Question

The water used in this experiment is sparged with nitrogen to remove any dissolved oxygen. The proton-coupled reduction of oxygen is shown in the following half reaction:

O2 + 4 H⁺ + 4 e^- = 2 H2O

a.Explain why sparging a solution with N2to remove dissolved oxygen is an equilibrium process.

b.Using the half reaction above, write the balanced equation for the reaction of ascorbic acid with oxygen. Show your work.

c.If you didn’t know whether or not O2is capable of oxidizing ascorbic acid, how would you determine this by consulting data? What data would you consult and what would you look for? (Simply trying the reaction is not an acceptable answer.)

Answer 1

a. The sparging method is actually based on Henry Law. According to Henry law, the solubility of a gas in liquid is proportional to the partial pressure of the gas in contact with liquid. When sparging is done, Nitrogen is introduced in the liquid, it forms small bubbles. When these bubbles are dispersed, the partial pressure difference develops between nitrogen and oxygen. This partial pressure forces the oxygen gas to leave the liquid. So, the nitrogen is used to remove oxygen from the liquid. As the process is occurring at the interface of liquid and gas, thus it is an equilibrium process.

b.

c. If, we don't know that the species are oxidizing or reducing, we need to see in the half reactions. The reaction in which the electron is gained is reducing reaction and the reaction in which electrons are lost is the oxidizing reaction. Here we can see in the half reactions, in case of O2, the electrons are gained, so the oxygen is itself getting reduced and oxidizing the other species. If we see the half reaction of ascorbic acid, the electrons are lost, which means that ascorbic acid is getting oxidised.

Another way to see if the O2 is reducing or oxidizing the other species, we need to check the reduction potentials of both the reactions. More negative is the reduction potential, better reducing agent it is. More positive value of reduction potential tells that it is better oxidising agent.