Water

12 Chemistry 2.6 rxn rates, eqm, acids and bases CR 07

Water

Water can behave as an acid or a base. Water contains H+ and OH- ions.

Water dissociates:H2O(l) H+

(aq) + OH-(aq)

Really H3O+ hydronium ion


Amphiprotic vs amphoteric

A substance that can donate or accept protons is called amphiprotic

Salts can act as acids and bases by donating or accepting H+ - usually H3O+ or OH- ions are formed – this is called amphoteric


acid base

HCl + H2O H3O+ + Cl-

Water is behaving as a base

donates proton

accepts proton


base acid

O2- + H2O OH-

Water is behaving as an acid

donates protonaccepts proton


Amphiprotic

Water is amphiprotic as it does not react with acids or bases to neutralise them – it does however donate or accept protons.

An amphoteric substance can neutralise an acid or a base


Acidic substancesWhen [H3O+] > [OH-] a solution is acidic An acidic substance is a substance that will either:

•dissolve in water to form an acidic solution OR

•Do not dissolve but react with bases and basic solutions


Basic substances

When [OH-] > [H3O+] a solution is basicA basic substance is a substance that will either:

• dissolve in water to form a basic solution OR

• Do not dissolve but react with acids and acidic solutions


Amphoteric substances

Amphoteric substances do not dissolve in water BUT do react with BOTH acidic and basic solutions.

Metal oxides and hydroxides are amphoteric


Al2O3 is amphoteric

Al2O3(s)+6H+(aq)2Al3+

(aq) +3H2O(aq)

Al2O3(s)+2OH-(aq)+3H2O 2Al(OH)4(aq)

+3H2O(aq)

evaporated NaAl(OH)4

Al2O3(s) has reacted with both an acidic solution and a basic solution

Water

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