Chapter 16

John A. SchreifelsChemistry 212

Chapter 16-1

8–1

Chapter 16

Acids and Bases


Chapter 16-2

8–2

Overview

• Acid – Base Concepts– Arrhenius – Brønsted – Lowry– Lewis

• Acid and Base Strengths– Relative Strengths of Acids and Bases– Molecular Structure and Acid Strength

• Self – Ionization of Water and pH– Self – Ionization of Water– Solutions of a Strong Acid or Base– The pH of a Solution


Chapter 16-3

8–3

Acid – base concepts

• Arhennius Concept:– acid: provides H+ ions (called a proton) in water.

HA(aq) H+(aq) + A(aq)– base: provides OH in water.

• Brønsted–Lowry Concept:– acid: donates H+.– base: accepts H+.

• No water or hydroxide required.– Base converted to its conjugate acid and develops properties that are those

of an acid.

E.g. Identify each as either an acid or base and determine its conjugate: and , F, H2CO3 and E.g. Identify the conjugate acids and bases in the reaction below:

• Amphoteric substance: a substance that can act as either an acid or base. (E.g. H2O in examples)

)aq(OH)aq(M)aq(MOHOH2

)aq(OH)aq(NH O(l)H (aq)NH 423

(aq)NH)aq(OH O(l)H )aq(NH 3324

3HCO 2

3CO 4HPO


Chapter 16-4

8–4

Acid – base concepts 2

• Lewis acid: electron pair acceptor• Lewis base: electron pair donor. .

• Lewis definition most general, then Brønsted-Lowery and finally Arhennius:

• E.g. • E.g.2 determine the Lewis acid and base in BF3 and

NH3 in the reaction:

• E.g.3 determine the Lewis acid and base in the following reaction:

Co3+(aq) + 6F(aq) [CoF6]3.

(aq)NH)aq(OH O(l)H )aq(NH 3324

233 )NH(Ag)aq(NH2)aq(Ag

3333 BFNHBFNH


Chapter 16-5

8–5

Acid/Base Strength

• Extent of acid/base reaction variable and depends upon the relative strengths of the acids and conjugate bases..

• Strong acids and bases react with other reactant to produce all product.

Common Strong Acids and Bases

Acids Bases

HClO4 LiOH

H2SO4 NaOH

HI KOH

HBr Ca(OH)2

HCl Sr(OH)2

HNO3 Ba(OH)2

Stronger acids and bases react to form weaker conjugate bases and acids.

)aq(A)aq(OH)l(OH)aq(HA 32

)aq(Cl)aq(OH)l(OH)aq(HCl 32


Chapter 16-6

8–6

Relative Strengths of Acids and Bases and Extent of Reaction

• The table of relative strengths of acids and conjugate bases can be used to predict if a reaction will produce product.

E.g. Which will produce product?

HNO3 + CN or HCN +

3NO

ClOHSSHClO

HNOFNOHF

or

or

233


Chapter 16-7

8–7

Factors Affecting Acid Strength

• Binary acids: – Bond strength is directly related to the acid strength (bond size).

• HI and HBr have larger bonds lengths and are more acidic than HF and HCl, even though fluorine is most electronegative.

– For bonds of similar size the acid strength is related to electronegativity difference.

E.g. bonds across a row more acidic towards right side of periodic table.

• Oxyacids are acidic substances that contain oxygen and some other nonmetal, e.g. HNO3, HOCl, etc.. Anything that affects the polarity of the O–H bond will affect the strength of the acid. – An increase in the electronegativity of an atom bound to oxygen

increases in polarity of the bond and makes it more acidic.– More oxygen = more polar.

E.g. determine relative acidity of HOI, HOBr and HOCl.

E.g. HClO4 is strongest acid in its oxyacid series.


Chapter 16-8

8–8

Autoionization of Water

• Water can act as both an acid and a base equilibrium is: H2O + H2O H3O+ + OH. Kw = [OH][H3O+] = 1.00 x 1014M2.

• Since [OH] = [H3O+] [H3O+] = 1 x 107 M (called a neutral solution)– Acidic [H3O+] > 1.00x107M– Neutral [H3O+] = 1.00x107M– Basic [H3O+] < 1.00x107M

• All acids/bases dissolved in water must obey equation for the ionization of water. – They either add H3O+ or OH to water.

• Most of the acids in this chapter will be stronger than water and add significantly to the hydronium ion concentration.E.g. the hydronium ion concentration of an acidic solution was 1.00x105 M. What was the [OH]?

• E.g. what is the hydronium ion concentration if the hydroxide concentration was 2.50x103 M?


Chapter 16-9

8–9

Strong acids and Bases

• A strong acid is completely dissociated in water. This leads to [H3O+] and [OH]. Eg: Calculate the [H3O+],[OH] and [Cl] for a 0.048 M HCl solution. Assume the contribution from water is negligible.E.g.2 Calculate the conc. of all ionic species as well as the pH if CNaOH = 0.080 M. E.g. 3 what is pH and [OH] of 0.125 M Ba(OH)2.

• [H3O+] of water is small compared to added [H3O+] from the acid and ignored in the calculation.

• A more rigorous treatment is:

– The last term is very small except when the concentration of the strong acid is very small.

– When the concentration of added acid is small compared withpure water, include the contribution from water.

E.g.4 estimate the [H3O+] of 108 M HCl.

OH3SA3total3 2]OH[]OH[]OH[


Chapter 16-10

8–10

The pH of a Solution

• pH = log[H3O+] and [H3O+] = 10pH – Acidic pH < 7.00– Neutral pH = 7.00– Basic pH > 7.00

E.g. determine the pH of a solution in which [H3O+] = 5.40x106 ME.g.2 determine the pH of a solution in which the [OH] = 3.33x103 ME.g.3 determine the pOH of a solution in which the [OH] = 3.33x103 M

E.g.4 Determine the [H3O+] if the pH of the solution is 7.35.• The term pX is defined in exactly the same way as pH. Eg.5 What is the pCa if [Ca2+] = 6.44x10-4

[H3O+][OH ] = 1.00x10 14

log{[H3O+][OH ]} = log{1.00x10 14}

pH + pOH = pKa = 14.00


Chapter 16-11

8–11

Methods of Measuring pH

• pH paper is used that has compounds in it which are change to different colors for different pH ranges.

• An colored indicator can be placed in the solution and its color correlated with pH.

HIn(aq) + H2O(l) H3O+(aq) + In(aq).

E.g. phenolphthalein is colorless in acid form but pink in basic form. – The pH at which they change color depends on their

equilibrium constant.

• More accurate and precise measurements are made with a pH meter. A combination of voltmeter and electrodes.

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