Lab Report: Effect of Electrolyte Concentration on Voltaic Cell Potential

Abstract

Redox reactions play a crucial role in generating electric currents in voltaic cells. This experiment investigates the impact of varying concentrations of negative terminal electrolyte (zinc sulfate, ZnSO4) on the potential difference in a voltaic cell. The hypothesis posits that lower concentrations of the negative terminal electrolyte will result in higher potential differences. Through experimentation, it was observed that as the concentration of ZnSO4 decreased, the potential difference in the voltaic cell increased, confirming the hypothesis. The proper setup of electrodes, a salt bridge, and a voltmeter ensured accurate measurements.

Introduction

Redox reactions, also known as oxidation-reduction reactions, involve the transfer of electrons from one reactant to another. In these reactions, two half-reactions occur: one reactant loses electrons (undergoes oxidation), while another gains electrons (undergoes reduction). Voltaic cells are devices where spontaneous redox reactions take place, generating electric currents. To harness this electron transfer for practical use, the electrons must flow through an external electrically conducting wire, rather than directly between the oxidizing and reducing agents.

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To prevent electrode polarization and facilitate the circulation of ions from the electrolyte, a salt bridge is utilized. Each electrode is connected to a voltmeter through clips and wires, allowing the measurement of the voltage generated by the redox reaction. The voltage reading is positive when the electrodes are correctly connected for a spontaneous reaction. For a redox reaction to occur, the species with the higher reduction potential must serve as the cathode.

Research Question

What is the effect of different concentrations of the negative terminal electrolyte (zinc sulfate, ZnSO4) on the potential difference in a voltaic cell?

Hypothesis

The lower the concentration of the negative terminal electrolyte (ZnSO4), the higher the potential difference in the voltaic cell.

Variables

Manipulated Variable: Concentration of negative terminal electrolyte (ZnSO4)

• Concentrations used: 1.0M, 0.10M, 0.010M, and 0.0010M ZnSO4
• Preparation method: Weighing the specified amount of ZnSO4 powder, dissolving it in distilled water, and adjusting the volume to 250ml in a volumetric flask for each concentration.

Responding Variable: Potential difference (measured with a voltmeter)

Fixed Variables:

• Type of electrode: Copper and zinc electrodes
• Size of electrodes: 5cm x 1cm
• Positive terminal electrolyte: 1.0M copper (II) sulphate (CuSO4)
• Type of salt bridge: Sodium nitrate (NaNO3)

Materials and Apparatus

Materials:

• Copper (II) sulphate powder
• Zinc sulphate powder
• Sodium nitrate powder
• Cotton string
• 0.5mm copper sheet
• 0.5mm zinc sheet
• Paper towel

Apparatus:

• 500ml beaker
• 100ml beaker
• 50ml beaker
• Voltmeter
• Connecting wires
• 100ml measuring cylinder
• Electronic balance
• Glass rod
• 250ml volumetric flask
• Meter rule
• Scissors

Procedure

Preparation of Zinc Sulphate Solution:

1. Weigh out 40.00g, 4.00g, 0.40g, and 0.04g of zinc sulphate powder separately.
2. Dissolve each amount of powder in distilled water in separate 50ml beakers.
3. Pour each zinc sulphate solution into labeled 250ml volumetric flasks.
4. Add distilled water to each volumetric flask to reach a total volume of 250ml and mix well.

Preparation of Salt Bridge:

1. Weigh out 100g of sodium nitrate powder and dissolve it in distilled water in a 50ml beaker.
2. Pour the sodium nitrate solution into a 250ml volumetric flask and dilute it to a total volume of 250ml.
3. Soak a 15cm cotton string in the sodium nitrate solution to create the salt bridge.

Preparation of Copper (II) Sulphate Solution:

1. Weigh out 40g of copper (II) sulphate powder and dissolve it in distilled water in a 50ml beaker.
2. Pour the copper (II) sulphate solution into a 250ml volumetric flask and dilute it to a total volume of 250ml.

Voltaic Cell Setup:

1. Measure 70ml of zinc sulphate solution prepared with different amounts of zinc sulphate powder.
2. Measure 70ml of 1.0M copper (II) sulphate solution.
3. Pour both solutions into separate 100ml beakers.
4. Cut out 5cm x 1cm copper and zinc sheets as electrodes.
5. Set up the voltaic cell as shown in the diagram.
6. Record three readings of the voltmeter.

Results

The potential difference (voltage) in the voltaic cell was measured for different concentrations of negative terminal electrolyte (ZnSO4). The results are shown in the table below:

Data Analysis

The data collected can be analyzed to observe the relationship between the concentration of ZnSO4 and the potential difference. The following formula can be used:

Potential Difference (V) = E° - (0.0592/n) * log([Zn2+])

Discussion

The results of this experiment indicate that as the concentration of the negative terminal electrolyte (ZnSO4) decreases, the potential difference in the voltaic cell increases. This supports the hypothesis that lower concentrations of ZnSO4 lead to higher potential differences. The phenomenon can be explained by the Nernst equation, which describes the relationship between ion concentration and electrode potential in electrochemical cells.

The Nernst equation for this experiment can be written as:

E = E° - (0.0592/n) * log([Zn2+])

Where:

• E is the cell potential
• E° is the standard cell potential
• n is the number of electrons transferred in the reaction
• [Zn2+] is the concentration of zinc ions
• [Cu2+] is the concentration of copper ions

According to the Nernst equation, as the concentration of zinc ions ([Zn2+]) decreases (as observed in lower concentrations of ZnSO4), the cell potential (E) increases. This explains why we observed higher potential differences in the voltaic cell as the concentration of ZnSO4 decreased.

Furthermore, the voltaic cell was set up with copper as the cathode and zinc as the anode. This arrangement ensured that the copper ions (Cu2+) were reduced at the cathode, while zinc ions (Zn2+) were oxidized at the anode. The reduction potential of copper ions is higher than that of zinc ions, leading to a positive cell potential and spontaneous redox reaction.

Conclusion

The experiment demonstrated that the concentration of the negative terminal electrolyte (ZnSO4) in a voltaic cell has a significant effect on the potential difference. As the concentration of ZnSO4 decreased, the potential difference increased, supporting the hypothesis that lower concentrations of the negative terminal electrolyte result in higher potential differences. This phenomenon is explained by the Nernst equation, which describes the relationship between ion concentration and electrode potential.

Recommendations

Further investigations could explore the impact of temperature on voltaic cell potential, as temperature can also influence the rate of redox reactions. Additionally, different combinations of electrode materials and electrolytes could be studied to understand their effects on cell potential and applications in practical devices.