Today’s lesson is brought to you by 7, 14, and the letters Kw, pH Kw: the ion-product constant of water Q: will pure distilled water conduct electricity A: As the demo shows, it will (slightly). • If water conducts electricity, ions must exist • Water exists as an equilibrium, which is referred to as the self-ionization of water: H 2 O + H 2 O ↔ H 3 O + (aq) + OH – (aq) Simplified reaction : H 2 O ↔ H + (aq) + OH – (aq) Kc = [H 3 O + ] [OH – ] [H 2 O] 2 Kw = or [H 3 O + ] [OH – ] Note: H + is just shorthand for H 3 O + Kc = [H + ] [OH – ] [H 2 O] 2 Kw = or [H + ] [OH – ] Kw • What is the value of Kw? • It has been measured at 1.0 x 10 –14 (25 °C) • Notice (pg. 600) that by definition: when the solution is [H + ] > [OH – ] acidic [H + ] < [OH – ] basic [H + ] = [OH – ] neutral • Pure water is neutral since [H + ] and [OH – ] must be identical (both come from one H 2 O) • As temperature increases Kw increases Q: Rewrite the equilibrium of water with heat as
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