1A03/1E03 Types of Reactions (2)1 CHEMISTRY - science of properties and transformations of matter...

1A03/1E03 Types of Reactions (2) 1

CHEMISTRYCHEMISTRY - science of properties and transformations of matter

CHEMICAL REACTIONS - central to CHEMSITRY

CLASSIFICATION of REACTIONS (Kotz, Ch. 4)

Precipitation

Oxidation-reduction

Acid-base

Gas forming


OXIDATION-REDUCTION REACTIONS

Cu (s) + 2 AgNO3 (aq) Cu(NO3)2 (aq) + 2 Ag(s)

Transfer of electrons:

Cu0 gives 2 electrons to 2 Ag+ to form Cu2+ and 2 Ag0

species that provides electrons …….. called the REDUCING AGENTspecies that accepts electrons …….. called the OXIDIZING AGENT

species that loses electrons is said to have been OXIDIZEDspecies that gains electrons is said to have been REDUCED

Chemists use the concept of OXIDATION NUMBER to recognize OXIDATION-REDUCTION reactionsHere Cu in Cu (s) O.N. = 0

Cu in Cu(NO3)2 O.N. = +2Ag in AgNO3 O.N. = +1Ag in Ag(s) O.N. = 0

(see Fig. 4.17 in Kotz)


RULES for OXIDATION NUMBERS O.N.(see Kotz, p. 191)

• Atoms in pure ELEMENT (Cu, I2, S8) 0

• Ions of single atom (Al#+) charge on ion

• Fluorine in compounds (NaF) -1

• other halogens in compounds (KCl, CsI) -1– EXCEPT when combined with F or O where O.N. = +1

• hydrogen in most compounds (HCl, HI) +1– EXCEPT metal hydrides (CaH2) where O.N. = -1

• oxygen in most compounds (MgO, Na2O) -2

– EXCEPT in peroxides (H2O2) where O.N. = -1

• the O.N. of all other elements in a compound are determined by

requiring that

SUM of O.N. of all elements = CHARGE on the compound


EXAMPLES: What is O.N. of S in H2SO4 ?

• H has an O.N. of• O has an O.N. of • 2 * O.N. (H) + 4 * O.N. (O) =

- overall charge of molecule is 0 therefore O.N. of S in H2SO4 is :

What is O.N. of N in HNO3 ?

• H has an O.N. of • O has an O.N. of • O.N. (H) + 3 * O.N. (O) =

- overall charge of molecule is 0, therefore O.N. of N in HNO3 is :

this shows the nitrate group: “ NO3 “ has a net O.N. of (and thus a charge) of THUS the O.N. of Ag in AgNO3 is


Another example of an OXIDATION-REDUCTION REACTION

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

Species Fe in Fe2O3 C in CO Fe C in CO2

O.N. +3 +2 0 +4

Thus:

Fe in Fe2O3 GAINS 3 electrons - Fe2O3 is the OXIDIZING AGENTC in CO LOSES 2 electrons - CO is the REDUCING AGENT

The iron in Fe2O3 was REDUCED to FeThe carbon in CO was OXIDIZED to CO2

NOTES:• oxidation-reduction reactions DO NOT require ions; can involve gases• OXIDATION NUMBERS do not represent actual charge

- rather they are a convenient book-keeping device which help to identify and classify reactions


ACID-BASE REACTIONS (see alsoKotz Ch 17 - sections 1 to 5 (pp. 794-804)

TYPES OF ACIDS/BASES

• Arrhenius:– ACID - substance that donates H+ in water

HCl H+(aq) + Cl- (aq)– BASE - substance that donates OH- in water

NaOH Na+ (aq) + OH- (aq)

– SALT - ionic product of an ACID - BASE reaction, composed of a

+ve CATION from the base and a -ve ANION from the acid

HCl (aq) + NaOH (aq) NaCl (aq) + H2O

ACID + BASE SALT + water


Brnsted - Lowry: ( sect 17.3, p. 707)

ACID - substance that donates H+ to another speciesHCO3

- (aq) H+(aq) + CO32- (aq)

BASE - substance that accepts H+ from another speciesNH3 + H2O NH4

+ (aq) + OH- (aq)

TYPICAL PROPERTIES of ACIDS and BASES

ACIDS• sour tase•corrosive•reacts with bases•turns natural dyes red•generates CO2 from limestone•generates H2 with metals

BASES•soapy feel•restores natural dyes to blue•reacts with acids


CONJUGATE ACID-BASE PAIRS

A pair of compounds that differ in composition by one H+

HBr (aq) + NH3 (aq) NH4+ (aq) + Br- (aq)

ACID BASE conjugate ACID conjugate BASE of NH3 of HBr

- H+

+ H+

ALL ACID-BASE REACTIONS involve TWO CONJUGATE ACID-BASE PAIRS

Acid(1) + Base(2) Conj-Acid(2)+ Conj-Base(1)

HCl + NH3 NH4+ + Cl-

H2O + NH3 NH4+ + OH-

CH3CO2H + H2O H3O+ + CH3CO2-


RELATIVE STRENGTHS of ACIDS and BASES

STRONG ACIDS - react completely with water to form H3O+ (aq)HCl (aq) + H2O H3O+ (aq) + Cl- (aq)

STRONG BASES - react completely with water to form OH- (aq)Li2O + H2O 2 Li+ (aq) + OH- (aq)

Weak ACIDS/ weak BASES only react partially with water

- an EQUILIBRIUM is formed : the conjugate ACID and the conjugate BASE are both present at the same time

WEAK ACID: (acetic acid a.k.a. vinegar)CH3CO2H + H2O CH3CO2

- (aq) + H3O+ (aq)

WEAK BASE:NH3 (g) + H2O NH4

+ (aq) + OH- (aq)


Relative Strengths of acids/bases depend on the solvent :

In water: H3O+ is the STRONGEST ACIDOH- is the STRONGEST BASE

Once an acid or base is fully reacted (dissociated) to form these species, one can no longer distinguish relative strengths. This is called theLEVELLING ACTION of water(The relative strengths of two strong acids can only be determined in non-aqueous solvents)

STRONG ACIDS: HCl, H2SO4, H3PO4, HNO3, HClO4

WEAK ACIDS: CH3CO2H, “H2CO3” (= CO2), HS, HCN, HF

WEAK BASES: NH3, Na2CO3

STRONG BASES: NaOH, Li2O, NaH, Ca(OH)2

Kotz, Table 17.3

In Br nsted acid-base reactions - H+ transfer occurs from STRONGER to WEAKER congugate acid-base pair


GAS FORMING REACTIONS

- some reactions generate gases. When carried out in an OPEN system, the escape of the gas acts as a DRIVING FORCE for the chemical reaction- loss of gas product pulls reaction to completion just as formation of a precipitate

e.g.CaCO3 (s) + 3 HCl (aq) CaCl2 (aq) + H2O (l) + CO2 (g)

Zn (s) + 2 HCl (aq) ZnCl2 (aq) + H2 (g)

NB. Each of these reactions can also be classified as one of the other 3 types of reactions - WHICH ONES ?


GAS FORMING REACTIONS - alternative classification

A. CaCO3 (s) + 3 HCl (aq) CaCl2 (aq) + H2O (l) + CO2 (g)

There is no precipitate and no change in O.N. (Ca stays as +2, Cl as -1) It is an ACID-BASE REACTION involving an intermediate H2CO3

called CARBONIC ACID, which immediately dissociates to H2O and CO2

2 H+ + CO32- “H2CO3 “ H2O + CO2

B. Zn (s) + 2 HCl (aq) ZnCl2 (aq) + H2 (g)

There is no precipitate. However there is an obvious change in O.N.It is also an example of an OXIDATION-REDUCTION REACTION

Zn goes from O.N. = 0 in the metal to O.N. = +2 in the chloride salt

H goes from O.N. = +1 in HCl (aq) to O.N. = 0 in the elemental gas


CHEMISTRY of HALOGENS

The Elements• Group 7A - all non-metals• diatomc molecules: F2 (gas), Cl2 (gas), Br2 (liquid), I2 (solid)• most stable oxidation state: -1• Undergo OXIDATION-REDUCTION reactions readily

with metals (all elements on right and middle of P.T.)

Trends:SIZE: I > Br > Cl > FElectron affinity: F > Cl > Br > IOxidizing strength F > Cl > Br > I

Compoundsmost stable form: HALIDES (A+X-)hydrogen halides: ACIDS: HCl (strong), others (weak)


prepared by electrochemical reaction (chlor-alkali process)

2 NaCl + H2O -------> Cl2 + 2 NaOH

• 8th most important industrial chemical by mass (1010 kg/yr)

IMPORTANT USES

• disinfectant of water (as hpyochlorous acid (HOCl(aq))

Cl2 + 2 H2O H3O+ + Cl- + HOCl (aq)

• bleach for textiles, paper, etc (as hypochlorite (OCl-)

Cl2 + 2 OH- H2O + Cl- + OCl- (aq)

• organochlorine compounds: plastics, drugs

ELECTROLYSIS

CHLORINE


CHLORINE: BANE or BOON ???Kotz, pp 88,89

Consumer friend . . . . . . . or environmental foe ??


SUMMARY: Key Concepts from “Types of Reactions” (Kotz Ch. 4)

•Classes of reactions: precipitationoxidation-reductionacid-basegas-forming

•Ionic versus non-ionic compounds•Solubility guidelines: Kotz, p. 168•Oxidation Number rules: Kotz, p. 191•Acid/base types: Arrhenius, Br nsted- Lowry• Typical chemistry of halogens

•Some reactions PROCEED TO COMPLETIONe.g. precipitation, gas forming, strong acid/strong base

•Some reactions establish an EQUILIBRIUM of reactants and productse.g. oxidation-reduction, weak acids, weak bases

LEARN TO BALANCE CHEMICAL EQUATIONS- both MASS and CHARGE balance are important


CHEMISTRY of DEMONSTRATIONS

Oxidation-Reductioniron and chlorine: 2 Fe (s) + 3 Cl2 (g) 2 FeCl3 (s) (black)

sugar and sulfuric acid: 2C6H12O6 (s) + 12 H2SO4 (aq)

6C (s) + 6 CO2 (g) + 24 H2O (l) + 12 SO2 (g)

Acid-Baseacid and limestone: 2 H+ (aq) + CaCO3 (s) Ca2+ (aq) + CO2 (g) + H2O(l)

acid and metal: 2 H+ (aq) + Zn (s) Zn2+ (aq) + H2 (g)

1A03/1E03 Types of Reactions (2)1 CHEMISTRY - science of properties and transformations of matter...

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