To understand the voltaic cell you've described, let's break it down step by step. A voltaic cell, or galvanic cell, converts chemical energy into electrical energy through spontaneous redox reactions. In your scenario, we have two compartments: one with a graphite rod in an acidic solution of potassium dichromate (K2Cr2O7) and chromium nitrate (Cr(NO3)3), and the other with a tin bar in a tin(II) nitrate (Sn(NO3)2) solution. The salt bridge, made of KNO3, allows ions to flow between the two compartments, maintaining electrical neutrality.
Diagram of the Voltaic Cell
While I can't provide a visual diagram, I can describe how to sketch it:
- Draw two vertical compartments, labeling the left one as the "Oxidation Half-Cell" and the right one as the "Reduction Half-Cell."
- In the left compartment, place a graphite rod (anode) dipping into the K2Cr2O7 and Cr(NO3)3 solution.
- In the right compartment, draw a tin bar (cathode) immersed in the Sn(NO3)2 solution.
- Connect the two compartments with a salt bridge, indicating the flow of ions.
- Label the anode (-) and cathode (+) to show the direction of electron flow from the anode to the cathode.
Balanced Chemical Equations
Next, let's write the half-reactions for each electrode and then combine them to get the overall reaction.
Oxidation Half-Reaction (Anode)
At the anode, the graphite rod facilitates the reduction of Cr(VI) ions from K2Cr2O7:
Cr2O7^2- + 14H^+ + 6e^- → 2Cr^3+ + 7H2O
This reaction shows that chromium is reduced from +6 to +3, consuming electrons in the process.
Reduction Half-Reaction (Cathode)
At the cathode, tin is oxidized:
Sn^2+ + 2e^- → Sn
Here, tin ions gain electrons to form solid tin, indicating a reduction process.
Overall Cell Reaction
To combine these half-reactions, we need to balance the number of electrons transferred. The oxidation of tin involves 2 electrons, while the reduction of chromium involves 6 electrons. Therefore, we multiply the tin reaction by 3:
3(Sn^2+ + 2e^- → Sn)
Now, the overall balanced equation becomes:
Cr2O7^2- + 14H^+ + 3Sn → 2Cr^3+ + 7H2O + 3Sn^2+
Cell Notation
Cell notation provides a shorthand way to represent the voltaic cell. It typically follows the format:
Anode | Anode Solution || Cathode Solution | Cathode
For your cell, the notation would be:
C | Cr2O7^2-, Cr(NO3)3 || Sn(NO3)2 | Sn
In this notation, "C" represents the graphite anode, and the double vertical line (||) indicates the salt bridge separating the two half-cells.
In summary, you've got a well-structured voltaic cell where the oxidation of tin and the reduction of chromium ions occur, generating electrical energy. This setup illustrates the principles of electrochemistry beautifully, showcasing how different metals and ions interact in a spontaneous redox reaction.
