Displacement as Redox

To show the true redox mechanism, chemists write an ionic equation. This strips away the spectator ions to reveal only the particles that actually gain or lose electrons. We can break this down even further into a half-equation, which explicitly shows the exact number of electrons (written as $e^-$) moving to or from a specific element.

Worked Example: Metal Displacement

Explain the redox processes occurring when solid zinc is added to copper(II) sulfate solution.

Step 1: Write the full chemical equation.

Step 2: Remove the spectator ions to form the ionic equation.

• The sulfate ion ($SO_4^{2-}$) remains unchanged in the solution.

Step 3: Write the oxidation half-equation.

• Zinc atoms are more reactive, so they lose two electrons to form zinc ions.

Step 4: Write the reduction half-equation.

• Copper(II) ions are less reactive, so they gain the two electrons to form neutral solid copper atoms.

(Practical Observations: The blue $Cu^{2+}$ solution would fade, and a reddish-brown solid of copper would form on the dissolving zinc).

Worked Example: Halogen Displacement

Halogens follow the exact same redox principles, but because they are non-metals, the more reactive element gains electrons instead of losing them.

Explain why the reaction between chlorine gas and sodium iodide solution is a redox reaction.

Step 1: Write the full chemical equation.

Step 2: Form the ionic equation by removing the sodium spectator ions ($Na^+$).

Step 3: Identify the reduction process.

• The more reactive chlorine molecules gain electrons to form chloride ions.

Step 4: Identify the oxidation process.

• The less reactive iodide ions lose electrons to form iodine molecules. Notice how the total charge is balanced on both sides of the arrow.