אשונ - wordpress.comjul 04, 2010 · + 12 e– 4 oh– + 12 h + 12 oh– 2 o reduction...

1 © Prof. Zvi C. Koren 19.07.10

4נושא 'חלק ב–תגובות כימיות


EXCEPTIONSSOLUBLE

COMPOUNDS

NoneSalts of alkali metals and of NH4

+

None

Salts of: NO3–

ClO3–

ClO4–

Ac–

Ag+, Hg22+, Pb2+Salts of Cl–, Br–, I–

Examples:

(NH4)2S(aq) 2NH4+ + S2–

Hg2(NO3)2(aq) Hg22+ + 2NO3

–

Hg(NO3)2(aq) Hg2+ + 2NO3

–

Ca3(PO4)2(s) (no change)

EXCEPTIONSPOORLY

SOLUBLE COMPOUNDS

alkali metals

and NH4+

(of course)

Salts of: CO32–

C2O42–

PO43–

CrO42–

S2–

Hydroxides OH–

Oxides O2–

Solubility of Ionic CompoundsGeneral Solubility Guidelines for Some Ionic Compounds in Water


Pb(NO3)2(aq) + K2CrO4(aq) PbCrO4(s) + KNO3(aq)

Double-Exchange, Double-Replacement:

Net Ionic Reactions

Gross Rxn.:

Ionic Rxn.: Pb2+ + 2NO3– + 2K+ + CrO4

2– PbCrO4(s) + 2K

+ + 2NO3–

Net Ionic Rxn.: Pb2+ + CrO42– PbCrO4(s)

Single-Exchange, Single-Replacement:

Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g)

2H+ + 2Cl– Mg2+ + 2Cl–

Mg(s) + 2H+ Mg2+ + H2(g)

Gross Rxn.:

Net Ionic Rxn.:

Precipitation Rxn.

Gas-Forming+

Redox

Spectator Ions

2

(continued)

Not all salts are relatively soluble in water. But all soluble salts dissociate into ions.


2NaNO3(aq) + K2CrO4(aq) Na2CrO4(aq) + 2KNO3(aq)

Double-Exchange, Double-Replacement ???

Gross Rxn.???:

Ionic Rxn. ???: 2Na+ + 2NO3– + 2K+ + CrO4

2– 2Na+ + CrO4

2– + 2K+ + 2NO3–

Net Ionic Rxn.: NOTHING: NO REACTION!!!

(continued)


Acid-Base Rxns. (Double-Exchange)

Strong Acid + Strong Base:

HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)Gross Rxn.:

H+ + Cl– + Na+ + OH– Na+ + Cl– + H2O(l)

H+ + OH– H2O(l)Net Ionic Rxn.:

Weak Acid + Strong Base:

HAc(aq) + NaOH(aq) NaAc(aq) + H2O(l)

Salt Water

HAc(aq) + Na+ + OH– Na+ + Ac– + H2O(l)

Net Ionic Rxn.: HAc(aq) + OH– Ac– + H2O(l)

Gross Rxn.:Salt Water

(continued)


Strong Acid + Weak Base:

2HCl(aq) + CaCO3(s) CaCl2(aq) +

“H2CO3(aq)”

Gross Rxn.:

2H+ + 2Cl– + CaCO3(s) Ca2+ + 2Cl– + H2O(l) + CO2(g)

2H+ + CaCO3(s) Ca2+ + H2O(l) + CO2(g)Net Ionic Rxn.:

Strong Acid + Weak Base:

HCl(aq) + NH3(aq) NH4+ + Cl– or NH4Cl(aq)

H+ + Cl- + NH3(aq) NH4+ + Cl-

Net Ionic Rxn.: H+ + NH3(aq) NH4+

Gross Rxn.:Salt

(Double-Exchange)

Base (salt)

(Single-Exchange)

SaltAcid


Fe2O3(s) + 3CO(g) 2Fe(s) + 3CO2(g)

+[O], oxidation

-[O], reduction

reducingagent

oxidizingagent

Redox (Reduction-Oxidation) Rxns.

In the beginning, Redox rxns. were defined in terms of actual O’s or H’s transferred.

C2H4(g) + H2(g) C2H6(g)

-[H], oxidation

+[H], reduction

reducingagent

oxidizingagent

O-transfer: From Fe2O3 to CO

H-transfer: From H2 to C2H4

[O]

[H]

But later, Redox rxns. were more generally defined in terms of electron-transfer even

if O’s or H’s were not actually involved. So, check the rxns. above.

How many e’s are

transferred in

each rxn.?


2Ag+ + Cu(s) 2Ag(s) + Cu 2+

–2e–, oxidation

+2e–, reduction

reducingagent

oxidizingagent

Redox (Reduction-Oxidation) Rxns. (continued)

So, Redox rxns. can be more generally defined in terms of electron-transfer

even if O’s or H’s were not actually involved.

e-transfer: From Cu to Ag+

Write 2 Half-Rxns.:

Red.: 2[Ag+ + e– Ag(s)]

Ox.: Cu(s) Cu2+ + 2e–

Total: 2Ag+ + Cu(s) 2Ag(s) + Cu2+

Oxidation: Oxidation # (“charge”) increases: e’s are lost

Reduction: Oxidation # (“charge”) decreases: e’s are gained

2e–


Koren’s “Law of the Jungle”

In the Jungle,

The Mighty Jungle,

The Lion

Sleeps Tonight …

(But when awakened he’ll roar)

LEO

GER!

LEO: Loss of Electrons is OxidationGER: Gain of Electrons is Reduction


Electronegativity

The ability of a bonded atom to draw electrons close to it.

Oxidation Numbers (or Oxidation States)

7A

H : HF : FH : Fδ+ δ-

O.N.:

• is an “invention” to explain Redox rxns.

• is a make-believe, virtual reality, electron-book-keeping charge on a bonded atom.

• indicates how electrons are shared among the various atoms bonded together.

• assumes that the more electronegative atom completely steals the electron(s) in a

bond, i.e., exaggerates the bond as ionic.

Na + F Na F+–Recall:

So, “oxidation number” of F is –1 and of H is +1:

Complete e-transfer

Full Charges

Partial e-transferPartial Charges

middle

Redox (Reduction-Oxidation) Reactions

(continued)


Rules for Determining Oxidation NumbersNote: The O.N. is the value on one atom of an element.

1. The O.N. of an atom in the molecule of a pure element is zero (0).

Examples: O in O2 or O3; S in S8; P in P4; Na in Na(s)

2. The O.N. of an atom in a monatomic ion is the same as its charge.

Examples: Cl–; Na+; Al3+

3. Some elements have the same oxidation numbers in ALL their compounds.

(a) Group IA Metals have an O.N. = +1 in all their compounds.

(b) Group IIA Metals have an O.N. = +2 in all their compounds.

(c) F has an O.N. = –1 in all its compounds.

4. Some elements have the same oxidation numbers in nearly all their compounds.

(a) H in covalent compounds is always +1; Examples: H2O; HCl; ...

H bonded to a IA or IIA metal is –1, of course. Examples: NaH; MgH2.

(b) O has an O.N. = –2 in most compounds. Examples: CO2; MgO; CH3OH ...

O as a peroxide is –1. Examples: H2O2; Na2O2,

O as a superoxide is –½. Example: KO2.

O with F has an O.N. of +2. Example: OF2.

5. The sum of all the O.N.’s equals the charge on the molecular species.

Work out the O.N. for each atom in the following species: CaCO3, SO42–, NH4

+


Koren’s Rules for Balancing Redox Rxns.

ָיָמהִִַמי ם.ִִי.אִ .מִ

...איזון :סיבה...הוספת :פעולהשלב

האטומים של היסוד העובר שינוי' מסקדםממ

לקטרוניםאאהאטומים של היסוד כלדרגות החימצון של

העובר שינוי

?וניםִיישל ( בתרכובות יוניות)אניונים /הקטיונים' מס

( O-ו H-חוץ מ)מאוזנים -היסודות הלא

ידרוקסידיהימןִאוִמוניִיימהבתמיסה ) +Hבעזרת , המטענים הכוללים

(בתמיסה בסיסית) –OH -או ב, (חומצית

מיםמים"O"אטומי ' מס

.(ים מאוזנים אוטומטית'H-ה: שים לב)

The Method:

:תגובה-לכל חצי

.מהתגובה הכוללת e-תגובה וחיסול ה-המשך עם חיבור שני חצאי


1. Balance & Complete the following rxn:

K2Cr2O7(aq) + Cu2O(s) CrCl3(aq) + CuCl2(aq) Is this a redox rxn?6+ 1+ 3+ 2+

oxidation

reduction

Write Two Half-Rxns.:

K2Cr2O7(aq) CrCl3(aq)

6+ 3+

oxidationCu2O(s) CuCl2(aq)

ימהִִמים.ִִי.א.מ

2+ 6 e- + 2 K+

+ 6 Cl-

H+

+ 14 H+ + 7 H2O

reduction

ימהִִמים.ִִי.א.מ1+ 2+

2 + 2 e-+ 4 Cl- + 2 H+ + H2O3x

K2Cr2O7(aq) + 3Cu2O(s) + 20H+ + 18Cl– 2CrCl3(aq) + 6CuCl2(aq) + 10H2O + 2K

+

e’s must cancelCheck atom-balance

& charge-balance

For Gross Rxn. (without ions): +2Cl– +2Cl–

K2Cr2O7(aq) + 3Cu2O(s) + 20HCl(aq) 2CrCl3(aq) + 6CuCl2(aq) + 10H2O(l) + 2KCl(aq)

For Net Ionic Rxn. (with ions):Cr2O7

2– + 3Cu2O(s) + 20H+

2Cr3+ + 6Cu2+ + 10H2O(l)Check atom-balance & charge-balance … again

REDOX Examples:

oxidizing agent

reducingagent


2. Balance & Complete the following rxn:

Disproportionation

or Auto-oxidation0 3– 3+

oxidation

reduction

Write Two Half-Rxns.:

P4(s) PH3(g)

0 3–

oxidationP4(s) PHO3

2–

ימהִִמים.ִִי.א.מ

4+ 12 e–

OH–

+ 12 OH–+ 12 H2O

reduction

ימהִִמים.ִִי.א.מ0 3+

4 + 12 e–+ 20 OH– + 8 H2O

2 P4(s) + 8 OH– + 4 H2O 4 PH3(g) + 4 PHO3

2–

Note: e’s cancel

Check atom-balance & charge-balance

Divide by 2:

Net Ionic Rxn.: P4(s) + 4 OH– + 2 H2O 2 PH3(g) + 2 PHO3

2–

REDOX Examples (continued):

P4(s) PH3(g) + PHO32–

Gross Rxn.(for example):

P4(s) + 4 NaOH(aq) + 2 H2O 2 PH3(g) + 2 Na2PHO3 (aq)


Summary of Types of Reactions

Formation

Decomposition, Dissociation

Combustion

Oxidation

Oxide-Forming

Gas-Forming

Acid-Base

Double-Exchange

Single Exchange

Precipitation

Net Ionic

Redox

אשונ - wordpress.comjul 04, 2010 · + 12 e– 4 oh– + 12 h + 12 oh– 2 o reduction...

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