The Effects of Current and Potential on Rate and Equilibrium (Grade 12 NSC Matric Physical Sciences): Revision Notes

The Effects of Current and Potential on Rate and Equilibrium

Current and rate of reaction

Understanding how current affects the rate of reaction is crucial when studying electrochemical cells. Current represents the flow of electrons, and this flow directly influences how quickly chemical reactions occur in both galvanic and electrolytic cells.

Galvanic cells

Let's examine how current affects reaction rates using the zinc-copper galvanic cell as our primary example. In this cell, two separate half-reactions occur simultaneously at different electrodes.

At the zinc electrode (anode), oxidation takes place:

$\text{Zn(s)} \rightarrow \text{Zn}^{2+}\text{(aq)} + 2\text{e}^-$

At the copper electrode (cathode), reduction occurs:

$\text{Cu}^{2+}\text{(aq)} + 2\text{e}^- \rightarrow \text{Cu(s)}$

The zinc electrode loses electrons, creating an excess of electrons on the zinc metal surface, whilst the copper electrode gains electrons, resulting in a deficit of electrons. This difference drives electron flow through the external circuit from the zinc anode to the copper cathode.

Electrolytic cells

In electrolytic cells, the relationship between current and reaction rate works differently but follows the same fundamental principle. When an external current is applied to an electrolytic cell, it forces a redox reaction to occur that wouldn't happen spontaneously.

The applied current causes the decomposition of chemical compounds through electrolysis. The crucial point is that increasing the applied current increases the rate of decomposition. More current means more electrons are being forced through the system per unit time, which accelerates the breaking down of chemical compounds into ions.

Potential difference, equilibrium and concentration

The potential difference across an electrochemical cell provides valuable information about the reaction's progress towards equilibrium and the concentrations of reactants and products.

Approaching equilibrium

Using our zinc-copper cell example again, we can observe what happens as the reaction progresses. Initially, the cell produces electricity because there's a significant concentration difference between reactants and products.

However, as the reaction continues, product concentration increases whilst reactant concentration decreases. This means the electron transfer rate gradually slows down. The cell is moving towards a state where the forwards and reverse reactions occur at equal rates.

Reaching equilibrium

When the chemical reaction finally reaches equilibrium, several important changes occur:

• The reaction stops converting chemical potential energy into electrical potential energy
• The concentrations of reactants and products become constant
• There is no excess or deficit of electrons on either electrode
• The potential difference becomes zero

Understanding the equilibrium state

At equilibrium, the cell is described as being "flat" - there's no longer any potential difference between the two half-cells. This means no current will flow because there's no driving force for electron movement.

Worked examples