21)Relationship between the free energy change g of redox reactions and the cell potential e

The cell potential E is a measure of whether a redox reaction is spontaneous. For the reaction A_red + B_ox → A_ox + B_red it equals

The Gibbs free energy change G is the quite general measure of reaction spontaneity and equals the free energy to do useful work. For the reaction

A + B → C + D,

Influence factors to change potentials

1. Oxidation and reduction ions concentration influence to redox potential.

2. Influence of solutions ionic strength

3. Influence of pH

1. Oxidation and reduction ions concentration influence to redox potential.

We can see that Potential depends [Ox]/]Red] ratio from Nernst equations, so we can change potentials to change [Ox]/]Red] concentration ratio. For example: Fe³⁺ + e⁻  Fe²⁺

2. Influence of solutions ionic strength

Change of ionic strength influences to change activity coefficient. So instead formal potential to replace for standard potential on the Nernst equation. For example:

Е⁰¹ – formal potential, it depends ionic strength and other electrolytes concentration.

3. Influence of pH.

1. Hydrogen ions join to react all of water solutions, so like that systems potential depends solutions pH.

2) Change of pH influences to change systems potential, and cause various

substance is formed. For example.

3) Formation of complex compound affect systems potential.

Redox pair components attend formation of additional complex components and to affect to potential. Because, formation of complex substance’s activity decrease. For example:

To add KCN to this system, Ag ion formed cyanide complex

There for free silver ions concentration is decreased, use Nernst equation, and got following equations.

4) Formation of precipitation component affect to potential. For example, NaCl solution is added Ag⁺/Ag redox pair, there

Therefor, silver ions concentration is decreased, and use Nernst equation to calculate change of potential.

22) Relationship beween e0 and the equilibrium constant k

Define direction of the redox reactions

Define direction of redox reactions is Defined by electric driving force (EDF) (electromotive force) and equilibrium constant, if EDF > 0, reaction occurs forward direction, if EDF < 0, reaction occurs reverse direction. For example.

Cupper ion – oxidation, iron ion – reduction.

EDF = E_Ox- E_Red= 0.34 – (-0.44) = 0.78 B ˃0

When set (reach) equilibrium, the redox pair potential gets to equal on oxidation – reduction reaction.

Electric driving force is just defined direction of reactions, but equilibrium constant is defined direction of reactions and completion.

К_redox⁰ ˃1 – forward reaction happened. К_redox⁰ ≥ 10⁸ – reaction occurs completely For example:

So reaction occurs completely.