Exam 3

The pH of this soft drink is 3.12. What is it's [OH-] ion concentration?

-Way 1: Kw= [H3O+]x[OH-] and [H3O+]= 7.59E-4 (from above) SO, [OH-]= Kw/ [H3O+] [OH-]= 1.0E-14/ 7.59E-4 [OH-]= 1.32E-11 -Way 2: 14= pH + pOH 14= 3.12 + pOH pOH= 14 - 3.12 = 10.88 [OH-]= 10^-pOH = 10^-10.88 [OH-]= 1.32E-11

Is there a relationship between pH and pOH?

-Yes 14= pH + pOH

Acid Ionization Constants

"Ka" Larger the Ka, the stronger the acid

Which of the following solutions are considered acidic? 1) H3O+= 3.6E-4 2) OH-= 4.8E-10 3) pOH= 12.0

- 1, 2 , and 3

Acids and Bases are relative terms under the Bronsted-Lowery perspective.

- A substance is defined as an acid or a base relative to the substance it is mixed with. - Usually we define substances relative to water.

Arrhenius Perspective

- Acid increases the hydrogen, H+ concentration when dissolved in water HCl (g) -----> H+ (aq) + Cl- (aq)

Polyprotic Acids

- Acids that can donate more than one proton

What is an Oxoacid?

- An acid that contains hydrogen, oxygen, and another element HO --- EOn "E" is non-metal element "n" = 0, 1, 2, or 3 - Common examples include: HNO3, H2SO4, or H3PO4 - Can compare with electronegativity or # of O atoms (more O = stronger acid)

What substances can behave as an acid and what substances can behave as a base under the Bronsted-Lowery perspective?

- Any substance that has hydrogen in its formula can potentially be a source of protons and therefore can behave as an acid. - Any substance that has a lone pair of electrons on a central atom can accept a proton and therefore can potentially behave as a base.

Arrhenius Perspective

- Bases increase the hydroxide ion, OH- when dissolved in water NaOH (s) ----> Na+ (aq) + OH- (aq)

What type of salt (acidic, basic, or neutral) is produced when HCN reacts with NH3? HCN + NH3 ---> NH4CN Ka(HCN)= 4.9x10^-10 Kb(NH3)= 1.8x10^-5

- Basic

Bases

- Bitter to taste - Slippery - Turn organic compounds colors (normally blue)

Parent Acid

- Derived by taking into account the charge of the anion and adding the appropriate number of protons to make the charge of that anion neutral. Ex) - anion is Cl^-1, think HCl - anion is SO4^-2, think H2SO4 - anion is ClO4^-1, think HClO4

Parent Base

- Derived by thinking of the corresponding metal hydroxide. If the salt is an ammonium salt, then subtract a hydrogen from the cation species to produce a neutral amine or ammonia. Ex) - cation is Na^+, think NaOH - cation is Mg^2+, think Mg(OH)2 - cation is NH4^+1, think NH3

Consider in another example, the Bronsted-Lowery reaction between acetic acid and water: CH3COOH + H2O <===> H3O+ + CH3COO-

- Equilibrium reaction. Equilibrium constant is 1.7E-5. CH3OO- is strongest base and CH3OOH is weaker acid.

Conjugate Acid-Base Pairs

- Every Bronsted-Lowry proton transfer reaction has two acid-base pairs if the reverse reaction is considered. HCl + H2O <===> H3O+ + CL- acid + base <===> acid + base

EX) Stronger acids? - H2SeO3 or H2SeO4 - H3PO4 or H3AsO4 - H2S or H2Te

- H2SeO4 - H3PO4 - H2Te

HONH3+ + IO3- <===> HONH2 + HIO3 K= 5.3E-6 -Which 2 are acids? -Based on K, what is the strongest acid?

- HONH3+ and HIO3 -Small K, so reactants favored, so strongest is HIO3

What is conjugate acid of the species, HS1-?

- Implies it is a base, so add 1 H+ and 1 to the overall charge -H2S

What is the conjugate base of the species, H2PO1-?

- Implies that H2PO1- is acid, so base would have 1 less H+ and make overall charge drop 1 -HPO4^2-

Qualitatively, how is the strength of acids compared? For example, reconsider the reaction between strong acid, HCl and H2O: HCl + H2O ---> H3O+ + Cl

- In this exchange, the stronger base would get the proton. The direction of the arrow tells us that H2O gets the proton and therefore, H2O is a stronger base than Cl-. - Cl- wants nothing to do with the proton when mixed with water. Thus, chloride is qualitatively a very weak base. - HCl is extremely strong when mixed with water. It's ability to donate a proton is very high.

Relationship between Ka and Kb:

- Ka(HA) x Kb(A-) = Kw - pKa + pKb = 14

What does Base Strength refer to?

- Measures the ability of a base to accept a proton. Stronger bases hold onto a proton whereas in comparison, weaker bases tend to lose the proton.

pH Scale

- Neutral if pH = 7.0 - Acidic if pH < 7.0 - Basic if pH > 7.0

pOH Scale

- Neutral if pOH = 7.0 - Acidic if pOH > 7.0 - Basic if pOH < 7.0

Hydrolysis Reaction

- Reaction of an ion with water to produce H3O+ of OH-

What does Acid Strength refer to?

- Refers to a measure of the ability of an acid to give up its proton. Stronger acids easily give up their protons. Weaker acids hold onto their protons to a greater extent

Base Strengths

- Refers to the ability of a substance to accept a proton. It is measured by the extent that a substance causes water to ionize to produce hydroxide, OH-. - Kb= base ionization constant - Kb= [HB+][HO-] / [B:]

Stoichiometric Reaction (K is infinitely large): Large K: Small K:

- SA + SB ----> WA + WB - SA + SB <===> WA + WB (favored side) - (favored side) WA + WB <===> SA + SB

Acids

- Sour to taste - React w/ active metals to produce hydrogen gas - Turn organic compounds colors (normally red)

Base Ionization Constant

- Strong base = larger Kb - Strong base = smaller pKb

Relationships:

- Stronger base = Weaker acid - Stronger acid = Weaker base - The weaker side is favored

What is the significance of the auto-ionization of water?

- The auto-ionization constant relationship is valid for all aqueous solutions including acidic, basic and neutral. - This implies that even though a solution may be acidic, there is still OH- present at equilibrium. Or conversely, even though a solution is basic, there is still H3O+present.

Salt Formation (way 1)

- The salt can be envisioned as being derived from a parent acid and a parent base. - The origin of the cation is derived from the parent base - The origin of the anion is derived from the parent acid.

Conjugate Acid-Base Pairs

- Two substances that are related by the loss or gain of a single hydrogen ion - They appear on opposite sides of the equation. - The acid member of the pair will have one more hydrogen atom in its formula and its charge will be +1 greater than the base member. HCl + H2O <===> H3O+ + Cl- a + b. a + B Pairs: HCl/Cl- H3O+/H2O

Bronsted-Lowry Perspective

- View acids and bases from the standpoint of a chemical reaction involving a transfer of a hydrogen ion, H+ (proton). - Acids = proton donors - Bases = proton acceptors

Can a substance behave as either an acid or a base?

- Yes. As long is it has a lone pair of electrons and a hydrogen atom in the formula Ex) H2O It has a lone pair of electrons on an oxygen atom and therefore it can accept a proton: H+ + H2O ---> H3O+ And it has hydrogen atoms in it's formula therefore, it can behave as a proton donor to another base: HO-H + :B ---> H:B+ + OH- A substance that can go either way is referred to as amphiprotic.

Four possible outcomes of salts being formed as a result of neutralization reaction:

1) A salt derived from a combination of a strong acid and a strong base is neutral. 2) A salt derived from a combination of a weak acid and a strong base is basic. 3) A salt derived from a combination of a strong acid and a weak base is acidic. 4) When confronted with a salt derived from a weak acidand a weak base, one must compare the acid ionization constant of the parent acid with the base ionization constant of the parent base.

Three Acid-Base Perspectives

1) Arrhenius Perspective: views in terms of ions formed when dissolved in H2) 2) Bronsted-Lowery Perspective: views in terms of a proton transfer reaction 3) Lewis Acid-Base Perspective: views in terms of an electron pair donation reaction

What steps are required to deduce whether a salt is considered acidic, neutral or basic?

1) Assess what parent acid and what parent base were used to make the salt in a neutralization reaction. 2) Qualitatively compare the strengths of the parent acid versus the strength of the parent base. The dominant substance will provide the character of the salt.

How is the strength of two acids compared from their structure?

1) Comparing acids that differ only by their charge and the number of protons in their formula. 2) Comparing binary acids across a period. 3) Comparing binary acids down a group. 4) Comparing oxoacids that differ by the central atom. 5) Comparing oxoacids with the same central atom but that differ by the number of oxygen atoms in their formula.

Relative to water, there are six strong acids:

1) HCL 2) HBr 3) HI 4) HNO3 5) HClO4 6) H2SO4

What acid strength does not refer to... The strengths of acids does not refer to:

1) How concentrated the acid is... 2) The pH of the aqueous solution... 3) How corrosive the aqueous solution is..

Six equations for interconverting H3O+, OH-, pH, and pOH

1) Kw= [H3O+]x[OH-] = 1.0x10^-14 2) pH= -log[H3O+] 3) [H3O+]= 10^-pH 4) pOH= -log[OH-] 5) [OH-]= 10^-pOH 6) 14= pH + pOH

Relative to water, there are eight strong bases:

1) LiOH 2) NaOH 3) KOH 4) RbOH 5) CsOH 6) Ca(OH)2 7) Sr(OH)2 8) Ba(OH)2

What factors can be used as arguments for comparing acid strengths?

1) The bond strength of the H-X bond. 2) The polarity of the H-X bond. 3) The stability of the resulting conjugate base.

Problems in Acid-Base Equilibrium Types you will encounter:

1)Given an initial concentration of a weak acid, and some change (as represented by the pH of the resulting solution), determine the acid ionization constant, Ka. 2)Given the initial concentration of a weak acid and the acid ionization constant, Ka determine the pH of the solution.

The pH of this soft drink is 3.12. What is it's pOH?

14= pH + pOH 14= 3.12 + pOH pOH= 14- 3.12 = 10.88

Write the chemical formula for the conjugate base of each of the following weak acids. A) HC2H3O2 B) HCN C) HSO3−

A) C2H3O2- B) CN- C) SO3^2-

Using the Brønsted-Lowry definition of acids and bases, identify the following. A) the conjugate acid of S2− B) the conjugate base of HBr C) the conjugate base of HCO3−

A) HS- B) Br- C) CO3^2-

EX). Acid Ka pKa HCN 4.9E-10 -- HF -- 3.18 H3BO3 7.3E-10 -- A) What is the pKa for HCN? B) What is the Ka for HF? C) What is the pKa for H3BO3?

A) pKa = -log(Ka) = -log(4.9E-10) = 9.31 B) Ka = 10^-pKa = 10^-3.18 = 6.6E-4 C) pKa = -log(Ka) = -log(7.3E-10) = 9.14

-Comparing acids that differ only by their charge and the number of protons in their formula. EX) which acid is stronger? H2CO3 or HCO3^1-

H2CO3 + H2O <===> H3O+ + HCO3^1- HCO3^1- + H2O <===> H30+ + CO3^2- Lesser charge is more stable H2CO3 is stronger

Auto-Ionization of Water

H2O + H2O --> H3O+ + OH- Water can react with itself Kw(298K)= [H3O+][OH-]= 1.0x10^-14

Which of the following substances below are most likely to be classified as a strong acid in water? HF HBr H2SO3 HC3H5O2

HBr

Both H2C2O4 and H2C4H2O5 ionize in water to a limited extent. Which of the conjugate bases, HC2O4− or HC4H2O5−, is the stronger base?

HC4H2O5-

Strong Acid- Weak Base Combination

HCl + NH3 ---> NH4Cl SA WB A

Strong Acid- Strong Base Combination

HCl + NaOH ---> NaCl + H2O SA SB N

Weak Acid- Weak Base Combination

HF + NH3 ---> NH4F WA WB A Ka(HF)= 6.8x10^-4 and Kb(NH3)= 1.8x10^-5 - Must compare Ka to Kb - Ka is greater the Kb so it results in an acidic salt

Write an equation that shows H2PO4− acting as an acid with water. Write an equation that shows H2PO4− acting as a base with water.

H_2PO_4^- + H_2O <=> HPO_4^-2 + H_3O^+ H_2PO_4^- + H_2O <=> H_3PO_4 + HO^1-

A Bronsted-Lowery Acid relative to water

Hydrogen Chloride, HCl: HCL + H2O ----> H3O+ (aq) + CL- Donor + Acceptor H3O+ is Hydronium ion

-Comparing binary acids across a period. -Comparing binary acids down a group.

Using electronegativity and bond strength

Bronsted-Lowry Perspective

Water is not necessary to produce an acid-base reaction in the Bronsted-Lowry perspective of acid-base theory. HCl (g) + NH3 (g) ----> NH4Cl (s)

pH Scale

pH= -log[H3O+] [H3O+]= 10^-pH

pKa Values

pKa = -log(Ka) Ka = 10^-pKa Smaller the pKa, the stronger the acid

pOH Scale

pOH= -log[OH-] [OH-]= 10^-pOH

Base-dissociation constants for several bases measured in aqueous solution at 25°C are shown in the table below. Arrange the bases in order from weakest to strongest. BaseKb (25°C) piperidine 1.3 ✕ 10−3 ethylamine 4.5 ✕ 10−4 pyridine 1.7 ✕ 10−9 triethylamine 5.7 ✕ 10−4

pyridine, ethylamine, triethylamine, piperidine

Which is the stronger acid, C5H5NH+ or H2C6H6O6?

H2C6H6O6

Identify the conjugate acid-base pairs in the reaction shown below. HCl + NH3 → Cl− + NH4+

Acid: HCl Conjugate Base: Cl= Base: NH3 Conjugate Acid: NH4+

A Bronsted-Lowery Base relative to water

Ammonia, NH3: NH3 (g) + H20 ----> NH4+ (aq) + OH- (aq) Acceptor + Donor OH- is Hydroxide ion NH4+ is ammonium ion

Weak Acid- Strong Base Combination

CH3COOH + NaOH --->Na+(CH3COO)- + H2O WA SB B

Identify the Brønsted-Lowry acid and base in the following reaction. PH3 + HF → PH4+ + F −

PH3= Base HF= Acid

The pH of this soft drink is 3.12. What is it's [H3O+] ion concentration?

[H3O+]= 10^-pH [H3O+]= 10^-3.12 [H3O+]= 7.59E-4

Acid-dissociation constants for several acids measured in aqueous solution at 25°C are shown in the table below. Arrange the acids in order from weakest to strongest. AcidKa (25°C) chloroacetic acid 1.3 ✕ 10−3 acrylic acid 5.6 ✕ 10−5 malonic acid 1.4 ✕ 10−3 phloroglucinic acid 2.1 ✕ 10−2

acrylic acid, chloroacetic acid, malonic acid, phloroglucinic