Topic 19: Oxidation and Reduction

19.1 Standard Electrode Potential

19.1.1 Describe the standard hydrogen electrode.

A hydrogen electrode consist of a platinum electrode covered with a fine powder of platinum around which H₂ is bubbled. Hydrogen then acts as an  electrode. Platinum does not react because it is inert. 

Hydrogen electrode is a reference relative to which all electrode potentials are measured. Hydrogen electrode’s electrode potential is set to zero.

Standard hydrogen electrode can be created only at the standard conditions for electrochemical cell: 25C ̊, concentration of 1mol∙dm^-3 and pressure of 1atm for gases.

19.1.2 Define standard electrode potential.

Standard electrode potential is a potential difference between a half-cell and the hydrogen half-cell at standard conditions for electrochemical cells (25C ̊, concentration of 1mol∙dm^-3 and pressure of 1atm for gases).

19.1.3 Predicting spontaneity of reactions using standard electrode potential values.

We can calculate spontaneity of a reaction by comparing standard electrode potentials of half-cells. The reaction we use is:

                                        

 

1.      If the value is positive and over +0.3, then the reaction is spontaneous.

2.      If the value is between 0 and +0.3, the reaction will likely be an equilibrium.

3.      If the value is negative, the reaction will not spontaneously occur.

19.2 Electrolysis

Electrolysis is the situation when redox cells are forced to run in reverse by attaching an electricity source to overcome the potential difference. In electrolysis, the half-cell with higher electrode potential is the cathode and the half-cell with lower electrode potential is the anode.

19.2.1 Predict and explain electrolysis products of aqueous solutions

Electrons always flow toward the half-cell with the highest E^o value. The half-cell with higher electrode potential is the cathode and the half-cell with lower electrode potential is the anode.

Species above water (when it is on the left) will not be oxidized, and species below water (when water is on the right) will not be reduced in an aqueous solution.

The substance with higer E^o value will be reduced, while the substance with lower E^o value will be oxidized. Reduction will happen at negative cathode and oxidation at the positive anode.

Inert electrodes do not interfere during redox reactions,  but metal electrodes can get involved by dissolving as ions – this is called electrode participation. 

When two species have similar standard electrode potentials, the concentration will decide which substance will be produced (reduced) at the cathode. 

E.g. NaCl solution: dilute – oxygen formed; saturated – chlorine gas formed

19.2.2 Determine relative amount of products formed during electrolysis.

The quantity of a product formed depends on: (1) charge of the ion, (2) current and (3) duration of electrolysis.

It will take twice as many moles of electrons to reduce a metal with charge of +2, than to reduce a metal of charge +1. 

Charge = Current x Time [amperes x seconds]; charge measured in coulombs

Moles of electrons = Charge/Fahrenheit Constant

It follows that the higher the current and the longer the time, more moles of electrons will be formed.

19.2.3 Use of electrolysis in electroplating

By electrolysis we can force a non-spontaneous reaction to happen. Charged ions will move toward negative cathode and metal at the positive anode will get oxidized and dissolved in solution. Thus, we place metal at the anode and the electroplated object at the cathode.