Electrochemistry

Objective A: to identify the parts of an electrochemical cell and to describe their function.

Exercises

Solutions

1. In the above diagram, the light bulb goes on after a short while.
2. With time, the copper strip grows while the zinc strip gets smaller. Meanwhile the blue colour of the copper solution fades.
3. For every mole of zinc that goes into solution, one mole of copper is deposited on the original copper strip.
4. No current is produced without a salt bridge.

Explanation:

Zinc undergoes oxidation:

Zn--> Zn⁺² + 2e

Since the metal turns into aqueous ions, this explains why the zinc strip shrinks. In the meantime, lost electrons flow toward the copper ions in the other beaker, lighting th ebulb in the process.

Electrons flow through the Cu strip where they attract the Cu⁺² ions and reduce them.

Cu⁺² + 2e --> Cu

The overall reaction is

Zn + Cu⁺² --> Zn⁺² + Cu

Zn = reducing agent; Cu⁺² = oxidizing agent

Why a salt bridge?

Without the KNO₃-salt bridge, excess positive charge(from Zn⁺²) would accumulate at the Zn anode and too much negative charge would accumulate at the Cu cathode (because Cu⁺² is being consumed). Nitrate (NO₃^-1) from the salt bridge compensates for the additional (+'s) formed by Zn, while K⁺¹ moves into the Cu⁺² beaker and replaces lost (+'s).