Standard Electrode Potentials

1. When a metal contacts a solution of its ions, an equilibrium establishes:

This equilibrium position determines the electrode's potential:

• Equilibrium left: Electron buildup on metal → negative potential

• Equilibrium right: Electron depletion from metal → positive potential
  
  

Three Types of Electrodes:

1. Metal electrodes: - metal in contact with its ions

2. Gas electrodes: - requires inert platinum electrode

3. Redox electrodes: - two oxidation states of same element

You cannot measure the potential of a single electrode - just like you cannot measure a single person's height without a reference point. We can only measure potential differences between two electrodes.

The Standard Hydrogen Electrode is assigned by convention and serves as our "reference" for all other measurements.

• Components of SHE:

• Platinum electrode (coated with platinum black for surface area)

• Hydrogen gas at 100 kPa pressure

• (usually 1M HCl)

• Temperature:

Half-equation:

Cell notation:

Measuring Against the SHE

The Formula:

When measuring unknown electrode potentials:

• SHE always on the left

• Test electrode on the right

Secondary Standards - Practical Alternatives

Since SHE is difficult to use (involves flammable gas), secondary standards are often used:

• Silver/silver chloride:

• Calomel electrode:

These are calibrated against SHE but much more convenient for routine use.

Setting Up Electrochemical Cells

Every electrochemical cell consists of:

• Two half-cells: Separate compartments where oxidation and reduction occur

• Electrodes: Conductive materials (usually metals) that allow electron transfer

• Electrolyte solutions: Ionic solutions that facilitate ion movement

• Salt bridge: Connection allowing ion flow between half-cells

• External circuit: Wire connecting the electrodes for electron flow

Why Do We Get a Voltage?

When two different metals are placed in solutions of their own ions, they have different tendencies to lose electrons (oxidize). The metal with the greater tendency to oxidize will:

• Release more electrons into the external circuit

• Create a negative charge buildup

• Become the negative electrode (anode)

The other metal will:

• Accept electrons from the external circuit

• Become the positive electrode (cathode)

• Allow reduction to occur

This difference in electron-losing tendency creates a potential difference (voltage) between the electrodes.

What is a Salt Bridge?

A salt bridge is typically a piece of filter paper or porous material soaked in an unreactive salt solution (commonly potassium nitrate, ).

Why Do We Need It?

Without a salt bridge, the cell would stop working almost immediately because:

1. As electrons flow through the external circuit, positive ions accumulate in one half-cell in this case, the left-hand half-cell

2. Negative ions accumulate in the other half-cell i.e. the right-hand half-cell

3. This charge buildup would prevent further electron flow

The salt bridge solves this by:

• Allowing anions (negative ions) to flow toward the anode (negative electrode) where oxidation occurs

• Allowing cations (positive ions) to flow toward the cathode (positive electrode) where reduction occurs

• Maintaining electrical neutrality in both half-cells

• Completing the circuit without allowing the solutions to mix

Why this happens:

• At the anode: Metal atoms lose electrons and become positive ions, creating excess positive charge

• Anions from the salt bridge flow in to balance this positive charge

• At the cathode: Positive ions gain electrons and are reduced, creating excess negative charge

• Cations from the salt bridge flow in to balance this negative charge

Why Not Just Use a Wire?

• Wires conduct electricity through movement of electrons only

• Solutions conduct electricity through movement of ions (both cations and anions)

• The salt bridge provides ionic conduction to complete the circuit

• Without ionic movement, charge buildup in the solutions would quickly stop the cell from working

Think of it this way: electrons flow through the external wire, but ions must flow through the salt bridge to maintain electrical neutrality in both half-cells. You need BOTH types of charge movement for the cell to function.

Salt Selection Criteria

The salt must be:

• Chemically inert - doesn't react with electrodes or solutions

• Highly soluble - ensures good ionic conductivity

• Unreactive ions - won't form complexes or precipitates

Examples of incompatible combinations:

• with copper systems: ions form complexes with ions

• with manganate systems: ions can be oxidized by (manganate is a strong oxidizing agent)

• with silver systems: precipitate would form

This is why is preferred - both and are relatively unreactive ions that:

• is very difficult to reduce (highly negative )

• is stable to oxidation under normal conditions

• Neither form complexes or precipitates with most electrode system

Why High Resistance?

A high-resistance voltmeter is essential because:

1. Minimizes current flow: Tiny current (nanoamps) means negligible reaction rates

2. Maintains near-equilibrium: Reactions occur extremely slowly

3. Accurate measurement: Voltage reading remains stable

4. Prevents cell discharge: Reactants aren't consumed during measurement

5. When measuring: Negligible current → reactions virtually stopped → stable voltage

6. When in use: Significant current → reactions proceed → voltage drops over time and eventually drops to 0V as the reactants are consumed