Reduction Oxidation Red-ox An introduction to electron transfer reactions

Oxidation-Reduction: A Reaction Oxidation: When a substances loses electrons. Reduction: When a substance gains electrons. Consider: Ca(s) + 2H+(aq)  Ca2+(aq) + H2(g). The neutral Ca(s) has lost two e- to 2 H+ to become Ca2+. We say Ca has been oxidised to Ca2+ At the same time 2 electrons are gained by 2 H+ to form H2 . We say H+ is reduced to H2 .

Redox Reaction with Air Consider the reaction of Ca with O2: 2Ca(s) O2(g)  2CaO(s) Ca is easily oxidised in air. On the left we see freshly cut Ca. On the right we see Ca with a dull surface. On the surface, there is a coat of CaO. Again, Ca(s) gains electron and is oxidised to Ca+2 And the neutral O2 has gained electrons from the Ca to become O2- in CaO. We say O2 has been reduced to O2-.

Electron Transfer and Terminology Lose electrons: Oxidation Gain electrons: Reduction. GER Leo

It Takes Two: Oxidation-Reduction In all reduction-oxidation (redox) reactions, one species is reduced at the same time as another is oxidised. Oxidizing Agent: The species which causes oxidation is called the oxidizing agent. The substance which is oxidised loses electrons to the other. The oxidizing agent is always reduced Reducing Agent: The species which causes reduction is called the reducing agent. The substance which is reduces gains electrons from the other. The Reducing agent is always oxidised

Oxidation of Metals with Acids It is common for metal to produce hydrogen gas when they react with acids. For example, the reaction between Mg and HCl: Mg(s) + 2HCI(aq)  MgCl2(aq) + H2(g) . In this rxn, Mg is oxidised and H in HCl is reduced. Note the change in oxidation state for these specie: Mg0  Mg+2 in MgCl2 & H+ in HCl  H0 in H2

Redox Rxn with Acid What determines whether the reaction occurs ? It is possible for metals to be oxidised with salt: Fe(s) + Ni(N03)2 (aq)  Fe(N03)2 (aq) + Ni (s) . Molecular Equation The overall ionic equation shows the redox chemistry: Fe(s) + Ni+2(aq)  Fe2+(aq) + Ni (s) Net ionic Equation In this reaction iron has been oxidised to Fe2+ while the Ni+2 has been reduced to Ni0. What determines whether the reaction occurs ?

The Activity Series Metals can be placed in order of their tendencies for losing electrons. This is called the activity series.

Competition For e- Transfer Consider: Na, Mg, Al, Metallic character decreases left to right. Metal tend to give up electrons. Now consider the reaction: Na + AlCl3  ??? (NaCl + Al) To determine if the reaction occurs, the question is to determine which metal has a greater affinity for electrons (or which is willing to lose e- ). Na is more willing to lose e- than Al Al is more willing to accept e- (less metallic) Conclude: The reaction occurs. 3Na + AlCl3  3NaCl + Al

Reading Activity Table A metal in the activity series can only be oxidised by a metal ion below it. In our example, Na is oxidised by Al. The metals at the top of the activity series are called active metals. The metals at the bottom of the activity series are called noble metals.

Example: Silver and Copper If we place Cu into a solution of Ag+ ions, will copper plate out of solution ? Cu(s) + 2AgNO3(aq)  ? [Cu(NO3)2(aq) + 2Ag (s)] or Cu (s) + 2Ag+ (aq)  ? [Cu2+(aq) + 2Ag (s)] Which metal is active? Which is noble ? g Cu g Ag \ Therefore, Cu 2+ ions is be formed because Cu is above Ag in the activity series. Copper Cu g Cu2+ + 2 e-Silver Ag g Ag+ + e-

Example: Redox Reaction B&L 4.47: Based on the activity series, what is the outcome of the following reaction ? b) Ag(s) + PbNO3 (aq)  ? c) Cr (s) + NiSO4 (aq)  ? e) H2 (g) + CuCl2 (aq)  ? f) Ba (s) + H2O (l)  ? b) Ag vs. Pb , Pb is more active, rxn not occurs c) Cr vs. Ni , Cr is more active, rxn occurs Cr (s) + NiSO4 (aq)  Ni (s) + CrSO4 (aq) d) H2 vs. Cu , H2 is more active, rxn occurs H2 (g) + CuCl2 (aq)  2HCl (aq) + Cu (s) e) Ba vs. H2 , Ba is more active, rxn occurs Ba (s) + H2O (l)  H2 (g) + Ba(OH)2 (aq)

Summary Redox - Oxidation/Reduction reaction Oxidation- Lose electron (LEO) Reduction- Gain electron (GER) Activity Series- Table showing elements’ relative ease of oxidation. Active M- Metal which prefers to lose e- and there fore prefer the oxidised form. Noble M- Metal which do not lose e- and therefore prefers the zeroth state.