Modern Theories of Acids & Bases. The Arrhenius and Bronsted-Lowry Theories. Acids & Bases. Acids and bases are special kinds of electrolytes. Like all electrolytes they break up into charged particles.
Svente Arrhenius (who first proposed the theory of ionization) identified acids as substances that ionize in water to produce hydronium ion.
HCl + H2O H3O+ + Cl-
Any substance that ionizes in water to produce hydronium ion can be classified as an acid.
HCl + H2O H3O+ + Cl-
HCl H+ + Cl-
However, it should be understood that H+ is an abbreviation for the hydronium ion. H+ ions DO NOT exist in water solution but are snatched up by water molecules to form hydronium ions.
Svente Arrhenius also identified bases as substances that ionize in water to produce hydroxide ion.
NaOH Na+ + OH-
Any substance that ionizes in water to produce hydroxide ion can be classified as a base.
Ionic substances that break up in solution to produce ions other than hydronium and hydroxide ions.
NaCl (s) Na+(aq) + Cl- (aq)
KNO3(s) K+ (aq) + NO3- (aq)
Li2SO4 (s) 2 Li+ (aq) + SO42-(aq)
Salts are made up of positive (metal) and negative (non-metallic or polyatomic) ions. The more familiar you become with Table E, the easier it will be for you to identify salts.
Identify each of the following as acids/bases/salts and show how they ionize:
Acid HC2H3O2H+ + C2H3O2-
Salt K2SO4 2K+ + SO42-
Base KOH K+ + OH-
Base LiOH Li+ + OH-
Acid HNO3 H+ + NO3-
All acids have the following properties:
Neutralize bases to form a salt and water
Have a sour taste (example: citric acid, vinegar)
React with active metals to produce a salt plus hydrogen gas.
Have pH’s less than 7
Because all acids have H3O+ ions present!
All bases have the following properties:
Neutralize acids to form a salt and water
Have a bitter taste (example: unsweetened chocolate, heroin)
Are slippery. React with fats/oils to form soap (saponification)
Have pH’s greater than 7 / Affect indicators
Because all bases have OH- ions present!
Another theory of acids & bases
A major problem with the Arrhenius definition of acids and bases is that it limits acids and bases to water (aqueous) solutions.
Since an acid requires water to ionize and form hydronium ions, there can be no Arrhenius acids unless water is involved as the solvent.
Bronsted-Lowry Acids/Bases can exist when no water is present
Arrhenius Acids/Bases only exist in water solutions.
All Arrhenius acids and bases can also be classified as Bronsted-Lowry acids and bases.
Bronsted and Lowry felt that this was too limiting, since there are many non-aqueous systems (no water is present).
They came up with the following definitions for acids and bases.
An acid is a proton (H+ ion) donor
A base is a proton acceptor
In the reaction below there are no Arrhenius acids or bases present (because no hydronium ions or hydroxide ions are formed).
However, the HCl is acting as a Bronsted-Lowry acid because it is giving a H+ ion to the NH3 (which is acting as a H+ ion acceptor - a base)
In the Bronsted-Lowry definition, substances are classified as acids or bases depending on how they behave in a given situation.
This means that the same substance can act as a acid in one reaction (by donating a proton) while acting as a base in another reaction
Since the water is giving up a H+ ion, it is acting as a Bronsted-Lowry acid. Since it does not form hydronium ions, it is NOT acting an Arrhenius acid.
For each of the following reactions identify any Bronsted-Lowry acids and bases.
Are any of the substances above amphoteric?
HCl (aq) H+(aq) + Cl-(aq)
HC2H3O2 H+ (aq) + C2H3O2-(aq)
Weak acids don’t completely break up because they go to equilibrium!
Many acid base reactions go to equilibrium, that is they have both a forward and reverse reactions
For instance, acetic acid (HC2H3O2) reacts with water to form hydronium ion and acetate ion.
However, the acetic acid only partially ionizes because a reverse reaction takes place preventing the forward reaction from reaching completion.
In the reverse reaction, the Hydronium ion acts as an acid (a proton donor) while the acetate ion acts as the base.
Every acid has a conjugative base
Every base has a conjugative acid
NaOH (aq) Na+(aq) + OH-(aq)
NH3 (aq) + H2 O NH4+ (aq) + OH-(aq)
Weak bases don’t completely break up because they go to equilibrium!
Salts can be classified as being:
How a salt is classified depends upon whether secondary reactions between the ions making up the salt and water form either hydronium or hydroxide ions.
The type of salt for a particular salt can be determined experimentally by testing the salt solution with universal indicator paper.
Typically, the pH values of salt solutions will be closer to 7 than that of acids or bases.
All salts can be considered to be formed from their “parent” acid and base by means of the neutralization reaction.
Acid + Base Salt + Water
The type of salt can be theoretically predicted based on the properties of the “parent” acid and the “parent” base.
Neutral salts are formed from a reaction between a strong acid and a strong base.
(Remember neutralization reactions are double replacement reactions.)
HCl + NaOH NaCl + HOH
(since HCl is a strong acid and NaOH is a strong base, NaCl is a neutral salt.)
Acidic salts are formed from a strong acid and a weak base.
HCl + NH4OH NH4Cl + HOH
(since HCl is a strong acid and NH4OH is a weak base, NH4Cl is an acidic salt.)
Basic salts are formed from a weak acid and a strong base.
HC2H3O2 + NaOH NaC2H3O2 + HOH
(since HC2H3O2 is a weak acid and NaOH is a strong base, NaC2H3O2 is a basic salt.)
In predicting the type of salt it is helpful to remember the three strong acids:
Most other acids are weak. Acetic acid (HC2H3O2) is the most common weak aciddiscussed in Regents chemistry.
For bases, the Group I hydroxides are all strong bases.
LiOH – lithium hydroxide
NaOH – sodium hydroxide
KOH – potassium hydroxide