentration
3. Connect the wires to each electrode (metal) and connect the wires though a voltmeter/ammeter
4. Saturate the salt bridge (filter paper) with Potassium Nitrate and insert each end into both half-cell solutions.
5. Record Results for both current and voltage
Hypothesis One
1. Setup the original Daniell Cell
2. After setting up and functioning the original Daniell Cell, change the concentration of the CuSO4 solution to 0.5M.
3. Record the results shown on the voltmeter and ammeter
4. After this, change the concentration of the CuSO4 solution to 1M
5. Record the results shown on the voltmeter and ammeter.
6. Repeat this with the zinc half cell, by changing the copper half cell back to 0.1M and increasing the zinc sulfate to 0.5M then 1M.
7. Record the results of the voltage and current for these two experiments.
Hypothesis Two (A)
1. Place two beakers side by side, filling them up half way with MgSO4 0.1M and CuSO4 0.1M.
2. Place the metals in their respective salts (sulfate solutions)
3. Connect the wires to each metal and plug the ends into the ammeter/voltmeter.
4. Now that the external circuit is present, saturate the salt bridge (in this case, filter paper) with Potassium Nitrate (KNO3) solution and place into each half cell.
5. Turn on the voltmeter/ammeter and record results.
Hypothesis Two (B)
1. Place two beakers side by side, filling them up half way with MgSO4 0.1M and MnO4 0.02M
2. Place the magnesium metal in its respective salt
3. Place an inert metal such as carbon in the MnSO4 solution
4. Connect the wires to each metal and plug the ends into the ammeter/voltmeter
5. Now that the external circuit is present, saturate the salt bridge (in this case, filter paper) with Potassium Nitrate (KNO3) solution and place into each half cell.
6. Turn on the voltmeter/ammeter and record results.
Hypothesis Three
1. Using a larger beaker, fill with the Cathode (Copper) electrolyte to around ½ full
2. Place the Porous Pot (which has been soaked in KNO3) in to the beaker and pour the other electrolyte (Zinc) inside.
3. Add the two electrodes to their distinguished salts.
4. Turn on the ammeter/voltmeter and record the results.
Super Cell
1. Repeat steps 1, 2 and 3 in hypothesis three.
2. Instead of using Copper and Zinc half cells, use MnO4 (Potassium Permanganate) and MgSO4 (Magnesium) with MgSO4 being the anode.
3. The concentrations for each half cells are as follows; KMnO4 0.02M and MgSO4 0.1M
4. Use an ammeter/voltmeter to receive the final data for the investigation.
Results Tables
Hypothesis One:
In reference to the Nernst Equation and Le Châtelier principle, our group believes that by having increased concentration in the cathode and a reduced concentration in the anode in each electrolyte will cause the voltage and current readings to increase and improve the Daniell Cell.
Voltage (V)
Current (mA)
Hypothesis Two:
According to the Standard Reduction Potentials for Half-Reactions table, our group hypothesises that by changing the half cells so that their E0 values are a greater distance apart that the voltage and current will show an increase and improve the Daniell Cell.
Voltage (V)
Current (mA)
Hypothesis Three:
After extensive research into different forms of salt bridges, our group believes that in using the porous pot salt bridge the decrease in resistance provided by the pot will increase the voltage and current readings of the original Daniell Cell.
Safety
During the investigation the group used five different types of electrolytes for the extent of the experiment. These five solutions were Zinc Sulfate, Copper Sulfate, Potassium Nitrate, Potassium Permanganate and Magnesium Sulfate. These five solutions yielded the property that meant they could be dissolved into water with Zinc, Potassium and Magnesium all forming a clear colour. Copper Sulfate forms a blue solution, whereas Potassium Permanganate forms a dark purple solution.
Throughout the duration of the investigation students must wear safety glasses along with a lab coat to protect their skin from any contact with the solutions. Keep the experimental area clear of books and other items that may cause an incidence of spilling or breaking equipment. In an emergency situation, such as contact with the eyes and skin, the patient must flush and wash away the solution for fifteen minutes and remove all contaminated clothing. If large amounts of any of these solutions are ingested do not induce vomiting and call for a physician immediately.
Conclusion and Discussion
In relation to the first hypothesis the results have supported what has been stated. From an in depth study of both the Nernst and Le Chatelier principles the hypothesis was made that by changing the concentration in the cathode and keeping the anode at its lowest concentration will show an increase in current compared to the original Daniell Cell. The average of the Daniell Cell’s voltage and current was 0.97 volts and 0.22 mA, then the concentration in the cathode and anode were increased and decreased giving positive results. When our hypothesis was tested the current, as expected, increased vastly going from an average of 0.22 → 0.50 mA by just changing the concentration. As well as the current the voltage also increased slightly going from 0.97 → 1.11 volts. This can be explained by using Ohm’s Law which is: which can be said as; when increasing I (current) as long as the resistance stays the same the voltage is also going to increase.
The second hypothesis was once again supported by encouraging results. In this case both the cathode and anode were getting changed at different times in hoping finding two half cells that gave the best voltage reading. The anode was changed first and by doing this the group achieved an improved reading, to complete the hypothesis the cathode was then changed which meant the new battery was using the two half cells that were the furthest away from each other that could be obtained in classroom conditions. The Daniell Cell’s starting voltage was 0.97 volts and when using Carbon + KMnO4 0.02M as the cathode and Copper + CuSO4 0.1M as the anode the reading we received was 2.84 V which was a much better results to the original Cell. Le Chatelier’s Principle can be used to explain the decrease in voltage (compared to the redox table) in the results involving these two half cells. Since the concentration in the cathode is lower in contrast to the anode the shift in equilibrium is towards the anode which then causes less of the solution used giving a lower voltage. The current was increased by 159.1 % which then can be clarified by Ohm’s Law which says that when increasing voltage the current will also increase when the resistance is kept at a constant.
The third and final hypothesis was in relation to the changing of the salt bridge which was hoping to bring about a change in the current flow. The group only tested one other type of salt bridge due to time constraints which proved very effective. The porous pot was a new type of salt bridge which caused the internal circuit to be much more efficient and also make the battery portable. The current reading given by using the porous pot salt bridge was far better then the group expected, going from 0.22 mA → 27 mA (12172 % increase) which was a very pleasing result firmly supporting our hypothesis. The voltage also showed an improvement of over 11 % which shows strong support again to our hypothesis.
Therefore our “Super Cell” was able to be formed by observing these result tables. Since the aim of this investigation was to find the best battery with the most current and voltage these changes must be made. The two half cells have to be as far away as possible on the redox table to cause the highest E0 value or voltage. The salt bridge that must be used is the porous pot as it provides a very high current reading and causes the battery to be more realistic as it will be portable. The last change is that the cathode must have the highest concentration for an extra increase in current. This factor was not testable for our super cell as the only concentration for the MnO4 was 0.02 M
Our “Super Cell” is then as follows
Mg(s) + MgSO4 (0.1M) & MnO4 (0.02M) + C(s)
Using Porous Pot soaked in KNO3
Voltage = 3.0740.001 V (216.91% difference)
Current = 1191 mA (53,990% difference)
These results show the vast improvement of the new “Super Cell” compared to the original Daniell Cell. The aim has therefore been attended to and completed and all three hypotheses have been supported with compatible results.
Evaluation
Although the results received were very positive and back up all hypotheses there were many limitations and errors that could have been taken care of in the future.
- The external circuits or the wires connecting each half cell almost all had rust on them which acts as a resistance causing a reduced voltage and current. The rust was also not consistent on each set of wires, which meant when the group used different wires the voltage and current readings differed each time.
- The volt and ammeters were very inaccurate at times and most of the time not giving a current reading which meant there was less time to carefully finish each experiment which then caused more inaccuracy in the results.
- Another error which occurred due to equipment, which was unfortunately unavoidable, was each of the solutions used. After each reading was taken we were to pour the solutions back into their containers. The solutions may have been contaminated whilst the cell was operating and as the solution was then poured back into the original container the contamination spread. This would eventually decrease the concentration of electrolyte and as such change the data, decreasing the voltage and current. To fix this error, the solutions should not be poured back into the containers again.
- Another error was the lack of trials within the experiment. Due to time constraints we complete the desired number of trials for each hypothesis. It is desirable to have as many tests as possible, one trial to establish the basis while the others are trying to be as accurate as possible. By not having a complete number of trials we limited the results that have been found. If we were able to complete more trials, we would have been able to have a fairer and more accurate result. We would be able to overcome this by having a longer time to conduct our trials. This would have given more time to be more precise with the work that was completed.
- A different filter paper salt bridge should have been used each time testing, since when used once the paper soaks up the electrolytes of each half cell which becomes inexact next time it’s put into use. The filter paper also dries out after a while which reduces the current of the cell, making the experiment more inaccurate as time progresses.
-
The group was only provided with 0.02 M KMnO4 which then forced us to not put one of our hypotheses into use in creating our “Super Cell” which was having the concentration in the anode lower then the concentration in the cathode.
-
Another error was the use of Carbon as the inert metal in the KMnO4 solution. Since Carbon is not completely inert the current was unable to flow, free of resistance from the Carbon, therefore reducing the current reading of the cell. The magnesium metal was also an issue as it tends to oxidise instantly with oxygen causing an oxide layer on the metal and also reducing the current reading of the cell.
Further investigation could possibly be to test many other hypotheses such as the surface area of the metals to make our results more thorough. More electrodes could have been added to the half-cells. This would create a cell with a larger surface area in which the reactions would take place. This would then allow a larger current and voltage as there would have been more electrons to create a higher current. There would have been a larger amount of area in which the electrons would be able to be taken from. The larger the surface area, more electrons are able to be attracted to the electrode, consequently producing more voltage and current. <Russian Chemical Views, 2009>
Cutting edge battery research and development have allowed improvements such as changing the metals in the half cell to still make it more efficient eg. Lithium. Lithium-ion batteries are incredibly popular these days. They're so common because, pound for pound, they're some of the most energetic rechargeable batteries available. Lithium batteries are disposable batteries that have lithium metal or lithium compounds as an anode. The term "lithium battery" comprises of many types of cathodes and electrolytes. The most common type of lithium cell used in consumer applications uses metallic lithium as anode and manganese dioxide as cathode, with a salt of lithium dissolved in an organic solvent. (Brain, 2008)
Improvements such as the electrodes being further apart in the redox table, eg lithium and silver would have been able to create a higher voltage. Again this is a problem because it is very unsafe as lithium is so reactive that it will react with the oxygen in the air. Therefore it may not be possible to create a Daniell Cell out of those materials. Further research into looking into ways of creating more reactive yet stable electrodes we could create a cell which supports the claims of the Redox table but is also able to be completed within a class room environment.
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