CHEMISTRY 161 Oxidation-Reduction Reactions Chapter 6

REVISION HCl(aq) + NaOH(aq) → NaCl(aq) + H 2 O(l) H + (aq) + OH - (aq) → H 2 O(l) (Arrhenius) HCl(aq) + NH 3 (aq) → NH 4 Cl(aq) H + (aq) + NH 3 (aq) → NH 4 + (aq) (Bronsted)

CHEMICAL REACTIONS a) precipitation reactions b) acid-base reactions (proton transfer) c) oxidation-reduction reactions (electron transfer) (redox reactions)

1. oxidation KEY CONCEPTS loss of electrons 2. reduction acceptance of electrons NUMBER OF ELECTRONS MUST BE CONSERVED

1. oxidation EXAMPLE 2. reduction !!!balance electrons!!! Na + Cl - Na  Na + + e Cl e  2 Cl - CaO, Al 2 O 3

substance that lost the electrons reduction agent substance that gained the electrons oxidizing agent oxidizing agent is reduced reducing agent is oxidized 2 Na + Cl 2  2 Na + Cl -

EXAMPLE 1 solid state reaction of potassium with sulfur to form potassium sulfide EXAMPLE 2 solid state reaction of iron with oxygen to form iron(III)oxide

OXIDATION NUMBER ionic compounds ↔ molecular compounds NaClHF, H 2 Na + Cl - ? electrons are fully transferred covalent bond charges an atom would have if electrons are transferred completely

HF H + + F - molecular compoundionic compound F - oxidation state -1 H + oxidation state +1 EXAMPLE 1

H2OH2O molecular compound ionic compound 2 H + + O 2- H + oxidation state +1 O 2- oxidation state -2 EXAMPLE 2

H2H2 molecular compound ionic compound H + + H - EXAMPLE 3 OXIDATION NUMBER OF FREE ELEMENTS IS ZERO

RULE 1 OXIDATION NUMBER OF FREE ELEMENTS IS ZERO H 2, O 2, F 2, Cl 2, K, Ca, P 4, S 8

RULE 2 monoatomic ions oxidation number equals the charge of the ion group I M + group II M 2+ group III M 3+ (Tl: also +1) group VII (w/ metal) X -

RULE 3 oxidation number of hydrogen +1 in most compounds (H 2 O, HF, HCl, NH 3 ) -1 binary compounds with metals (hydrides) (LiH, NaH, CaH 2, AlH 3 )

RULE 4 oxidation number of oxygen -2 in most compounds (H 2 O, MgO, Al 2 O 3 ) -1 in peroxide ion (O 2 2- ) (H 2 O 2, K 2 O 2, CaO 2 ) -1/2 in superoxide ion (O 2 - ) (LiO 2 )

RULE 5 oxidation numbers of halogens F: -1 (KF) Cl, Br, I: -1 (halides) (NaCl, KBr) Cl, Br, I: positive oxidation numbers if combined with oxygen (ClO 4 - )

RULE 6 charges of polyatomic molecules must be integers (NO 3 -, SO 4 2- ) oxidation numbers do not have to be integers -1/2 in superoxide ion (O 2 - )

MENUE 1.oxidation states of group I – III metals 2.oxidation state of hydrogen (+1, -1) 3. oxidation states of oxygen (-2, -1, -1/2, +1) 4.oxidation state of halogens 5.remaining atoms

oxidizing agents OCl - Cl - ????? EXP1

reducing agent 2 Na + 2 H 2 O  H NaOH EXP2

NO NO 2 NO + NO - NO 2 - NO 3 - PO 4 3- SO 4 2- SO 3 SO 2 KO 2 K2OK2O BrO - KClO 4

1.redox reactions 2. oxidation versus reduction 3. oxidation numbers versus charges 4. calculation of oxidation numbers REVISION

TYPES OF REDOX REACTIONS 1.combination reactions A + B → C 2. decomposition reactions C → A + B 3. displacement reactions A + BC → AC + B 4. disproportionation reactions

1.combination reactions A + B → C two or more compounds combine to form a single product S 8 (s) + O 2 (g) → SO 2 (g) 1.oxidation numbers 2. balancing charges

MENUE 1.oxidation states of group I – III metals 2.oxidation state of hydrogen (+1, -1) 3. oxidation states of oxygen (-2, -1, -1/2, +1) 4.oxidation state of halogens 5.remaining atoms

2. decomposition reactions C → A + B breakdown of one compound into two or more compounds HgO(s) → Hg(l) + O 2 (g) 1.oxidation numbers 2. balancing charges KClO 3 (s) → KCl(s) + O 2 (g)

3. displacement reactions A + BC → AC + B an ion or atom in a compound is replaced by an ion or atom of another element 3.1. Hydrogen displacement 3.2. Metal displacement 3.3. Halogen displacement

3.1. Hydrogen displacement group I and some group II metals (Ca, Sr, Ba) react with water to form hydrogen Na(s) + H 2 O(l) → NaOH + H 2 (g) less reactive metals form hydrogen and the oxide in water (group III, transition metals) Al(s) + H 2 O(l) → Al 2 O 3 (s) + H 2 (g)

3.1. Hydrogen displacement even less reactive metals form hydrogen in acids Zn(s) + HCl(aq) → ZnCl 2 (aq) + H 2 (g) EXP3

Li K Ba Ca Na Mg Al Zn Cr Fe Cd Co Ni Sn Pb H Cu Hg Ag Pt Au activity series of metals displace H from water displace H from steam displace H from acids

Li K Ba Ca Na Mg Al Zn Cr Fe Cd Co Ni Sn Pb H Cu Hg Ag Pt Au likes to donate electrons does not like so much to donate electrons EXP4

3.2. Metal displacement V 2 O 5 (s) + 5 Ca(s) → 2 V(s) + 5 CaO(s) TiCl 4 (g) + 2 Mg (l) → Ti(s) + 2 MgCl 2 (l)

3.3. Halogen displacement F 2 > Cl 2 > Br 2 > I 2 reactivity (‘likes’ electrons) Cl 2 (g) + 2 KBr(aq) → 2 KCl(aq) + Br 2 (l) 00+1 Br 2 (g) + 2 KI(aq) → 2 KBr(aq) + I 2 (s)

4. disproportionation reactions an element in one oxidation state is oxidized and reduced at the same time H 2 O 2 (aq) → 2 H 2 O(l) + O 2 (g) Cl 2 (g) + 2 OH - (aq) → ClO - (aq) + Cl - (aq) + H 2 O(l)

SUMMARY 1.combination reactions A + B → C 2. decomposition reactions C → A + B 3. displacement reactions A + BC → AC + B 4. disproportionation reactions

Homework Chapter 4, p problems