acids and bases

Chap 15 Acids and BasesKey components Bronsted-Lowery Acids and Bases Conjugate Acid-Base Pairs pH Scale Strong and Weak Acids and Bases Ka, Kb, and Kw

Salt Solutions Lewis Acids and Bases Lab: Indicators and Titration Curve.

What are acids? What are bases?

Acids: A solution which contains more H+ than OH-.

Base: A solution which contains more OH- than H+.

Write the Equilibrium Equation for ionization of water.Write the equilibrium expression for

this ionization. Is pure water a good conductor

electricity? Why? In a given moment ionization take

place at a rate of 1 out of 8 hundred million molecules. Then what?

Autoionization

Water is excluded in the equilibrium expression for aqueous solutions.New Constant is formed because of

this. Kw = ion product constant

Kw= [H+][OH-]

Kw= [H+][OH-] = 1.0 x10-14

–Memorize this expression

Thus:

Acid: greater [H+]Base: greater [OH-]Neutral: [H+] = [OH-] Go to overhead 1

The Proton in Water

H3O+ Hydronium ion

H+ Hydrogen ion on board notes 5

Practice Problems Chapter 15Page 641-644

Bronsted-Lowery (4,6,8) Dissociation of water (10) pH Scale (16-26 Even) Weak acids/Weak bases (32,34,36) Ka and Kb (42,44,46,48,52,54) Diprotic and Polyprotic Acids (60,62) Structure and Strength (66,68) Acid-Base Properties for Salts (76,78,79) Oxides and hydroxides (84,86) Lewis Acid and Bases (90,92)

Bronsted -Lowery Acids and Bases.Acid: a substance capable of

donating a proton.Base: a substance capable of

accepting a proton.HCl + H2O H3O+ + Cl-

–Who is the acid, who is the base?

NH3 + HCl NH4+

+ Cl-

-Who is the acid, who is the base?

NH3 + H2O NH4+

+ OH-

–Who is the acid, who is the base?

Conjugate Acid-Base Pairs

Conjugate: Joined together as a pair.

In any acid-base reaction, we can tie together the conjugate acid base pairs.

» Overhead 3-6

Amphoteric: A substance that is capable of acting as either acid or a base.

» whiteboard

Conjugate Acid Base Strength

The more easily a substance gives up a proton, the more difficult it is for that conjugate base to accept that proton.

–The stronger the acid the weaker the conjugate base.

The more easily a base accepts a proton, the less readily its conjugate acid gives up a proton.

–The weaker the acid the stronger its conjugate base.

– Overhead 7-8

The pH ScalepH = power hydrogen ionBased on a negative

logarithm with a base of 10.

It is only negative so that the pH turns out positive.

Logarithms

Logarithm = the exponent 10 was raised to to achieve the number.

• log10 = 1 (101)• log 100 = 2 (102) • log 49.23 = (101.692229836)

AntilogarithmReverse of log process.Antilogarithm= The number that

corresponds to what 10 was raised to.

•Antilog 1= 10•Antilog 2= 100•Antilog 1.69 = 49.23

pHpH = -log[H+] What is the pH of neutral

solution? pH = -log(1.0x10-7) =-(-7.00)Answer is 7 (just plug log in

and remember to change from a neg to a positive).

[H+] = 1.0 x 10-3: What is the pH?

pH= -log(1.0 x 10-3) = -(-3.00) = 3.00

What is the pH of a solution in which [OH-] = 2.0 x 10-3?

[H+] = Kw/[OH-] = 5.0 x 10-12

pH = -log(5.0 x 10-12) = 11.30

How do you find the hydrogen ion concentration when given the pH?To obtain the [H+] when given

pH, we must take the antilog of a -pH.

What is [H+] if the pH is 3.80?pH= -log[H+] = 3.80 log[H+] = -3.80 (mult both sides by a neg 1)

[H+] = antilog(-pH) antilong(-3.8)

[H+] = antilog(-3.8) =10-3.80

10-3.80 = 1.6 x 10-4 M Review [H+] = antilog(-pH) pH = -log[H+]

pOH

pOH = -log[OH-]pH + pOH = -log Kw

=14

–Thus pH + pOH = 14

Strong Acids and Strong bases

Strong electrolytes: Solute that completely ionizes in solution and conducts electricity

Acids: HCl, HBr, HI, HNO3, HClO4

Bases: NaOH, KOH,Ca(OH)2, Alkali and Alkaline earth’s

Strong Acids and Strong Bases

HA(aq) H+(aq)

+ A-(aq)

BOH(aq) B+(aq)

+ OH-(aq)

Notice the one-way arrow.

What is the pH of a 0.011 M solution of Ca(OH)2 ?

Weak Acids partially ionized

HA(aq) H+(aq) + A-

(aq)

Notice the arrowThis is an equilibrium

reaction and all the rules of equilibrium can be applied to this equation

Equilibrium Expression

Ka = Acid- Dissociation Constant

The larger the value of Ka the stronger the acid.

Calculating Ka

A student prepared a 0.10 M solution of formic acid, HCHO2, and measured its pH at 25oC to be 2.38.

–Calculate Ka.

–What percentage of formic acid is ionized.

Niacin Overhead Question page 7

pH calculations of Weak AcidsCalculate the pH of a 0.30

M Solution of acetic acid at 25o C. What percent ionizes?

Calculate the pH of a 0.20 M solution of HCN. Percent ionization?

What is the pH of a 0.10 M Solution of niacin?

Calculate the percentage of HF molecules ionized in –a) 0.10 M HF solution –b) 0.010 M HF solution –According to LeChatlier’s principle what is the result of diluting this solution by a factor of 10. Why?

Calculate the percentage of niacin molecules ionized in – in a 0.010 M solution.

– in a 0.001 M solution.

Polyprotic Acids

An acid with more than one proton that can ionize.

Ka1, Ka2, Ka3

H2SO3 H+ + HSO3- Ka=

HSO3- H+ + SO3

- Ka=

Calculate the pH and concentration of oxalate ion, [C2O4

2 -], in a 0.020 M solution of oxalic acid, H2C2O4.

Note: Two Parts:»1. 103 will determine pH »2. Use concentrations of the ions

in Ka1 to determine the concentration of the oxalic ion.

Consider polyprotic acids: Is the second ionization always less than the first ionization? Why?

Weak BasesWeak base + H2O

Conjugate Acid + OH- Most common:NH3 + H2O NH4

+ + OH-

What is the equilibrium Expression for this equation?

Calculate the [OH-] in a 0.15M solution of NH3?Kb= 1.8 x 10-5

What is the pOH?What is the pH?What is the relationship

between Kb and pH?

Which of the following compounds should have the highest pH as a 0.05 M solution?Pyridine: Kb = 1.7 x 10-9

Methylamine:=Kb 4.4 x 10-4

Nitrous Acid:=Ka= 4.5 x10-4

Types of Weak BasesTwo Types

–1. Neutral substances that contain an atom with lone pair of electrons that can serve as a proton acceptor.

–NH3, NH2CH3, Caffeine, codeine, amphetamine, etc

Types of Weak Bases–2. Anions of weak acids

(Conjugate Bases)

– HS-

– CO32-

ExampleNaClO

Na+ ClO-

Why is ClO- a base?

A solution is made by adding solid sodium hypochlorite to enough water to make a 2L solution. If the solution has a pH of 10.50, how many moles of NaClO were added to the water?

A solution of NH3 in water has a pH of 10.50. What is the molarity of the solution?

Relation Between Ka & Kb

What happens when we add an acid to a base?

HCl + NaOH H2O + NaCl water salt

What happens when we add an acid reaction to a base reaction?

NH4+ NH3 + H+

NH3 + H2O NH4+ + OH-

Net ionic Equation H2O H+ + OH-

K = [H+][OH-]/[H2O]

Remember Reaction 1 + Reaction 2 = Reaction 3 (sum)K1 x K2 = K3 (products) Ka x Kb = [H+][OH-]all the other species in

the products cancel out and Kw = [H+][OH-] Kw = Ka x Kb

Don’t Get Freaked Out

pKa + pKb = pKw = 14.00(-logKa)+(-logKb) = (-logKw)

pH + pOH = 14.00

Calculate Kb for F-. Calculate Ka for NH3.

Which of the following anions has the largest base-dissociation constant, PO4

3-, NO2-, or

N3-? Overhead 21

5 points: Acid Base Properties of Salts

1. Salts can be acids or bases and are always completely ionized.

2. Anions of salts derived from weak acids are able to hydrolyze H2O. What does this mean?

A- + H2O HA + OH-

5 Points: Acid Base Properties of Salts

3. Anions of strong acids do not hydrolyze water. IE: NO3

- Thus, no [H+] so pH is not influenced.

5 Points: Acid Base Properties of Salts

4. Some salt anions derived from acids have ionizable protons and can be an acid or a base. IE: HSO3

- = amphoteric

Predict whether the salt Na2HPO4 will form an acidic or basic solution when dissolved in water. Overhead 22

Predict whether the dipotassium salt of citric acid, K2HC6H5O7, will form an acidic or basic solution in water.

What can you do? Indicate whether each of the following

substances will form an acidic, basic, or neutral solution.

A. KC2H3O2

B. NaHCO3

C. CH3NH3Br

D. KNO2

E. NH4CN

5 Points: Acid Base Properties of Salts

5. All cations save alkali and heavy alkaline earths(Ca2+, Sr2+, Ba2+, Ra2+) act as weak acids in water solutions and they also hydrolyze water.

5 Points of Acid Base Properties of Salts

Ionize/ no hydrolyze Li+

Na+ With this in mind, K+ Ca+ the presence of Rb+ Sr+ any of these ionsCs+ Ba+ in solution does Fr+ Ra+ not influence pH.

Consider Aluminum

AlCl3

pH of a solution of a saltWe can predict qualitatively

the pH by considering the cations and anions from which the salt was composed.

This can only be done by considering a “Normal Salt”.One without ionizable proton.

ie: Not normal NaH2PO4

NaHCO3

4 Rules for predicting pH of a salt.

1. Salts derived from a strong base and strong acid have a cation and anion that do not hydrolyze. pH= 7

2. Salts derived from a strong base and a weak acid have an ion that acts as a relatively strong conjugate base. pH>7

3. Salts derived from a weak base and a strong acid have a cation that acts as a relatively strong conjugate acid. pH<7

4. Salt derived from a weak base and a weak acid where both the cation and anion hydrolyze. pH- depends on the extent to which ion hydrolyzes.

Use all 9 rules to solve for the following problems.(1-5)explains why (6-9) work.

1. Salts ionize 2. Anions of weak acids hydrolyze water

creating an OH-(base) 3. Anions of strong acids do not hydrolyze.

(pH- not influenced) 4. Amphoteric anions can act as an acid or a

base. 5. Cations (save heavy alkaline earths/alkali)

hydrolyze and act as weak acids.

9 Rules 6. Salts derived from a strong acid and

strong base do not hydrolyze. pH=7 7. Salts derived from a strong base and

weak acid= anion that acts a relatively strong conjugate base. pH>7

8. Salts derived from a strong acid and weak base= cation that acts as a relatively strong conjugate acid. pH<7

9. Salts derived from a weak acid and weak base= ? Depends on Ka and Kb.

List the following solutions in order of increasing pH

0.1 M Co(ClO4)

0.1 M RbCN0.1 M Sr(NO3)2

0.1 M KC2H3O2

Of the following pairs of salts, which one form the more acidic 0.010 M solution1. NaNO3 or Fe(NO3)3

2. KBr or KBrO3. CH3NH3Cl or BaCl24. NH4NO2 or NH4NO3

Stomach

Stomach

Which solution will be acidic?

1. LiBr 2. NaBr 3. NH4NO2

4. Fe(NO3)3

5. NaCN

Acid Base behavior and Chemical Structure

What is polarity?What is electronegativity?What is bond Energy?

Hydrogen atoms can act as acids, bases or neither.

HCl = AcidNaH = BaseH-C in CH4

= Neutral

The Strength of an acid depends on three factors. 1. Polarity of H-X bond.2. Strength of H-X bond.3. Stability of Conjugate

Base. make a conclusion about stable conjugate base at the end of this section.

Overhead # 30

1. Metal hydrides are bases or neutral.

2. Non-metal hydrides are neutral or acidic. (save NH3)

Overhead # 30

3. Vertical row of non-metals has a tendency toward increasing acidity with increasing atomic number.

Oxyacids- When O-H bonds and additional O are bound to a central atom.O-H can be an acid or a

base.

General Rule 1 1. As the ability of Y to attract electrons

increases, the acidity of the substance will also increase. 2 reasons – A. Y’s EN: O - H is more polar and thus

favors the loss of H+.– B. Because the conjugate base is usually

an anion, its stability generally increases as the electronegativity of Y increases. The reverse reaction is not going to happen that easily due to Y’s EN. Example

General Rule #2

For oxyacids that have the same central atom Y, acid strength increases as the number of oxygen atoms attached to Y increases. Note: This is true of the addition of any relative electronegative anion.

Arrange the compound according to increasing acid strength.

1. AsH3, HI, NaH, H2O

2. H2SeO3, H2SeO4,, H2O

In each of the following pairs, choose the compound that leads to the more acidic (or more less) solution.

1. HBr or HF 2. PH3 or H2S

3. HNO2 or HNO3

4. H2SO3 or H4SiO4

Carboxyl Acids

COOH- Group

Formic Acid- ant and wasp acid Benzoic Acid Acetic Acid

What happens to the acidity if CH3COOH is changed to CF3COOH?

Lewis Acids and Bases

Lewis Acids: Electron pair acceptor.

Lewis Base: Electron Pair donor.

All species that were classified as a Bronsted-Lowery/Arrhenius acid or base can also be classified Lewis Acid or base.

However, Lewis Acids and bases have an advantage. More species can be classified as Lewis

Acids or a Bases because we are not referring to only H+ ions(Protons).

To see the big picture you need the Lewis structure for viewing.

For Example: H+ + NH3 NH4+

Other Examples

CO2 + H2OCarbonic Acid= dead fish.

NH3 + BF3

Fe3+ in Water