Electrochemistry

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Common Misconceptions

3 min read

- What you are getting from a statement might not be necessarily true. Let us burst some of the common misconceptions.

The ions of an electrochemical cell move only due to the presence of electrodes

An electrochemical cell is always described by the movement of ions towards the oppositely charged electrodes.

While this may look like the only movement in an electrochemical cell, very often we tend to overlook the movement of ions in the electrochemical cell due to a concentration gradient. This gradient results in a motion known as diffusion.

To sum it up, the ions moving towards the opposite electrodes due to the electrostatic force of attraction are very small in number. The majority of the movement is because of the diffusion of charged particles from regions of higher concentrations to regions of lower concentration.

A galvanic cell doesn't work without the salt bridge
We are likely to see a diagram of a galvanic cell like this one in our textbooks.

Does it mean that the galvanometer connected to a galvanic cell will not give any reading without the salt bridge? Let's find out.

Initially, when the electrons flow from the negative to the positive terminal, we see a potential difference and a reading is recorded by the galvanometer.

But as time goes by, the anodic compartment will have an excess of positive charge and the cathodic compartment an excess of negative charge.

This results in a potential difference that is in a direction opposite to the potential difference between the two electrodes. The two opposing potentials cancel each other out and we get a zero reading. This brings in the need for a salt bridge - to maintain the electrical neutrality between the two compartments.

So, a galvanic cell can work without the salt bridge, but it's short lived.

A positive cell potential indicates spontaneity; the reaction requires no external energy to begin.
If we look at the this expression

G_{c e l l}^{0} = - n F E_{c e l l}^{0}

, a positive value of

E_{c e l l}^{0}

ensures that the value of

G_{c e l l}^{0}

would be negative and the reaction will be spontaneous.

But does it mean that such a reaction never require any external energy?

The magnitude of

G_{c e l l}^{0}

does not say anything about the reaction speed or rate.

Despite a positive value of

E^{0}

, the reaction between gasoline and oxygen(fuel cell) is extremely slow. This is because this reaction has a large activation energy,

E_{A}

So, it may require some energy initially for reactants to overcome the activation energy barrier. As a matter of fact, even though a tremendous amount of energy is released during the reaction between gasoline and oxygen, it still needs a spark to overcome the barrier.

After crossing this barrier, the reaction will be spontaneous and does not require a constant supply of energy to proceed further.