The feasibility of reactions 188-189

Question 1

Explain how the standard electrode potential value may not be valid when measuring a cell?

Answer 1

• Non-standard conditions alter the value of an electrode potential.
• From le Chatelier’s principle, on increasing the concentration of Cu2+(aq):
• the equilibrium opposes the change by moving to the right.
• electrons are removed from the equilibrium
• the electrode potential becomes less negative, or more positive.

A change in electrode potential resulting from concetration changes means that predictions made on the basis of the standard value may not be valid.

Question 2

Why will some reactions not take place if even if the two standard electrode potentials of two half cells show a feasible reaction?

Answer 2

• A reaction may have a very high activation energy and so the rate of reaction will be slow.
• The actual conditions used for a reaction may be different from the standard conditions used to measure E* values. This will affect the value of the electrode potential.
• Standard electrode potentials apply to aqueous equlibria - many reactions take place under very different conditions.

Question 3

What is the general rule about standard electrode potentials that allow us to predict if a reaction will actually take place, even if the standard electrodes of the half cells work?

Answer 3

As a general working rule:

• the larger the difference between E values, the more likely it is that a reaction will take place.
• If the difference between E values is less than 0.4V, then a reaction is unlikely to take place.