6.4- Explaining Acids and Bases

Arrhenius’ TheoryArrhenius’ Theory

acids ionize in water to produce hydrogen ionsbases dissociate in water to produce hydroxide ions

Arrhenius Theory WeaknessesArrhenius Theory Weaknesses

does not explain why: NH3(aq) acts as a base, or why CO2(g) can act as an acid some gases like sulfur dioxide and nitrogen dioxide produce acidic solutions

some substances act as both acids and bases

Revised Arrhenius TheoryRevised Arrhenius Theory

includes water in the reactionacids react with water to produce hydronium (H3O+

(aq))

HCl(aq) + H2O(l) ---> H3O+(aq) + Cl-

(aq)

hydronium is a hydrogen ion bonded onto a water molecule

..

bases are substances that react with water to produce hydroxide ions. This explains why ammonia acts as a base.

NH3(aq) + H2O(l) ---> OH-(aq) + NH4

+(aq)

also explains why molecules such as CO2(g), SO2(g), and NO2(g)

act as acids.Two step reaction:

CO2(g) + H2O(l) ---> H2CO3(aq)

H2CO3(aq) + H2O(l) ---> H3O+(aq)

+ HCO3-(aq)

also explains why some ions can act as both acids and bases.

eg. HCO3-(aq) and HPO4

2-(aq)

HCO3-(aq) + H2O(l) ---> H3O+

(aq) + CO32-

(aq)

HCO3-(aq) + H2O(l) ---> OH- (aq)+ H2CO3(aq)

Neutralization ReactionWe have seen neutralization reactions to look

like thisHCl(aq) +NaOH(aq) NaCl(aq) +H2O(l)

According to the modified Arrehius theory, we can now state that neutralization is the reaction between hydronium ions ( from the acid) and hydroxide ions ( from the base) to produce water

H3 O+ (aq) +OH- (aq) 2H2 O(l)

AssignmentRead page 248 -253Do page 251 #1,2Do page 253 #4,5,6