a new chapter: 17 Acids & Bases

The basics:

• it’s just equilibrium

• same K expressions, with new names: Ka, Kb

• same equilibrium calculations

What’s new:

• the concept of Kw

• p-functions (pH, pKa, pKw)

• pH scale

What do you recall about Acid /Base Chemistry?

Acid /Base Chemistry in the Garden

Soil Acidity determines Flower colorPlant pigments: anthocyanin

Anthocyanin, red formAcid form: AH

Anthocyanin, blue formBase form, A-

The chemist-gardener: how to make your hydrangeas Pink or Blue

Pink form = protonated anthocyanin, acid form AH

Need acid: gardeners use ferrous sulfate

Reaction:

The chemist-gardener: how to make your hydrangeas Pink or Blue

Blue form = deprotonated anthocyanin, base form A-

Need base: gardeners use lime

Reaction:

The chemist-gardener: how to choose reagents for your garden

Think about it: why use ferrous sulfate?why not use HCl?

Reaction differences:

Equilibrium Constants in Acid Base Chemistry

Ka: acid dissociation constant describes how much H+ (H3O+) formed

General Acid Reaction:

Equilibrium Constants in Acid Base Chemistry

Kb: base dissociation constant describes how much OH- formed

Equilibrium Constants in Acid Base Chemistry

The magnitude of Keq ( so either Ka or Kb)determines the strength of acid or base

HCl is a strong acid: so Ka is very large, Ka = 108, what is [H3O+]?

Ferrous ion, Fe(2+) is a weak acid: so Ka is small, Ka = 10-10. Now what is [H3O+]?

Takes Two to Tango! Acid Base Conjugate Species

AH + H2O A- + H3O+acid conjugate

basebase conjugate

acid

Takes Two to Tango! Acid Base Conjugate Species

B + H2O BH+ + HO-base conjugate

acidacid conjugate

base

Takes Two to Tango! Acid Base Conjugate Species

B + AH BH+ + A-base conjugate

acidacid conjugate

base

Dual Personality of Water: both an acid and a baseAmphoteric (or amphiprotic)

The Autoionization reaction of water.

2 H20 H3O+ + OH-

Keq =

Kw

Because Kw = [H3O+][OH-] = 10-14 and Kw is a constant: then

When [H3O+] = [OH-] = 10-7 M, the solution is neutral

[H3O+] [OH-]

Kw

When [H3O+] > [OH-], [H3O+] > 10-7 M,the solution is acidic

[OH-] < 10-7 M

[H3O+] > 10-7 M

Kw

When [OH-] > [H3O+], [OH-] > 10-7 M, the solution is basic

Because Kw = [H3O+] x [OH-] must be 10-14

[OH-] > 10-7 M

[H3O+] < 10-7 M

Because all these exponential numbers are a pain:i.e. Kw = [H3O+][OH-] = 10-14

[H3O+] = [OH-] = 10-7 M (in neutral water)

Ka = 10-10

we will use: The P-Functionp of (a number) = -log10(a number)

so “pH” means p of [H3O+], or -log10 [H3O+]

so the pH of neutral water, where [H3O+] = 10-7 M is pH 7

The P-Function and K’s

p of (Keq) = -log10Keq = pKeq

p of (Ka) = -log10Ka = pKa

p of (Kw) = -log10Kw = pKw

Examples: Keq = 10-10 the pKeq = 10

Ka = 1.20 x 10-5 the pKeq = 4.92

Kw = 10-14 the pKw = 14

The P-Function simplifies exponential numbers

Ka = 1.20 x 10-5 , pKeq = 4.92

[H3O+] = 4.20 x 10-5 M, pH = 4.376

[OH-] = 6.66 x 10-6 M, pOH = 5.176(note

number of sig figs)

Examples converting from p-function: if pH = 7.47,

[H3O+] = 10-7.47 M= 3.39 x 10-8 M,

if pKa = 4.92, Ka = 10-4.92 = 1.20 x 10-5

P-Function simplifies a large range of numbers: graphically

10 1 10-1 10-3 10-5 10-7 10-9

10-11 10-13 10-14

[H3O+], M

-1 0 1 3 5 7 9 11 13 pH

Note that on a p-scale, the smaller the p-number, the larger the actual number

converts to a simpler scale

Apply the P-function to each side

p of Kw = p of [H3O+][OH-] = p of 10-14

Working in P-Functions can simplify problems

Recall Kw = [H3O+][OH-] = 10-14

-log Kw = -log ( [H3O+][OH-] )= -log 10-14

pKw = pH + pOH = 14-log Kw = -log [H3O+] + ( -log [OH-] ) = -log 10-14

Kw = 10-14

Now apply this equation: pKw = pH + pOH = 14

to this picture

[H3O+] [OH-]

pKw = 14

Now apply this equation: pKw = pH + pOH = 14

to this picture

pH pOH

pKw

When the solution is acidic[H3O+] > 10-7 M, pH < 7 : pH is small

Because pKw = pH + pOH must be 14

pH < 7 pOH > 7

pKw

When the solution is ____________[H3O+] __10-7 M, pH ___ 7 : pH is ________

pOH is_______

pH is_______

Fill in the blanks!