Chapter 16Acids & Bases

Acid-Base Theory

• How would you describe an acid or a base?– Similar questions about acids and bases have

been asked ever since their discovery in the 1200’s

– Because of these curiosities, several theories were generated in attempts to clarify what makes an acid an acid (or a base a base)

Acid-Base Theory - Arrhenius• Arrhenius defined the

terms acid and base as follows:– An acid is a substance that

produces H+ ions when dissolved in water (now described as hydronium rather than H+).

– A base is a substance that produces OH- ions when dissolved in water

Hydronium Ion (H3O+) or (H+(aq))

Acid-Base Theory - Arrhenius

• We can demonstrate these definitions as follows:

HNO3(l) H+(aq) + NO3-(aq)

H2O

Ca(OH)2(s) Ca2+(aq) + 2OH-(aq)H2O

Acid-Base Theory - Arrhenius

• Are there any errors, or oversights, in the Arrhenius Theory?

– Consider ammonia, NH3, which is a well known base:

– What happens when ammonia is placed in water?

NH3(g) ????H2O

Brnsted-Lowry Theory

• Bronsted and Lowry came up with an alternative definition for acids and bases

Bronsted Acids & Bronsted Bases

• A bronsted acid is a substance from which a proton (H+) can be removed (proton donor).

• A bronsted base is a substance that can remove a proton from an acid (proton acceptor).

Conjugate Acids & Bases

• A conjugate base is formed when an acid releases a proton (H+).

• A conjugate acid is formed when a base accepts a proton (H+).

Acids and Conjugate Bases (Page 491)

Bases and Conjugate Acids (Page 491)

Bronsted Acid – Base Reactions

• An acid and a base that are related by the gain and loss of a proton are called a conjugate acid-base pair.

• Every acid has associated with it a conjugate base.

• Likewise, every base has associated with it a conjugate acid.

Conjugate Acid – Base Pairs

Conjugate Acid – Base Pairs

Water is amphoteric

Amphoteric

• A substance that may either gain or lose a proton is amphoteric.

• In other words it can act like either a Bronsted acid or a Bronsted base.

• Amphoteric substances include hydrogen containing anions, hydroxides of some metals (especially those near the metal/nonmetal dividing line).

What ion do you think is present in the solution below?

• For every ion in a solid or solution there must be an ion of opposite charge, a counter ion, in order for the solid or solution to be electrically neutral.

Cu2+

• Hydronium ion is the conjugate acid of water and hydroxide ion is the conjugate base of water.

Self – ionization (autoionization) of water

Water can react with itself in a process known as self – ionization (autoionization).

Self – ionization of water• Occurs to a very slight extent (≈2 in every

billion water molecules self – ionize at 25°C).

Write the equilibrium expression for this reaction.

Ion Product of waterKw = [H3O+][OH-]

Kw = [H3O+][OH-] = 1.0 x 10-14 (at 25°C)°

Kw = [H3O+][OH-] = 1.0 x 10-12 (at 100°C)°

Is this reaction exothermic or endothermic? Support your answer.

• How does adding and acid (i.e. HCl) to water affect [H3O+], [OH-] and the Kw?

• How does adding and base (i.e. NaOH) to water affect [H3O+], [OH-] and the Kw?

• Weak acidsWeak acids are much less than 100% ionized in are much less than 100% ionized in

water.water.

• One of the best known is acetic acid (CHOne of the best known is acetic acid (CH33COCO22H)H)

Weak AcidsWeak AcidsWeak AcidsWeak Acids

A

• Strong Base:Strong Base: 100% dissociated in water.100% dissociated in water.

NaOH (aq) → NaNaOH (aq) → Na+ + (aq) + OH(aq) + OH- - (aq)(aq)

Strong BasesStrong Bases

• Weak base:Weak base: less than 100% ionized in less than 100% ionized in waterwater

• One of the best known weak bases is One of the best known weak bases is

ammoniaammoniaNHNH3 3 (aq) + H(aq) + H22O (l) ↔O (l) ↔ NH NH44

+ + (aq) + OH(aq) + OH- - (aq)(aq)

Weak BasesWeak BasesWeak BasesWeak Bases

CH3CO2H is added to H2O

• Write the reaction that occurs.

• Write the equilibrium expression for the reaction.

Equilibrium Constants Equilibrium Constants for Weak Acidsfor Weak Acids

Equilibrium Constants Equilibrium Constants for Weak Acidsfor Weak Acids

• A Weak acid has a KA Weak acid has a Kaa

• The higher the KThe higher the Kaa the stronger the acid the stronger the acid

NH3 is added to H2O

• Write the reaction that occurs.

• Write the equilibrium expression for the reaction.

Equilibrium Constants Equilibrium Constants for Weak Basesfor Weak Bases

Equilibrium Constants Equilibrium Constants for Weak Basesfor Weak Bases

• A Weak base has a KA Weak base has a Kbb • The higher the KThe higher the Kbb the stronger the base the stronger the base

Appendix F and G in the front of your notebook contain many Ka

and Kb values.

• This is not a complete listing of weak acids and weak bases however it does seem that there are more weak acids.

Weak Acids

Name Chemical Formula KA

Acetic acid HC2H3O2 1.80 x 10-5

Acetylsalicylic acid HC9H7O4 3.2 x 10-4

Benzoic acid HC7H5O2 6.0 x 10-5

Chloroacetic acid ClCH2COOH 1.40 x 10-3

Formic acid HCOOH 1.77 x 10-4

Hydrocyanic acid HCN 4.00 x 10-10

Hypochlorous acid HOCl 4.00 x 10-8

Hydrofluoric acid HF 7.20 x 10-4

Hypoiodous acid HOI 2.30 x 10-11

Lactic Acid HC3H5O3 1.5 x 10-4

Nicotinic acid HC6H4NO2 1.40 x 10-5

Nitrous Acid HNO2 4.5 x 10-4

We can use Ka and Kb values to rank the strength of weak acids and weak bases.

Weak Acids

Name Chemical Formula KA

Acetic acid HC2H3O2 1.80 x 10-5

Acetylsalicylic acid HC9H7O4 3.2 x 10-4

Benzoic acid HC7H5O2 6.0 x 10-5

Chloroacetic acid ClCH2COOH 1.40 x 10-3

Formic acid HCOOH 1.77 x 10-4

Hydrocyanic acid HCN 4.00 x 10-10

Hypochlorous acid HOCl 4.00 x 10-8

Hydrofluoric acid HF 7.20 x 10-4

Hypoiodous acid HOI 2.30 x 10-11

Lactic Acid HC3H5O3 1.5 x 10-4

Nicotinic acid HC6H4NO2 1.40 x 10-5

Nitrous Acid HNO2 4.5 x 10-4

Which weak acid below is the strongest?

The higher the Ka the stronger the acid.

Why does a higher Ka value indicate that an acid is stronger?

For two acids of the same molarity the one with the higher Ka is stronger because it dissociates more. This increases the concentration of the products

(more specifically hydronium).

Which weak base below is the strongest?

Weak Bases

Name Chemical Formula KB

Ammonia NH3 1.80 x 10-5

Diethylamine (CH3CH2)2NH 8.6 x 10-4

Methylamine CH3NH2 4.2 x 10-4

Pyridine C5H5N 1.5 x 10-9

Quinoline C9H7N 6.3 x 10-8

Trimethylamine (CH3)3N 7.4 x 10-5

Why does a higher Kb value indicate that an base is stronger?

For two bases of the same molarity the one with the higher Kb is stronger because it dissociates more. This increases the concentration of the

products (more specifically hydroxide).

Relative Strengths of Conjugate Pairs

Ka x Kb = Kw = 1 x 10-14

• This equation is used to determine the Ka or Kb value for an acid or base within a conjugate pair.

• Teacher Example: Determine the Kb of acetate ion.

• (See Appendix F) Ka CH3CO2H = 1.8 x 10-5

• 1.8 x 10-5 x Kb = 1 x 10-14

• Kb CH3CO2- = 5.6 x 10-10

What is the Kb of PO43-?

• Ka HPO42- = 3.6 x 10-13

• For any conjugate pair:

Ka x Kb = Kw = 1 x 10-14

• Kb for PO43- = 2.8 x 10-2

For the conjugate pair: HPO4

2- & PO43-

• See Appendix F and G in the front of your notebook

• Ka HPO42- = 3.6 x 10-13

Ka x Kb = Kw = 1 x 10-14

• Kb for PO43- = 2.8 x 10-2

Conjugate Pair Acid – Base Strength Comparison

CH3CO2H and CH3CO2-

•Ka CH3CO2H = 1.8 x 10-5

•Kb CH3CO2- = 5.6 x 10-10

HPO42- and PO4

3-

•Ka HPO42- = 3.6 x 10-13

•Kb for PO43- = 2.8 x 10-2

Is NH4+ a stronger acid than HCN? Support your answer.

Acid Base Neutralization

Acid + Base → Salt + Water

• These reactions go to completion. (→)

Acid Base Neutralization

Acid + Base → Salt + Water

HCl + NaOH → NaCl + H2O

2HNO3 + Ba(OH)2 → Ba(NO3)2 + 2H2O

• salt (an ionic compound containing the cation from a base and the anion from an acid)

Acid – Base Neutralization

• Assume stoichiometric amounts of acid and base.

1. A strong acid and a strong base form a neutral solution.

2. A strong acid and a weak base form an acidic solution.

3. A weak acid and a strong base form a basic solution.

4. A weak acid and a weak base can form an acidic, basic, or neutral solution.

5. The salt formed in the neutralization reaction is responsible for the observations listed above.

Neutralization Reaction

– A neutralization reaction occurs when an acid and a base react to form water and a salt (an ionic compound containing the cation from a base and the anion from an acid)

NaOH + HCl → NaCl + H2O

KOH + CH3CO2H → KCH3CO2 + H2O

Is the solution that results from each reaction acidic, basic or neutral?

Neutralization Reaction

NaOH + HCl → NaCl + H2O (neutral)

KOH + CH3CO2H → KCH3CO2 + H2O (basic)

Why are the solutions that result from neutralization reactions acidic, basic or neutral?

NaCl is a neutral salt and

KCH3CO2 is a basic salt

Acidic, Basic, and Neutral Salts

*

All anions other than those from strong acids or HSO4- and H2PO4

-*All cations other than those from strong bases* *

Cations from groups I and II

* *

A basic ion reacts as a base when added to water.A basic ion reacts as a base when added to water.

Na2CO3

Na+

CO32-

An acidic ion reacts as an acid when added to water.An acidic ion reacts as an acid when added to water.

KHSO4

K+

HSO4-

Is the salt acidic, basic, or neutral?Write an equation to support your

answer if the salt is acidic or basic.

NH4I

Is the salt acidic, basic, or neutral?Write an equation to support your

answer if the salt is acidic or basic.

KClO4

Is the salt acidic, basic, or neutral?Write an equation to support your

answer if the salt is acidic or basic.

Na3PO4

Is the salt acidic, basic, or neutral?Write an equation to support your

answer if the salt is acidic or basic.

Zn(NO3)2

Is the salt acidic, basic, or neutral?Write an equation to support your

answer if the salt is acidic or basic.

AlCl3

Is the salt acidic, basic, or neutral?Write an equation to support your

answer if the salt is acidic or basic.

NH4NO2

Acid – Base Strength

• We have already discussed some methods for comparing strengths of acids and bases:

1. Percent ionization

2. Ka or Kb comparison

• Example HCl is a stronger acid than CH3CO2H

(Even though it may sound odd. This is the same as saying that CH3CO2H is a stronger base than HCl).

Strengths of Binary Acids (H#X)

• Binary Acid Examples: HCl, H2O, NH3, CH4

• Binary acid strength is based on:1. Bond Polarity (Electronegativity)

2. Bond Strength

Strengths of Binary Acids (H#X)

Rule 1

When moving from left to right in a period the acid strength increases with the increasing electronegativity of “X”. This is because the H X bond becomes more polarized.

• Example: HCl is a stronger acid than H2S.

Strengths of Binary Acids (H#X)

Strengths of Binary Acids (H#X)

Rule 2

When moving down a group the acid strength increases because the H – X bond is weaker and easier to break.

• The H – F bond is very strong and hard to break and therefore HF is weaker than the other acids.

• The H – I bond is the weakest and easiest to break and therefore HI is the strongest acid.

Confused?

• These rules seem to contradict each other but if we look at the bond energies it may make more sense.

• Bond energy is a greater factor in a group than in a period.

BondBond Bond EnergyBond Energy

(kJ/mol)(kJ/mol)

H – C 415

H – F 569

H – I 295

• Even though HI is the strongest acid we consider HCl, HBr, and HI all to be strong acids.

• This is because of the leveling effect.

The Leveling Effect

• Some solvents such as water, can make these acids appear to be the same strength as they all ionize ≈ 100%.

• This is known as the leveling effect.

• This leveling effect can also be seen with the strong bases.

Strengths of Oxyacids (HOX)

• Oxyacid examples: HClO, H2SO4, NaOH

• Oxyacid strength is based on:1. Electronegativity of “X”

2. The number of oxygen atoms

Strengths of Oxyacids (HOX)

Rule 1• As the electronegativity of “X” increases the

strength of the oxyacid increases.• This can be explained by the electron withdrawing

power of oxygen compared to “X”. • Oxygen is an “electron bully”

Strengths of Oxyacids (HOX)

Na – O – H ••

••

Strengths of Oxyacids (HOX)

Strengths of Oxyacids (HOX)

Strengths of Oxyacids (HOX)

Rule 2

For oxyacids with the same element “X”. The acid strength increases as the

number of oxygen atoms increases.• This can be explained by the electron withdrawing

power of oxygen. • Oxygen is an “electron bully”

Strengths of Oxyacids (HOX)

Mono-, Di-, and Triprotic AcidsAcids can be classified as to the number of

hydrogen atoms that they can give up (labile protons).

• Monoprotic acids contain “1” ionizable H.– HCl, HNO2, CH3CO2H

• Diprotic acids contain “2” ionizable H’s.– H2SO4, H2S, H2CO3

• Triprotic acids contain “3” ionizable H’s.– H3PO4, H3AsO4

The Stepwise Dissociation of Phosphoric Acid

H3PO4 (aq) + H2O(l) H2PO4-(aq) + H3O+

(aq)

H2PO4-(aq) + H2O(l) HPO4

2-(aq) + H3O+

(aq)

HPO42-

(aq) + H2O(l) PO43-

(aq) + H3O+(aq)

For polyprotic acids each successive ionization is weakerFor polyprotic acids each successive ionization is weaker

a

Practice

Practice

Practice

Practice

Practice

Practice

Practice

Which is the stronger acid H2C2O4 or HC2O4ˉ ?

Practice

Which is the stronger acid H2C2O4 or HC2O4ˉ ?

This is in your notebookRules for Strengths of Acids and Bases

1. Strong acids such as HCl are strong because they dissociate completely in water forming

a high yield of H3O+ ions. Strong bases such as NaOH are strong because they dissociate

completely in water forming a high yield of OH- ions. Even though the strengths of strong acids and strong bases can vary somewhat we do not see these differences in aqueous solution because water “levels” or evens out the differences in the strengths of these substances because of its ability to easily donate or accept H+ ions when in solution with strong acids or strong bases. This is known as the “levelling effect” of water.

2. Weak acids and bases only ionize slightly forming a much lower yield of H3O

+ or OH- ions. Normally the ionization of these substances is less than 5% in water and therefore weak acids and bases are primarily in unionized molecular form when in aqueous solution. The strengths of weak acids can be compared from their Ka values and the strength of weak bases from their Kb values. The higher the Ka the stronger the acid and the higher the Kb the stronger the base.

3. Binary acids (HX) consist of one or more H+ ions combined with a single anion. They increase in strength within a row in the periodic table as element X moves towards the elements in group 17 (left to right in the periodic table). This is because as we move in this direction the H – X becomes more polarized making it more likely for H+ ions to form (H X). This explains why HF is a stronger acid than H2O.

As element X moves down within a group in the periodic table the H – X bond becomes weaker and therefore easier to break. The easier the bond is to break the more likely the substance is to release H+ ions and therefore become a stronger acid. This explains why HCl is a stronger acid than HF.

It should be obvious from these two rules for binary acids that bond strength is less of a factor within a period and more of a factor within a group when it comes to the strengths of binary acids.

4. Oxyacids such as HNO3 (HOX) become stronger acids as the electronegativity of the

element “X” (the element other than hydrogen or oxygen) increases. All oxyacids within their Lewis structure have H bonded to O bonded to X (H – O – X). Of these three elements the oxygen will have the greater ability to withdraw electrons from the H or X. It will preferentially withdraw electrons from the element (either H or X) which has the lower electronegativity. The higher the electronegativity of the element X the more likely the O is to withdraw electrons from H causing the bonds in H – O – X to polarize as (H O – X). This will cause H+ to form making the substance more acidic. This is why HClO3 is stronger than HBrO3.

As X becomes less electronegative the H – O bond becomes less polarized until eventually X becomes less electronegative than H causing O to withdraw electrons from X rather than H. This causes the bonds to polarize such that OH- forms, (H – O X), creating a base such as NaOH.

In oxyacids with the same element “X”, the acid strength increases as the number of electron withdrawing oxygens in the molecule increases. As the number of oxygen atoms in the molecule increases there is a greater tendency to polarize the O – H bond to produce H+, (H O – X). For example HClO4 is a stronger acid than HClO3.

Volumetric Analysis (Titration)

(Standard solution)(Standard solution)

Titration of a 40.00mL sample of HTitration of a 40.00mL sample of H33POPO44 solution requires solution requires

35.00mL of 0.1500M KOH to reach the end point. 35.00mL of 0.1500M KOH to reach the end point. Determine the molar concentration of the HDetermine the molar concentration of the H33POPO44..

2KOH + H3PO4 → K2HPO4 + 2H2O

0.06563M H3PO4

10.0 grams of vinegar (containing acetic acid, CH10.0 grams of vinegar (containing acetic acid, CH33COCO22H) is H) is

titrated with 65.40mL of 0.150 M NaOH to reach an endpoint. titrated with 65.40mL of 0.150 M NaOH to reach an endpoint.

What is the percent by mass of acetic acid in the vinegar?What is the percent by mass of acetic acid in the vinegar?

10.0 grams of vinegar (containing acetic acid, CH10.0 grams of vinegar (containing acetic acid, CH33COCO22H) is H) is

titrated with 65.40mL of 0.150 M NaOH to reach an endpoint. titrated with 65.40mL of 0.150 M NaOH to reach an endpoint.

What is the percent by mass of acetic acid in the vinegar?What is the percent by mass of acetic acid in the vinegar?

5.89%