Redox Reactions

Electron Transfer Reactions Electron transfer reactions are oxidation-reduction or redox reactions. Results in the generation of an electric current (electricity) or be caused by imposing an electric current. Therefore, this field of chemistry, in which electron transfer reactions are investigated is often called ELECTROCHEMISTRY.

Oxidation and Reduction MUST go TOGETHER!!!

2Mg (s) + O 2 (g)  2MgO (s) Mg: -has gained oxygen. - has been oxidized. -is the reducing agent (since it removes the oxygen---OXYGEN REMOVER) O 2 : -has been removed. - has been reduced. -is the oxidizing agent (since it gives / is the source of oxygen– OXYGEN DONATOR)

CuO(s) + H 2(g)  Cu(s) + H 2 O(l) Oxidized:Reduced: Oxidizing agent:Reducing agent:

REDOX REACTIONS CAN OFTEN BE OBSERVED AS SIGNIFICANT COLOR CHANGES.

e.g. Fe(s) + CuSO 4(aq)  FeSO 4 (aq ) + Cu(s)

Another way to define oxidation and reduction OIL RIG x i d a t i o n s o s e e d u c t i o n s a i n

2Mg (s) + O 2 (g) 2MgO (s) 2Mg 2Mg e - O 2 + 4e - 2O 2- Oxidation (lose e - ) Reduction (gain e - )

Terminology for Redox Reactions OXIDATION—loss of electron(s) by a species; increase in oxidation number; increase in oxygen.OXIDATION—loss of electron(s) by a species; increase in oxidation number; increase in oxygen. REDUCTION—gain of electron(s); decrease in oxidation number; decrease in oxygen; increase in hydrogen.REDUCTION—gain of electron(s); decrease in oxidation number; decrease in oxygen; increase in hydrogen. OXIDIZING AGENT—electron acceptor; species is reduced. (an agent facilitates something; ex. Travel agents don’t travel, they facilitate travel)OXIDIZING AGENT—electron acceptor; species is reduced. (an agent facilitates something; ex. Travel agents don’t travel, they facilitate travel) REDUCING AGENT—electron donor; species is oxidized.REDUCING AGENT—electron donor; species is oxidized. OXIDATION—loss of electron(s) by a species; increase in oxidation number; increase in oxygen.OXIDATION—loss of electron(s) by a species; increase in oxidation number; increase in oxygen. REDUCTION—gain of electron(s); decrease in oxidation number; decrease in oxygen; increase in hydrogen.REDUCTION—gain of electron(s); decrease in oxidation number; decrease in oxygen; increase in hydrogen. OXIDIZING AGENT—electron acceptor; species is reduced. (an agent facilitates something; ex. Travel agents don’t travel, they facilitate travel)OXIDIZING AGENT—electron acceptor; species is reduced. (an agent facilitates something; ex. Travel agents don’t travel, they facilitate travel) REDUCING AGENT—electron donor; species is oxidized.REDUCING AGENT—electron donor; species is oxidized.

e.g. Formation Silver Crystals

All “combustion” reactions and “metal+acid” reactions are REDOX!

ELECTROLYSIS Electrolysis is the process in which electrical energy is used to break down a substance. In electrolysis, any liquid that contains IONS is used since it conducts the electricity. This liquid is called ELECTROLYTE. By means of the movement of both anions and cations, the electrical current is carried in the electrolyte.

electrolytic cell An electrolytic cell consists of two electrodes in a molten salt or solution. Source of direct electrical current: pushes e - into cathode & pulls e - from anode (Where reduction occurs) (Where oxidation occurs) PA ! Positive Anode Electrodes (usually GRAPHITE or PLATINIUM) are inert solids that conduct the electricity but do NOT react.

Electrolysis can be conducted for molten forms and aqueous solutions of ionic compounds.

ELECTROLYSIS OF MOLTEN IONIC COMPOUNDS The compound is broken down to its constituent elements.

Electrolysis of molten lead(II) bromide

-In the liquid, the ions move to the electrode of opposite charge. -At the cathode (-), Pb 2+ ions accept electrons and form Pb(s) on the electrode. -At the anode(+), Br - ions lose electrons and form Br 2 gas. -Electrons flow from anode to the the (+) terminal of the battery and from (-) terminal of the battery to the cathode. -Electrons carry the current in the external circuit (the wires and electrodes). -Ions carry the current in the liquid. -The electrodes remain unchanged. PbBr 2 (l)  Pb(s) +Br 2 (g) ANODECATHODE 2Br -  Br 2 + 2e - Pb e -  Pb Br 2(g) Pb(s)

Because of high melting points of metals, electrolysis of molten salts requires very high temperatures.

GENERAL RULE FOR THE ELECTROLYSIS OF MOLTEN IONIC COMPOUNDS… At the ANODE----ELEMENTAL FORM OF THE NONMETAL At the CATHODE---- ELEMENTAL FORM OF THE METAL will be obtained.

Electrolysis of molten salts Overall electrolytic cell rxn: AlCl 3(l)  Al(s) + 3/2Cl 2(g) C(graphite) electrode Al deposits C(graphite) electrode Al +3 Molten AlCl 3 Cathode:Al +3 (l)+3e -  Al(s)

Exercise Which substances will be obtained at anode and cathode? C(graphite) electrode C(graphite) electrode Molten KF

ELECTROLYSIS OF AQUEOUS SOLUTIONS OF IONIC COMPOUNDS In aqueous solutions, we have to consider water as well since the water itself produces ions.

Tiny % of water split up into ions…

Rules for the electrolysis of a solution Tendency to be oxidized at anode (+): Halides in concentrated solutions (Cl -, Br -,l - )> H 2 O -When halides are oxidized at the anode, their elemental forms are obtained. -- When water is oxidized at the anode, Oxygen is obtained. IMPORTANT!!! If the solution is dilute, oxygen is obtained at the anode.

Rules for the electrolysis of a solution Tendency to be reduced at cathode (-): Cu 2+, Ag + > H 2 O > Ions of of other metals -When Cu 2+, Ag + are reduced at the cathode, their elemental forms are obtained. -- When water is reduced at the cathode, hydrogen is obtained. IMPORTANT!!! The more reactive the metal, the more it likes to exist as ions.

The electrolysis of aqueous NaCl(brine) is an important for the production of chlorine and NaOH.