H2 Chemistry 12 - Electrochemistry: EMF Calculation from Standard Electrode Potentials

Step 1 - State the problem

Calculate the standard cell potential for the electrochemical cell made from zinc and copper half-cells.

Step 1 - State the problem

The standard electrode potential of zinc two plus slash zinc is minus zero point seven six volts.

Step 1 - State the problem

The standard electrode potential of copper two plus slash copper is plus zero point three four volts.

Step 1 - State the problem

Determine which electrode is the anode and which is the cathode, and write the overall cell equation.

Step 2 - Identify anode and cathode

The half-cell with the more negative electrode potential is oxidised - it becomes the anode.

Step 2 - Identify anode and cathode

Zinc has the more negative value at minus zero point seven six volts, so zinc is the anode.

Step 2 - Identify anode and cathode

Copper has the more positive value at plus zero point three four volts, so copper is the cathode.

Step 2 - Identify anode and cathode

Remember: oxidation occurs at the anode, and reduction occurs at the cathode.

Step 3 - Calculate the standard cell EMF

The standard cell EMF equals E cathode minus E anode.

Step 3 - Calculate the standard cell EMF

That is plus zero point three four minus the bracket minus zero point seven six.

Step 3 - Calculate the standard cell EMF

Zero point three four plus zero point seven six equals plus one point one zero volts.

Step 3 - Calculate the standard cell EMF

A positive EMF confirms that the reaction is feasible under standard conditions.

Step 4 - Write the overall cell equation

Add the two half-equations together.

Step 4 - Write the overall cell equation

The electrons cancel because both half-equations involve two electrons.

Step 4 - Write the overall cell equation

The overall equation is: zinc solid plus copper two plus ions give zinc two plus ions plus copper solid.

Step 4 - Write the overall cell equation

Zinc is oxidised and copper two plus is reduced. Electrons flow from the zinc electrode to the copper electrode through the external circuit.

Step 5 - Link EMF to Gibbs free energy and common pitfalls

The standard Gibbs free energy change is related to EMF by the equation delta G equals minus n F E.

Step 5 - Link EMF to Gibbs free energy and common pitfalls

Here n is two moles of electrons, F is the Faraday constant at ninety-six thousand five hundred coulombs per mole, and E is one point one zero volts.

Step 5 - Link EMF to Gibbs free energy and common pitfalls

Delta G equals minus two times ninety-six thousand five hundred times one point one zero, which is minus two hundred and twelve kilojoules per mole. The negative value confirms spontaneity.

Step 5 - Link EMF to Gibbs free energy and common pitfalls

Common mistake: using E anode minus E cathode instead of E cathode minus E anode. Always subtract the anode from the cathode.