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SCH 4U 1. Recall: Grade 9 Science Which subatomic particles are transferred in a chemical reaction? Ex. 2 Na (s) + Cl 2(g) → 2 NaCl (s) Electrons! 2.

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Presentation on theme: "SCH 4U 1. Recall: Grade 9 Science Which subatomic particles are transferred in a chemical reaction? Ex. 2 Na (s) + Cl 2(g) → 2 NaCl (s) Electrons! 2."— Presentation transcript:

1 SCH 4U 1

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

3 Electron Transfer Theory Zn (s) + 2 HCl (aq) → ZnCl 2(aq) + H 2(g) Zn (s) → Zn 2+ (aq) 2H + (aq) → H 2(g) Zn 2+ (aq) + 2 Cl - (aq) + 2e - *These are called half-reactions! 3

4 Zn → Zn e - 2H + + 2e - → H 2 Zn + 2H + → Zn 2+ + H 2 Zn (s) + 2HCl (aq) → ZnCl 2(aq) + H 2(g) *Notice half-reactions are balanced by both mass and charge 4 What happened to the chlorine?

5 Terminology: 5 Reduction = process in which electrons are GAINED Oxidation = process in which electrons are LOST i.e. Zn → Zn e - i.e. 2H + + 2e - → H 2

6 Hmm, how will I remember this? 6 LEO the lion says GER! Lose e- = Oxidation Gain e- = Reduction

7 Ag + (aq) + e - → Ag 0 (s) Cu 0 (s) → Cu 2+ (aq) + 2e - 7 GER = Reduction LEO = Oxidation 2Ag + (aq) + Cu(s) → 2Ag (s) + Cu 2+ (aq) 222 Ag (s) Cu (s) *Total number of e- gained must equal the total number of e- lost

8 The reduction and oxidation reactions occur simultaneously... 8 Often called “redox” reactions Chemical reactions involving a transfer of electrons

9 Ex. 1 Write and label two balanced half-reaction equations for the rxn of zinc metal with aqueous lead (II) nitrate. Zn (s) + Pb(NO 3 ) 2(aq) → Pb (s) + Zn(NO 3 ) 2(aq) Zn (s) → Zn 2+ (aq) + 2e - Pb 2+ (aq) + 2e - → Pb (s) 9 LEO = Oxidation GER = Reduction

10 Oxidation States Useful (necessary!) when redox reactions involve molecules and polyatomic ions. → a method of “electron bookkeeping” 10 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 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

11 Example: Water

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

13 Need to know: Atom or IonOxidation Number Examples All atoms in elements0Na, Cl 2 Hydrogen in all compounds+1H in HCl *Except H in hydridesH in LiH Oxygen in all compounds-2O in H 2 O *Except O in peroxidesO in H 2 O 2 All monatomic ionscharge on ionNa + is +1; S 2- is -2 13

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

15 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! 15 An oxidation (LEO) = increase in ox. number A reduction (GER) = decrease in ox. number

16 Example: Identify the oxidation and reduction in the following reaction: 2Cr 2 O 7 2- (aq) + 16H + (aq) + 3C 2 H 5 OH (aq) → 4Cr 3+ (aq) + 11H 2 O (g) + 3CH 3 COOH (aq) Which species’ ox. numbers have changed? x + 7(-2) = -2 x = x + 6(+1) + (-2) = 0 x = x + 4(+1) + 2(-2) = 0 x = 0 0 Chromium atoms in Cr 2 O 7 2- are reduced (GER) while carbon atoms in C 2 H 5 OH are oxidized (LEO)

17 Homework p. 656 # 7-11 p.659 # p. 662 # 18 & 19 p. 663 # 7 17


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