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13.5 The effects of current and potential on rate and equilibrium

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13.5 The effects of current and potential on rate and equilibrium (ESCRB)

Current and rate of reaction (ESCRC)

A galvanic cell

Let's think back to the \(\text{Zn}\)-\(\text{Cu}(\text{s})\) electrochemical cell. This cell is made up of two half-cells and the reactions that take place at each of the electrodes are as follows:

\(\color{blue}{\text{Zn(s)} \to \text{Zn}^{2+}\text{(aq) + 2e}^{-}}\)

\(\color{red}{\text{Cu}^{2+}\text{(aq) + 2e}^{-} \to \text{Cu(s)}}\)

  • At the zinc electrode, the zinc metal loses electrons and forms \(\text{Zn}^{2+}\) ions. The electrons are concentrated on the zinc metal while the \(\text{Zn}^{2+}\) ions are in solution.

  • At the copper electrode, the copper ions gain electrons and forms solid copper.

  • This means that there is an excess of electrons on the zinc anode, and a deficit of electrons on the copper cathode.

  • Electrons will flow from an area of high concentration to an area of low concentration. Therefore the electrons on the zinc anode flow through the external circuit towards the more positive copper cathode.

  • The larger the difference between the excess and the deficit of electrons, the faster the electrons will flow and the greater the current will be.

  • The faster the electrons will flow, the greater the rate of the reaction must be.

  • Therefore the larger the current, the faster the rate of the reaction.

An electrolytic cell

  • Conversely, in an electrolytic cell a redox reaction takes place when a current is applied.

  • This redox reaction is the decomposition of a chemical compound (electrolysis).

  • The rate of this decomposition (into ions) is increased when the current applied is increased.

Potential difference, equilibrium and concentration (ESCRD)

Again we can use the zinc-copper cell as an example.

When the chemical reaction between the zinc and the copper slows, the increase in product concentration, and the decrease in reactant concentration, slow too. This means that the electron transfer rate will decrease.

When the chemical reaction in the cell stops:

  • The reaction is no longer converting chemical potential energy to electrical potential energy.

  • The concentrations of the reactants and products have become constant and equilibrium has been reached.

  • There is no excess or deficit of electrons on the electrodes, and the potential difference of the cell is \(\text{0}\).

  • A potential difference of \(\text{0}\) means that the current is \(\text{0}\).

So the potential difference across a cell is related to the extent to which the cell reaction has reached equilibrium. When equilibrium is reached, the potential difference of the cell is zero and the cell is said to be 'flat'. There is no longer a potential difference between the two half-cells, and no more current will flow.