A little help on chem theories =D

Question

how does each theory define acids and bases??

Arrhenius-

Brønsted-Lowry-

Lewis-

help pleaseeeee =D

Answer 1

1) According to the Arrhenius definition, an acid is any substance, which when dissolved in water, tends to increase the amount of . An example is HCl:

HCl(g) yeilds: H+(aq) + Cl- (aq)

An Arrhenius base is any substance, which when dissolved in water, tends to increase the amount of OH. An example is NaOH:

NaOH(S) yeilds: Na+(aq) + OH-

These definitions are correct but not general enough to include the wide range of acid and base substances which are known to exist. In addition, they rely on the use of water as a solvent, which is also too narrow.

2)the Brønsted-Lowry acid-base theory is an acid-base theory. It was proposed independently by Johannes Nicolaus Brønsted and Thomas Martin Lowry in 1923. In this system, an acid is defined as any chemical species (molecule or ion) that is able to lose, or "donate" a hydrogen ion (proton), and a base is a species with the ability to gain or "accept" a hydrogen ion (proton). It follows that if a compound is to behave as an acid, donating a proton, there must be a base to accept the proton. So the Brønsted-Lowry concept can be defined by the reaction

acid + base  conjugate base + conjugate acid.

The conjugate base is the ion or molecule remaining after the acid has lost a proton, and the conjugate acid is the species created when the base accepts the proton. The reaction can proceed in either forward or backward direction; in each case the acid donates a proton to the base.

Water is amphoteric and can act as an acid or as a base. In the reaction between acetic acid, CH3CO2H, and water, H2O, water acts as a base.

CH3CO2H + H2O  CH3CO2- + H3O+

The acetate ion, CH3CO2-, is the conjugate base of acetic acid and the hydronium ion, H3O+, is the conjugate acid of the base, water.

The case of boric acid exemplifies the usefulness of the Brønsted-Lowry concept as the acid does not in fact dissociate, but does effectively donate a proton to the base, water.

B(OH)3 + 2H2O  B(OH)4- + H3O+

Water can also act as an acid, for instance when it reacts with ammonia. The equation given for this reaction is:

H2O + NH3  OH- + NH4+

in which H2O donates a proton to NH3. The hydroxide ion is the conjugate base of water acting as an acid.

A strong acid, such as hydrochloric acid, dissociates completely. A weak acid, such as acetic acid, may be partially dissociated; the acid dissociation constant, pKa, is a quantitative measure of the strength of the acid.

A wide range of compounds can be classified in the Brønsted-Lowry framework: mineral acids and derivatives such as sulfonates, phosphonates, etc., carboxylic acids, amines, carbon acids, 1,3-diketones such as acetylacetone, ethyl acetoacetate or Meldrum's acid and many more.

A Lewis base, defined as an electron-pair donor, can act as a Brønsted-Lowry base as the pair of electrons can be donated to a proton. This means that the Brønsted-Lowry concept is not limited to aqueous solutions. Any donor solvent, S, can act as a proton acceptor.

AH + S:  A- + SH+

Typical donor solvents used in acid-base chemistry, such as dimethyl sulphoxide or liquid ammonia have an oxygen or nitrogen atom with a lone pair of electrons that can used to form a bond with a proton.

A Lewis acid, defined as an electron-pair acceptor, need not be a Brønsted-Lowry acid. For example, the magnesium ion, Mg2+ can accept electron pairs from water molecules, as in the reaction

Mg2+ + 6H2O → Mg(H2O)62+

but no protons are exchanged.

3) The modern way to define a Lewis acid and base:

Acid: an electron acceptor.

Base: an electron donor.

A "Lewis acid" is any atom, ion, or molecule which can accept electrons and a "Lewis base" is any atom, ion, or molecule capable of donating electrons. However, a warning: many textbooks will say "electron pair" where I have only written "electron." The truth is that it sometimes is an electron pair and sometimes it is not.

It turns out that it may be more accurate to say that "Lewis acids" are substances which are electron-deficient (or low electron density) and "Lewis bases" are substances which are electron-rich (or high electron density).

Several categories of substances can be considered Lewis acids:

1) positive ions

2) having less than a full octet in the valence shell

3) polar double bonds (one end)

4) expandable valence shells

Several categories of substances can be considered Lewis bases:

1) negative ions

2) one of more unshared pairs in the valence shell

3) polar double bonds (the other end)

4) the presence of a double bond

Answer 2

As you can see from Krissi's nice cut and paste from:

http://www.nyu.edu/classes/tuckerman/hon...

and from:

http://en.wikipedia.org/wiki/Br%C3%B8nst...

Google is a good place to find these answers.