Acids and Bases: Introductory Concepts

Arrhenius

...acids increase the ______ when dissolved in H2O.

...bases increase the ______ when dissolved in H2O.

e.g., HCl and NaOH

--

[H+]

[OH–]

useful concept, butlimited to aq. solns.

Svante Arrhenius(1859–1927)

Bronsted-Lowry acids:

Bronsted-Lowry bases:

-- H+ and H3O+ are used interchangeably

-- B-L concept is NOT limited to aqueous solutions

can transfer H+ to othersubstances (i.e., theyare proton donors)

can accept H+ from othersubstances (i.e., theyare proton acceptors)

closer to reality;“hydronium ion”

faster to write;“hydrogen ion” H3O+H+

Johannes Bronsted(1879–1947)

:Thomas Lowry(1874–1936)

e.g., NH3 in above example

Arrhenius and Bronsted-Lowry definitions overlap,in many cases.NH3(aq) + H2O(l) NH4

+(aq) + OH–(aq)

One substance can’t be a Bronsted-Lowry acid unless...

-- the acid must be able to lose H+

-- the base must have a nonbonding pair of e– that can bind with the H+

A. base

B-L base

another simultaneously acts as a B-L base.

(causes [OH–] to )

(accepts H+)B-L acid(donates H+)

H–N–H

H

:H+

[ ]+

Arrhenius Bronsted Lowry

Amphoteric substances can be acids or bases,depending on the reaction conditions.

HCl(aq) + H2O(l)

NH3(aq) + H2O(l)

H3O+(aq) + Cl–(aq)

NH4+(aq) + OH–(aq)

B-L base

B-L acid

NH3 is another example.

When a B-L acid, it morphs into NH2– on the P side.

“ “ “ base, “ “ “ NH4+ “ “ “ “ .

[ ]+

[ ]–

and the acid has an extra H+.

-- In acid-base equilibria, protons are donated in forward and reverse reactions.

-- The two substances in a conjugate acid-base pair differ by a H+...

HNO2(aq) + H2O(l) NO2–(aq) + H3O+(aq)

ACIDCONJ.BASE

baseconj.acid

In every acid-base rxn, the position of eq. favors thetransfer of H+ from stronger acid to stronger base.

STRONGERACID

weaker(conjugate)

acid

STRONGERBASE

weaker(conjugate)

base

-- Strong acids / bases easily / ____ H+.

-- Weak acids / bases do NOT easily / ____ H+.

donate

donate

accept

accept

-- The stronger a/n acid / base,

the weaker its conj. base / acid.