Concentration Cell

North Carolina School of Science and Mathematics
15 Dec 201103:18

Summary

TLDRThe experiment involves setting up an electrochemical cell using two copper half-cells with different copper sulfate solution concentrations (0.1 M and 0.2 M). A salt bridge connects the cells, and the voltmeter shows a positive voltage with oxidation happening in the lower concentration cell. As copper ions are exchanged, the concentrations in both half-cells equalize, causing the voltage to drop to zero. Adding sodium sulfide to the higher concentration half-cell precipitates copper ions, reversing the cell polarity and changing the voltage to negative.

Takeaways

  • 🔋 The experiment involves setting up electrochemical cells using two copper half-cells with different concentrations.
  • ⚡ The two copper half-cells consist of copper metal electrodes and copper sulfate solutions of 0.1 M and 0.2 M concentrations.
  • 🔗 A salt bridge containing potassium nitrate connects the two half-cells, allowing ion exchange.
  • 📊 The initial voltage reading on the voltmeter is positive 6.1 millivolts.
  • 🔴 The red lead is connected to the 0.1 M solution (anode), while the black lead is connected to the 0.2 M solution (cathode).
  • ➡️ Oxidation occurs in the left half-cell (lower concentration), where copper metal forms copper ions to increase the ion concentration.
  • ⬅️ Reduction occurs in the right half-cell (higher concentration), where copper ions are reduced to copper metal, decreasing the ion concentration.
  • 🕒 Over time, the copper ion concentrations in the two half-cells will equalize, and the voltage will drop to zero.
  • 🧪 Adding sodium sulfide to the right half-cell precipitates copper ions, reducing the copper ion concentration below that of the left half-cell.
  • 🔄 The addition of sodium sulfide causes the right half-cell to become the anode and the left half-cell to become the cathode, reversing the polarity and producing a negative voltage.

Q & A

  • What are the two copper half-cells made of in the experiment?

    -The two copper half-cells consist of a copper metal electrode and a copper sulfate solution.

  • What are the concentrations of copper sulfate in the two half-cells?

    -The solution in the right half-cell is 0.2 M, while the solution in the left half-cell is 0.1 M.

  • What role does the salt bridge play in the electrochemical cell?

    -The salt bridge, containing potassium nitrate, connects the two half-cells and allows the flow of ions to maintain charge balance during the reaction.

  • Why is there a positive voltage reading when the red lead is attached to the 0.1 M solution and the black lead is attached to the 0.2 M solution?

    -The positive voltage occurs because the red lead, attached to the 0.1 M solution, acts as the anode where oxidation occurs, while the black lead attached to the 0.2 M solution acts as the cathode where reduction occurs.

  • What causes oxidation to occur in the left-half cell?

    -Oxidation takes place in the left-half cell because the copper ion concentration is lower in that half, prompting copper metal to oxidize into copper ions to balance the concentration.

  • What happens in the right half-cell during the reaction?

    -In the right half-cell, copper ions are reduced to copper metal, which decreases the copper ion concentration in that solution.

  • Why does the voltage eventually go to zero in this electrochemical cell?

    -The voltage drops to zero as the copper ion concentrations in both half-cells equalize, indicating that no more net electron flow occurs between the two sides.

  • What effect does adding sodium sulfide to the right half-cell have on the cell?

    -Adding sodium sulfide precipitates copper ions in the right half-cell, reducing the copper ion concentration below that of the left half-cell, which causes the anode and cathode to switch.

  • How does the cell's voltage change after adding sodium sulfide?

    -The voltage becomes negative because the right half-cell, where sodium sulfide was added, becomes the anode, and the left half-cell becomes the cathode.

  • What is the long-term impact of precipitating copper ions in one half-cell on the electrochemical reaction?

    -By reducing the copper ion concentration in one half-cell, the flow of electrons reverses, altering the direction of oxidation and reduction, until a new equilibrium is reached or the reaction ceases.

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関連タグ
ElectrochemistryCopper cellsVoltage changeConcentration effectOxidationReductionElectrodesChemical reactionsPrecipitationLaboratory experiment
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