Electrochemistry: Electric Cells SCH 4U

Recall: Grade 9 Science Electrons! Which subatomic particles are transferred in a chemical reaction? Electrons! Ex. 2 Na(s) + Cl2(g) → 2 NaCl(s)

Electron Transfer Theory Zn(s) + 2 HCl(aq) → ZnCl2(aq) + H2(g) Zn(s) → Zn2+(aq) 2H+(aq) → H2(g) Zn2+(aq) + 2 Cl-(aq) + 2e- + 2e- *These are called half-reactions!

Zn → Zn2+ + 2e- 2H+ + 2e- → H2 Zn + 2H+ → Zn2+ + H2 What happened to the chlorine? Zn → Zn2+ + 2e- 2H+ + 2e- → H2 Zn + 2H+ → Zn2+ + H2 Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g) *Notice half-reactions are balanced by both mass and charge

Terminology: i.e. 2H+ + 2e- → H2 i.e. Zn → Zn2+ + 2e- Reduction = process in which electrons are GAINED i.e. 2H+ + 2e- → H2 Oxidation = process in which electrons are LOST i.e. Zn → Zn2+ + 2e-

LEO the lion says GER! Hmm, how will I remember this? Lose e- = Oxidation LEO the lion says GER! Gain e- = Reduction

Ag+(aq) + e- → Ag0(s) Cu0(s) → Cu2+(aq) + 2e- 2 2 GER = Reduction LEO = Oxidation Cu(s) 2Ag+(aq) + Cu(s) → 2Ag(s) + Cu2+(aq) *Total number of e- gained must equal the total number of e- lost Ag(s)

Often called “redox” reactions The reduction and oxidation reactions occur simultaneously ... Often called “redox” reactions Chemical reactions involving a transfer of electrons Q: Are all reactions redox reactions? NO Example of a non-redox rxn: Pb(NO3)2(aq) + Na2SO4(aq) ---> 2NaNO3(aq) + PbS(s) Double displacement, oxidation states NOT changing

Ex. 1 Zn(s) → Zn2+(aq) + 2e- Pb2+(aq) + 2e- → Pb(s) Write and label two balanced half-reaction equations for the rxn of zinc metal with aqueous lead (II) nitrate. Zn(s) + Pb(NO3)2(aq) → Pb(s) + Zn(NO3)2(aq) Zn(s) → Zn2+(aq) + 2e- Pb2+(aq) + 2e- → Pb(s) LEO = Oxidation GER = Reduction

Oxidation States Oxidation Number Useful (necessary!) when redox reactions involve molecules and polyatomic ions. → a method of “electron bookkeeping” Oxidation Number a positive or negative # corresponding to the apparent charge an atom in a molecule/ion would have if e- pairs in covalent bonds belonged entirely to the more electronegative atom So far just looked at atoms and monatomic ions To explain/describe oxidation and reduction of molecules and polyatomic ions

Example: Water -2 Oxygen atom: EN = 3.1 Hydrogen atom EN = 2.1 +1 +1

Note:. To avoid confusion with charges, oxidation Note: To avoid confusion with charges, oxidation numbers are written with the sign BEFORE the number. i.e. oxidation number vs. charge +2 2+

Need to know: Atom or Ion Oxidation Number Examples All atoms in elements Na, Cl2 Hydrogen in all compounds +1 H in HCl *Except H in hydrides -1 H in LiH Oxygen in all compounds -2 O in H2O *Except O in peroxides O in H2O2 All monatomic ions charge on ion Na+ is +1; S2- is -2

Examples *Remember: sum of oxidation numbers must equal the total charge What is the oxidation number of carbon in methane, CH4? CH4 What is the oxidation number of manganese in a permanganate ion, MnO4-? MnO4- -4 +1 x + 4(+1) = 0 x = -4 *Remember: sum of oxidation numbers must equal the total charge Because just a systematic way of counting electrons x + 4(-2) = -1 x = +7 +7 -2

Oxidation Numbers in Redox Rxns If the ox. number of an atom/ion changes in a chemical rxn, it is a redox reaction! → electron(s) transferred! An oxidation (LEO) = increase in ox. number A reduction (GER) = decrease in ox. number

Example: +6 -2 +1 -2 -2 +1 2x + 6(+1) + (-2) = 0 x = -2 +3 +1 -2 -2 +1 Identify the oxidation and reduction in the following reaction: 2Cr2O72-(aq) + 16H+(aq) + 3C2H5OH(aq) → 4Cr3+(aq) + 11H2O(g) + 3CH3COOH(aq) Which species’ ox. numbers have changed? +6 -2 +1 -2 -2 +1 2x + 6(+1) + (-2) = 0 x = -2 +3 +1 -2 -2 +1 2x + 7(-2) = -2 x = +6 Det. blood alcohol content from breathe or blood: involves reaction of sample with dichromate ion in an acidic environment If ethanol is present, produces chromium (III) ions, water, and acetic acid x + 4(+1) + 2(-2) = 0 x = 0 Chromium atoms in Cr2O72- are reduced (GER) while carbon atoms in C2H5OH are oxidized (LEO)

Homework p. 656 # 7-11 p.659 # 12-16 p. 662 # 18 & 19 p. 663 # 7