Understanding Electron Flow in EMF Cells: Anode to Cathode or Cathode to Anode?

[Raghav Gupta]

Homework Statement

A variable , opposite external potential $E_{ext}$ is applied to the cell Zn | Zn²⁺ (1M) || Cu²⁺ (1M) | Cu, of potential 1.1V . When E_ext < 1.1 V and E_ext > 1.1V , respectively electrons flow from :
1. anode to cathode and cathode to anode
2. cathode to anode and anode to cathode
3. cathode to anode in both cases
4. anode to cathode in both cases

Homework Equations

NA

The Attempt at a Solution

At anode oxidation happens and at cathode reduction.
So electrons are moving from anode to cathode when E_ext < 1.1 V and cathode to anode when E_ext > 1.1 V
So option 1 is looking correct.
In answer key option 4 that is anode to cathode in both cases is given.
Why?

 

[Merlin3189]

My guess is this is almost a "trick" question,
The anode is defined as the electrode where electrons leave a cell and the cathode where electrons enter a cell. So no matter which way the current flows, external to the cell electrons always flow from anode to cathode.
When you switch the battery, you switch the names of the electrodes.

 

[Raghav Gupta]

My guess is this is almost a "trick" question,
The anode is defined as the electrode where electrons leave a cell and the cathode where electrons enter a cell. So no matter which way the current flows, external to the cell electrons always flow from anode to cathode.
When you switch the battery, you switch the names of the electrodes.

Thanks, got the tricky part.

 

[James Pelezo]

Look at a table of Reduction Potentials (here's one for quick reference: http://chemunlimited.com/Table of Reduction Potentials.pdf ), in all cases electrons flow from the more negative reduction potential to the more positive reduction potential ... ALWAYS. Choose any two half reactions and note the E^o-values. The more negative value is the anode (site of oxidation) and the more positive value cathode (site of reduction). I might suggest studying how the respective ion concentrations in each half cell affect the voltage of the Galvanic/Voltaic Process. That is, what voltage does the cell have if [reducing agent ion] < [oxidizing agent ions] and visa versa. If [oxidizing agent ions] = [reducing agent ions] what is the cell voltage? Happy charging...