Acids and Bases Section 8.4: K w and the pH scale Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

O H H+ O H H O H HH O H - + [] + Acid-Base Properties of Water H 2 O (l) H + (aq) + OH - (aq) H 2 O + H 2 O H 3 O + + OH - acid accepts 2 e’s donates H + produces H + or H 3 O + conjugate base base donates 2 e’s accepts H + produces OH - conjugate acid GD 18.1 C 15.2 autoionization of water

Write the equilibrium constant expression for the autoionization of water. K c =

H 2 O (l) H + (aq) + OH - (aq) The Ion Product of Water K c = [H + ][OH - ] [H 2 O] [H 2 O] = constant K c [H 2 O] = K w = [H + ][OH - ] H 2 O (l ) H 3 O + (aq) + OH - (aq) K c = [H 3 O + ][OH - ] [H 2 O] 2 [H 2 O] = constant K c [H 2 O] 2 = K w = [H + ][OH - ]

The Ion Product of Water The ion-product constant (K w ) is the product of the molar concentrations of H + and OH - ions at a particular temperature. At 25 0 C K w = [H + ][OH - ] = 1.0 x when [H + ] = [OH - ] when [H + ] > [OH - ] when [H + ] < [OH - ] Solution Is neutral acidic basic 15.2

The Ion Product of Water The ion-product constant (K w ) is the product of the molar concentrations of H + and OH - ions at a particular temperature. At 25 0 C K w = [H + ][OH - ] = 1.0 x therefore [H + ] = [OH - ] = Do these numbers (14, 7) have significance? 15.2

The Ion Product of Water The ion-product constant (K w ) is the product of the molar concentrations of H + and OH - ions at a particular temperature. At 25 0 C K w = [H + ][OH - ] = 1.0 x Note: The autoionization of water is an endothermic process. What will happen to the K w if the temperature increases? equil shifts to the right – more H + and OH - produced… K w increases… pH decreases water is still neutral… but pH is slightly less than 7 at higher temps 50°C [H + ] = [OH - ] = 3.05 x ;pH is 6.5 heat + H 2 O (l) H + (aq) + OH - (aq)

What is the concentration of OH - ions in a HCl solution whose hydrogen ion concentration is 1.3 M? 15.2

What is the concentration of OH - ions in a HCl solution whose hydrogen ion concentration is 1.3 M? K w = [H + ][OH - ] = 1.0 x [H + ] = 1.3 M [OH - ] = KwKw [H + ] 1 x = = 7.7 x M 15.2

pH – A Measure of Acidity pH = - log [H + ] [H + ] = [OH - ] [H + ] > [OH - ] [H + ] < [OH - ] Solution Is neutral acidic basic [H + ] = 1 x [H + ] > 1 x [H + ] < 1 x pH = 7 pH < 7 pH > 7 At 25 0 C pH[H + ] 15.3

pH can be measured with universal indicator pH probe

15.3 pOH = -log [OH - ] [H + ][OH - ] = K w = 1.0 x log [H + ] – log [OH - ] = pH + pOH = Note: the focus doesn’t always have to be on the [H + ]. You can also consider the [OH - ]

15.3 pH [H + ] [OH - ] universal indicator RedOrangeGreenBluepurple descriptio n Common example Laboratory dilute acid (1 M HCl) Vinegar, acid rain Pure waterMilk of magnesia, household ammonia Laboratory dilute alkali (1 M NaOH)

The pH of rainwater collected in a certain region of the northeastern United States on a particular day was What is the H + ion concentration of the rainwater? The OH - ion concentration of a blood sample is 2.5 x M. What is the pH of the blood? 15.3

The pH of rainwater collected in a certain region of the northeastern United States on a particular day was What is the H + ion concentration of the rainwater? pH = - log [H + ] [H + ] = 10 -pH = = 1.5 x M The OH - ion concentration of a blood sample is 2.5 x M. What is the pH of the blood? pH + pOH = pOH = -log [OH - ]= -log (2.5 x )= 6.60 pH = – pOH = – 6.60 =

What is the pH of a 2 x M HNO 3 solution? What is the pH of a 1.8 x M Ba(OH) 2 solution? 15.4

What is the pH of a 2 x M HNO 3 solution? HNO 3 is a strong acid – 100% dissociation. HNO 3 (aq) + H 2 O (l) H 3 O + (aq) + NO 3 - (aq) pH = -log [H + ] = -log [H 3 O + ] = -log(0.002) = 2.7 Start End M 0.0 M What is the pH of a 1.8 x M Ba(OH) 2 solution? Ba(OH) 2 is a strong base – 100% dissociation. Ba(OH) 2 (s) Ba 2+ (aq) + 2OH - (aq) Start End M M0.0 M pH = – pOH = log(0.036) =

Homework … due ___________ Read Section 8.4 – The pH scale (pp ) Answer Ex 8.4 p 216 #1-8