ELECTROCHEMISTRY REDOX REACTIONS Oxidation-reduction equilibria

The loss of electrons of a substance is called oxidation, and the gain of electrons is reduction A reaction in which both oxidation and reduction is occurring is called a redox reaction. These are very common; as one substance loses electrons the other substance accepts them. The substance which loses electrons is also known as the reducing agent, or reductant, and the substance which accepts the electrons is called the oxidizing agent, or oxidant. The oxidizing agent is always being reduced in a reaction; the reducing agent is always being oxidized. OXIDATION-REDUCTION

Oxidation numbers Rules for the assigning of oxidation numbers All species in their elemental form are given the oxidation number of zero. All monoatomic ions have the same oxidation number as the charge on the ion. e.g. Mg 2+ has the oxidation number of +2. All combined hydrogen has an oxidation number of +1 (except metal hydrides where its oxidation number is -1). All combined oxygen has an oxidation number of -2 (except peroxides where the oxidation number is -1). In polyatomic species, the sum of the oxidation numbers of the element in the ion equals the charge on that species The oxidation number of a non-ionic polyatomic compound is zero

Redox Reactions We have charge separation: the metal -, the liquid + And electric potential forms between 2 places with opposit charges Cu 2+ H2OH2O 2e- Electrode: is an electrical conductor Which is a piece of metal immersed Into the solution of the ion of the metal Piece of metal- Cu

Electrode potential: ε, in Volts ε : electrode potential ε 0 : standard electrode potential z : # of charge(s) of ion The electric potential depends on the ion c.c. of the solution the temperature the pressure the substance Formula of Electrode potential: ε The potential difference between the solution and the surface of the metal

Calculations of electrode potentials of simple electrodes:

How can 2 metal electrode potentials be positive and negative? All the non-metals give anion electrode such as Cl - All metals give positive electrode But what about the electrode potentials??? The electrode potential is not measurable directly We have to connect it to an other electrode which is called reference electrode and we can measure only the difference between the two electrodes The reference electrode is the normal hydrogen electrode

Normal hidrogen electrode: at 25 0 C 1 atm pressure 1 M c.c. of H + Normal hydrogen electrode H+H+ H2H2 Gas phase with H 2 gas having a pressure 1 atm Liquid phase with 1 M H + Platinum 0 0 The electrode potential of the Hydrogen is ZERO e-e-

Standard electrode potential scale Electrode 1 : normal H electrode ≡ 1 M [H+] Electrode 2 : standard normal metal electrode ≡ 1 M [metal] H Cu Ag Hg O Li Zn Standard electrode potential: ε 0 Is a measured potential difference between a metal electrode (which is immersed into its 1 M ionic solution at 25 o C at 1 atm pressure ) and the normal hydrogen electrode The values of the standard electrode potentials are in tables

1. oxidation 2. reduction These half reactions can’t take place separately because electron source is needed That’s why they are connected! Because of the 2 system is connected the half reactions can be separated physically Only for redox reactions! Half reactions: Battery redox reactions

The half reaactions are separated but they are connected with a wire The electrons migrate through the wire And the salt bridge compensate the charges

Daniel cell

Negative pole - (in this case : Zn) source of the electrons – it is the negative pole -That is the electrode which will give electrons to the other electrode -If 2 electrodes are connected, the electrode with the lower potential will be the negative pole That is the negative electrode of the system where oxidation takes place - anode -The negative electode therefore is a reducing system

Pozitive pole + ( In this case : Cu) -That is the electrode which is the electron acceptor -If 2 electrodes are connected the electrode with the higher potential will be the pozitive pole the pozitive electrode of the system where reduction takes place - cathode -The pozitive electode therefore is a oxidazing system The negative and pozitive poles ( and the standard potential values) show the way of the electron migration

The lower the electrode potential the higher the tendency is to reduce other systems that is to be oxidazed A metal with lower standard potential value can reduce the ions of an other metal with higher standard potential value e.g. But In case of the battery reaction one of the reactant is in elementary form Standard potential shows that a given electrode is an oxidazing or a reducing system ε Zn o = (- pole) ε Cu o = (+ pole)

The electromotive force The difference between the potentials of 2 electrodes is called Electromotif force ( e. m. f. ) The electromotive force must be pozitive Voltic cell When 2 electrodes are connected with each other they are called as voltic cell Which is called battery e.m.f. = ε ’larger’ – ε ’smaller’ e.g. E.m.f. = ε Cu – ε Zn = (-0.76) = V

Voltic cell = battery The Zn- Cu voltic cell is called Daniel cell The application of electrochemistry is the production of batteries When 2 electrode are connected during the reaction energy is Released ( e. m. f) the voltic cell is used as energy source = Battery When 2 electrodes are connected E can take out from this system H Cu Ag Hg O Li Ca Mg Zn (V)

Battery To make a longlife, small and cheap battery we need electrodes with a very small and very large st. electr. pot. The battery is exhausted after a lot of using…. Why??? Usages of batteries: the electrode potentials change because of the changing of its ion c.c.

In case of the battery reaction one of the reactant is in elementary form But Half reactions: Neither of components are in elementary form Redox electrodes oxidation Reduction

Redox electrodes Redox electrode: 2 different ionic form of the same element are in the same solution

Redox electrodes Oxidazed - pole Reduced + pole The standard electrode potentials always give the direction of the reaction!!!! Lower electrode potential: - pole; oxidation; (Cu) Reducing agent Higher electrode potential + pole; reduction; ( Fe) Oxidazing agent

Redox electrodes