REDOX REACTIONS & ELECTROCHEMISTRY

What is REDOX? Many of the reactions you have performed this year have been REDOX reactions even though you may not have known it. So, now you may well ask,what are they? REDOX reactions are those in which electrons are transferred from one species to another (i.e. atom, ion, etc.). Electron transfer involves, you guessed it, production of electricity.

What does “REDOX” stand for? RED = reduction, which occurs when a species (ion or atom) gains an electron OX = oxidation, which occurs when a species (ion or atom) loses an electron

How can I remember what REDOX means? Remember LEO the Lion says GER LEO ~ lose electrons = OXIDATION GER ~ gain electrons = REDUCTION LEO - GER

Now for some examples of REDOX REACTIONS.... #1 Fe + 2HCl → H2 + FeCl2 #2 H2O2 + Ni2+ → H2O + Ni3+ #3 H2 + Cl2 → 2HCl Note in #1 that the oxidation number (charge) for the iron (Fe) has gone from zero to 2+. Note in #1 that the oxidation number (charge)for the hydrogen (H) has gone from 1+ to zero. Your task is to determine the oxidation number changes in #2 and #3.

Look again at the reaction in #1 Fe + 2HCl → H2 + FeCl2 Let's “dissect” this reaction. The entire reaction consists of two “half” reactions Fe → Fe2+ + 2e The iron atom has LOST two electrons, therefore,it has oxidized (LEO). 2H1+ + 2e → H2 The hydrogen ions have GAINED two electrons, therefore,they have reduced (GER). NOTE: The chlorine ions have not changed; they are “spectator” ions.

Just as all reactions must balance, so too the number of electrons transferred must balance. Fe + 2HCl + 2e → H2 + FeCl2 + 2e There can be no REDUCTION without OXIDATION. You can't have one without the other. There is no net change in the number of electrons in a REDOX reaction. NOTE: The species that accepts the electrons is the “oxidizing agent.” Why? Because is is facilitating the oxidation of some other species. NOTE: The species that donates the electrons is the “reducing agent.” Why? Because is is facilitating the reduction of some other species.

Where do I find REDOX reactions. Are they only in the lab. No Where do I find REDOX reactions? Are they only in the lab? No! Fire is a REDOX reaction. Rusting is a REDOX reaction.

Why is the Statue of Liberty Green? Because of a REDOX reaction. The Statue of Liberty was colored when it was first made. The statue reacted with water and carbon dioxide in the air to form a green “patina” The copper carbonate and copper hydroxide are both insoluble in water. 2Cu(s) + H2O(g) + CO2(g) + O2(g) → CuCO3(s) + Cu(OH)2(s)‏

Balancing REDOX reactions It is possible to balance REDOX reactions and half reactions through a series of steps that are found on p. 90 of Masterson and Hurley. You will not be expected t o balance these equations here, but expect to do lots of them if you take chemistry in college. You should be able to tell whether a half reaction is oxidation (LEO) or reduction (GER).

ELECTROCHEMISTRY Electrochemistry is the study of the interconversion between electrical and chemical energy. All the reactions in electrochemistry are REDOX reactions. Electrochemistry takes place in one of two types of electrochemical cells. VOLTAIC ELECTROLYTIC

Voltaic cell ~ a spontaneous reaction that generates electrical energy. The cell must be designed so that oxidation occurs at one electrode (anode) and reduction occurs at the other electrode (cathode). Notice the “salt bridge” connecting the two halves and completing the circuit.

Electrolytic cell ~a non-spontaneous redox reaction made to occur by pumping electricity into the system The electron pump (electrical source) pushes electrons into the system at the cathode and removes them from the anode. Reduction occurs at the cathode (- electrode) because it attracts (+) cation Oxidation occurs at the anode (+ electrode) because it attracts (-) anions The REDOX reaction that occurs in an electrolytic cell is called “electrolysis” which means to break up with electricity

Electroplating is one use of electrolytic cells People often “bronze” their children’s baby shoes or otherwise have them coated with a metallic coating for posterity. The shoes are covered with a graphite paste which acts as the cathode and dipped in a metallic ion solution.