Acids and Bases: Introductory Concepts Arrhenius...acids increase the ______ when dissolved in H 2 O....bases increase the ______ when dissolved in H 2 O. e.g., HCl and NaOH -- [H + ] [OH – ] useful concept, but limited to aq. solns. Svante Arrhenius (1859–1927)

Bronsted-Lowry acids: Bronsted-Lowry bases: -- H + and H 3 O + are used interchangeably -- B-L concept is NOT limited to aqueous solutions can transfer H + to other substances (i.e., they are proton donors) can accept H + from other substances (i.e., they are proton acceptors) closer to reality; “hydronium ion” faster to write; “hydrogen ion” H3O+H3O+ H + Johannes Bronsted (1879–1947) : Thomas Lowry (1874–1936)

e.g., NH 3 in above example Arrhenius and Bronsted-Lowry definitions overlap, in many cases. NH 3 (aq) + H 2 O(l) NH 4 + (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+H+ [] + Arrhenius BronstedLowry

e.g., NH 3 in above example Arrhenius and Bronsted-Lowry definitions overlap, in many cases. NH 3 (aq) + H 2 O(l) NH 4 + (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+H+ [] + Arrhenius BronstedLowry

Amphoteric substances can be acids or bases, depending on the reaction conditions. HCl(aq) + H 2 O(l) NH 3 (aq) + H 2 O(l) H 3 O + (aq) + Cl – (aq) NH 4 + (aq) + OH – (aq) B-L baseB-L acid NH 3 is another example. When a B-L acid, it morphs into NH 2 – on the P side. “ “ “ base, “ “ “ NH 4 + “ “ “ “. [] + [] –

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 +... HNO 2 (aq) + H 2 O(l)NO 2 – (aq) + H 3 O + (aq) ACID CONJ. BASE base conj. acid

In every acid-base rxn, the position of eq. favors the transfer of H + from stronger acid to stronger base. STRONGER ACID weaker (conjugate) acid STRONGER BASE 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.