Electrochemistry

C12-1-09 REACTIVITY AND OXIDATION NUMBERS OUTCOME QUESTION(S): C12-1-09 REACTIVITY AND OXIDATION NUMBERS Outline the development of our understanding of oxidation and reduction reactions. Develop an activity series and predict spontaneity of reactions. Determine the oxidation numbers for atoms in compounds and ions. Identify reactions as redox or non-redox and identify species involved. Vocabulary & Concepts 

Reduction: originally meant the removal of oxygen from a compound. Oxidation: originally meant the combining of oxygen with other elements. - rusting or burning are common examples of oxidation Reduction: originally meant the removal of oxygen from a compound. - there is a decrease or reduction in the mass of the material as the oxygen is removed Oxidation 2 Mg(s) + O2(g) → 2 MgO(s) Reduction CuO(s) + C(s) → Cu(s) + CO2(g)

SO2 + H2O → H2SO3 Cu(s) + 2 AgNO3(aq) → CuNO3(aq) + 2 Ag(s) Not all chemical reactions proceed the same way: Some chemical reactions involve rearrangement of atoms as new bonds are formed: SO2 + H2O → H2SO3 Compounds might look different here, but every element was able to keep the same number of electrons as before Other reactions also involve electron transfer: Cu(s) + 2 AgNO3(aq) → CuNO3(aq) + 2 Ag(s) You might not be able to tell, but elements here have lost electrons and others have gained electrons as the new bonds have formed

Oxidation-Reduction Reactions – (REDOX) Chemical changes that occur when electrons are transferred from one reactant to another Oxidation – now means an atom loses electrons Reduction – now means an atom gains electrons These reactions happen in pairs and cannot be separated – if one atom loses electrons, another gains "LEO says GER” Losing Electrons is Oxidation Gaining Electrons is Reduction

Magnesium is oxidized Oxygen is reduced 2 Mg (s) + O2 (g) → 2 MgO (s) Mg – neutral Mg2+ – ion O – neutral O2– – ion Magnesium is oxidized Oxygen is reduced … as … (losing) (gaining) Mg → Mg2+ + 2e- O + 2e- → O2- These are called “half reactions” - literally the reaction split in two and will be important throughout the unit

Why do redox reactions occur? Elements fight for electrons in bonds Stronger pulls electrons away from weaker Stronger reduced (gains), weaker oxidized (loses) Not really this simple – it depends on EN and the situation of the compound and reaction What happens when you place a piece of aluminum foil in copper solution? Cu2+(aq) + Al(s)  ?

Oxidation: Al(s) → Al3+(aq) + 3e– +2 +3 Cu2+(aq) + Al(s) → Cu(s) + Al3+(aq) Oxidation: Al(s) → Al3+(aq) + 3e– (losing 3 electrons) Reduction: Cu2+(aq) + 2e– → Cu(s) (gaining 2 electrons) Cu2+ ions can oxidize Al metal: Cu is a stronger attractor of e- This is a spontaneous reaction - occurs automatically with NO added energy needed

A reaction of nickel in a lead solution? Activity Series: List of metals in order of reactivity (willingness to lose elections) used to predict products in a reaction of metals A reaction of nickel in a lead solution? +2 +2 Pb2+(aq) + Ni(s) → Pb(s) + Ni2+(aq) Oxidation: Ni(s) → Ni2+(aq) + 2e– 2e– (losing) Reduction: Pb2+(aq) + 2e– → Pb(s) 2e– (gaining) Notice the patterns that exist in the created half-reactions (electron placement, phases changing)

Activity Series Construction Lab Click on the TV to be taken to the link…

C12-1-09 REACTIVITY AND OXIDATION NUMBERS CAN YOU / HAVE YOU? C12-1-09 REACTIVITY AND OXIDATION NUMBERS Outline the development of our understanding of oxidation and reduction reactions. Develop an activity series and predict spontaneity of reactions. Determine the oxidation numbers for atoms in compounds and ions. Identify reactions as redox or non-redox and identify species involved. Vocabulary & Concepts