Daniell cell (galvanic cell): electrons, ions, salt bridge and cell potential over time

Close the circuit of a zinc–copper Daniell cell and watch the electrons, the ions and the salt bridge, the zinc electrode dissolve and copper deposit, and the cell potential fall as the concentrations change. Free for every student.

Also searched as: daniell cell · daniell cell · galvanic cell · voltaic cell · salt bridge function · electrochemical cell anode cathode

About this experiment

Aim

To see how a Daniell cell produces an electric current from a redox reaction: where the electrons flow, what the salt bridge does, what happens at each electrode and why the cell potential falls with time.

Theory

A galvanic (voltaic) cell turns the energy of a spontaneous redox reaction into electrical energy. In the Daniell cell a zinc rod stands in ZnSO₄ solution and a copper rod in CuSO₄ solution. Zinc is the anode (negative): Zn → Zn²⁺ + 2e⁻ (oxidation). Copper is the cathode (positive): Cu²⁺ + 2e⁻ → Cu (reduction). The electrons flow through the wire from zinc to copper, and the conventional current flows the other way.

The salt bridge (a U-tube with KCl or KNO₃ in a gel) completes the circuit. As Zn²⁺ ions are produced the left solution becomes positive, and as Cu²⁺ ions are used up the right solution becomes negative. Anions move through the bridge towards the anode and cations towards the cathode and keep both solutions neutral. Without the salt bridge the charge builds up and the current stops at once.

The cell potential is E°cell = E°(Cu²⁺/Cu) − E°(Zn²⁺/Zn) = 0.34 − (−0.76) = 1.10 V at 1 mol/L. As the cell works, [Zn²⁺] rises and [Cu²⁺] falls, so Q = [Zn²⁺]/[Cu²⁺] increases and E falls according to the Nernst equation. When E reaches 0 the cell is "dead". The zinc rod loses mass and the copper rod gains mass: 65.4 g of Zn dissolves for every 63.5 g of Cu deposited (2 mol of electrons, 193 000 C).

Formulas used

• anode: Zn → Zn²⁺ + 2e⁻
• cathode: Cu²⁺ + 2e⁻ → Cu
• cell: Zn | Zn²⁺ (aq) ‖ Cu²⁺ (aq) | Cu
• E°cell = 0.34 − (−0.76) = 1.10 V
• E = 1.10 − (0.0592/2) log([Zn²⁺]/[Cu²⁺])
• moles reacted = I t / (2F)

Procedure: what to do in the virtual lab

1. With the circuit closed, watch the electrons move from Zn to Cu and the ions move in the salt bridge.
2. Read which electrode loses mass and which gains mass. Note the colour of the copper sulphate solution.
3. Remove the salt bridge. What happens to the current?
4. Open the circuit (switch off) and read the open-circuit potential. Then close it again with a small resistance.
5. Wait until the cell runs down and read the graph of E against time.

Observations to make

• Electrons flow from the zinc electrode (anode) to the copper electrode (cathode) in the wire.
• Anions drift towards the zinc side and cations towards the copper side in the salt bridge.
• The zinc rod gets thinner, the copper rod thicker, and E decreases slowly, then faster as Cu²⁺ is used up.
• Without the salt bridge no steady current flows.

What you can change in this experiment

• Starting concentration of Zn²⁺ (mol/L)
• Starting concentration of Cu²⁺ (mol/L)
• External resistance (Ω)
• Circuit closed (switch on) (on / off)
• Salt bridge in place (on / off)

Viva questions on Daniell cell (galvanic cell)

Questions an examiner may ask in the Class 12 chemistry practical. Run the experiment in the lab to find the answers yourself, then check them in the Learn tab.

1. What is the function of the salt bridge?
2. Which electrode is the anode in the Daniell cell and what happens there?
3. Why does the EMF of the Daniell cell fall with time?
4. In which direction do electrons and current flow?

Frequently asked questions

Syllabus: Class XII Chemistry, Chapter 2 — Electrochemistry, §2.2 (galvanic cells, the Daniell cell, salt bridge)

What you do in the lab

Change the controls and watch the chemistry behave in real time in 3D. Live readouts and graphs show the numbers behind it, and the Learn tab has the theory, procedure, observations and viva questions. Works on a phone or a laptop, in English and Hindi.

Topics: Daniell cell · galvanic cell · voltaic cell · salt bridge · electrochemical cell · anode and cathode · cell potential · electrochemistry

More from this unit

• Conductivity of strong and weak electrolytes
• Electrolytic refining of copper
• Daniell cell and the Nernst equation
• Electrochemistry: cell potential, Nernst equation, ΔG and equilibrium constant
• Electrolysis of brine (chlor-alkali)
• Electrolysis of copper sulphate (inert electrodes)
• Electrolysis of potassium iodide
• Electrolysis of water

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