Chem HHS unit 10 review

Acid

• “acidus” meaning sour

• compounds that produce hydrogen ions (H⁺) when dissolved in H₂O

• H⁺ is, in reality, a single proton (very tiny)

• Ex: HNO₃, SO₄, H₃PO₄, HC₂H₃O₂

• Qualities:

  • Fizzes: metal reaction, antacid, marble rock (CaCO₃)

  • Red: pH paper and bromothymol blue

  • Clear/nothing: phenolphthalein

  • Feel test: watery

  • Taste test: sour

Base

• produces hydroxide (OH^-) ions when dissolved in H₂O

• Ex: KOH, LiOH, Mg(OH)₂ - all metals

• Qualities:

  • Blue: pH paper and bromothymol blue

  • Nothing: metal reaction, antacid, and marble rock (CaCO₃)

  • Pink: phenophthalein

  • Feel test: slippery

  • Taste test: bitter

Classify the following as an acid, base, or salt

1. Ca(OH)₂

2. K₂Cr₂O₇

3. HBr

4. Al₂O₃

5. H₂C₄H₄O₆

6. RbOH

7. HOH

8. Al(OH)₃

1. base

2. salt

3. acid

4. salt

5. acid

6. base

7. water

8. base

For Ca(OH)₂ above, if it comes to equilibrium in water and [Ca²⁺] = 0.016 M, [OH^-] = 0.023 M, what is the K_b at room temp?

[.016] x [.023]² = 8.5 × 10^-6

Acid/Base reactions

Acids and bases (double replacement) neutralize each other to make liquid water and aqueous salt

acid(H⁺) + base(OH^-) ———→ water (HOH) + salt

Write the products of these acid/base double replacement reactions:

1. HCl (aq) + NaOH (aq) ——>

2. H₂SO₄ + LiOH ——>

3. H₂SO₄ + Al(OH)₃ ———>

• don’t forget to balance and add the state (aq, l, s, etc.)!

Formulas

You start with a 0.048 M solution of HNO₃. Show how it ionizes. Also, what is its [H+], [OH-], pH, and pOH?

strong acid or base

• nearly completely ionizes in water

  • HCl, HNO₃, H₂SO₄ —- large K_a

  • NaOH, KOH —- large K_b

• Ex: HCl ———> H⁺ + Cl^- k_a = 1.3 × 10⁶

0% 100%

• assumed to completely dissociate in water

• The larger K_a —> more H⁺ ions at equilibrium —> the stronger the acid (lower pH)

• ph scale:

  • acid: 0-2

  • base: 12-14

weak acid or base

• ionizes only slightly in water

  • citric acid, H₃PO₄, HC₂H₃O₂ —— small K_a

  • Ca(OH)₂, Mg(OH)₂ —— small K_b

Ex: HC₂H₃O₂ -------------> H⁺ + C₂H₃O₂^- K_a = 1.8 x 10^-5

99% 1%

• dissociate only to a limited extent

• The smaller K_a —> fewer H⁺ ions at equilibrium —> the weaker the acid (higher pH)

• Since strong acids essentially dissociate 100%, only acids have Ka values associated with them

  • The dissociation of weak bases is treated the same way, except instead of Ka, we use K_b (base dissociation constant)

• need to know k_a to find pH

• ph scale:

  • acid: 3-6

  • base: 8-11

Calculate the pH of a 1.00 M solution of acetic acid, HC₂H₃O₂

**HC₂H₃O₂ is a weak acid with an acid dissociation constant of 1.8 x 10 ^-5

pH = 2.4

What is the Ka for benzanoic acid (HC₇H₅O₂) if [HC₇H₅O₂] = .500 M and the pH = 2.60?

1.3 × 10^-5

The pH of a 0.40 M solution of ammonia, NH₃OH is 11.4 (K_b = 1.8 x 10^-5)

The pH of a 0.40 M solution of KOH is 13.6 (strong base, 100% ionizes)

How do the differences in pH values demonstrate the difference between a strong vs. weak base?

Even though the strong and weak base have the same initial concentration, because the strong base dissociates completely, it has a greater [OH^-] present in solution. This explains why it has a high PH compared to the weak base.

Polyprotic acids

• monoprotic acid – any acid that contains 1 ionizable H⁺

  • ex: HCl, HNO₃, HC₂H₃O₂

  • HC₂H₃O₂ (l) -----> H⁺ (aq) + C₂H₃O₂^- (aq)

• diprotic acid – an acid that releases 2 H⁺

  • ex: H₂SO₄, H₂CO₃

• triprotic acid – an acid that releases 3 H⁺

  • ex: H₃PO₄, H₃BO₄

Provide each dissociation step for phosphoric acid—H₃PO₄

• K_a1 = 6.9 × 10^-3

• K_a2 = 6.2 × 10^-8

• K_a3 = 6.9 × 10^-13

At equilibrium, rank each chemical species (besides H₃O⁺, H₂O, and OH^-) by concentration

Two concepts of acids and bases

Bronsted Acids & Bases

Conjugate Acid-Base Pairs

Conjugate Acid-Base Pairs examples

1. PO₄^-3 (aq) + H₃O⁺ (aq) ==== HPO₄^2- (aq) + H₂O (l)

2. CN^- (aq) + H₃O⁺ (aq) ==== HCN (aq) + H₂O(l)

Metals and Water

Metals and water examples

1. Na (s) + H₂O (l) ——>

2. Ga (s) + H₂O (l) ——>

• don't forget to balance!

Hydronium ion

Use these formulas: pH = -log [H⁺]

[H⁺] x [OH^-] = 1.0 x 10^-14

1. pH = 1.46

2. pH = 8.07

3. [H+] = 9.52 x 10^-14 M

4. [H+] = 1.9 x 10^-12 M

5. pH = 3.17

6. [H+] = 1.3 x 10^-9 M

7. [OH-] = 2.0 x 10^-5 M

H⁺ and OH^- in beaker review

• In any solution, there pH will always be some amount of H+ and OH

• How much H+ there is compared to OH- will determine how acidic or basic the solution is

Ex: specific numbers don’t matter, all that matters is amount shown

• pH = 7 —→ when solution is neutral: H+ = OH-

  • 4 H+ and 4 OH- drawn in beaker

• pH = 6 —→ when solution is slightly acidic: H+ » OH-

  • 12 H+ and 1 OH- drawn in beaker

• pH = 8 ——> when solution is slightly basic: H+ « OH-

  • 1 H+ and 10 OH-

a) You have a beaker of water that has a [H+] = 10^-7 M. Calculate the [OH^1-] = ___________

The pH = ______ The pOH = ______

b) Now use your values of [H+] and [OH1-] to make a particle diagram simulating how many H+ and OH1- ions are found in the beaker of water below. When you draw the H+ and OH1- ions, be mindful of how many H+ ions there would be compared to how many OH1-

1. [OH-] = 10^-7

2. pH = 7

3. pOH = 7

a) You have an acid that has a [H+] = 0.000048 M. Calculate the [OH1-] = ______________

The pH = ______ The pOH = ______

b) Now use your values of [H+] and [OH1-] to make a particle diagram simulating how many H+ and OH- ions are found in the beaker of water below: (of course you can’t show exact numbers of particles because of the exponential nature of pH and logarithms, just show one having more and the other having less)

1. [OH-] = 2.0 x 10^-10

2. pH = 4.3

3. pOH = 9.7

a) You have a base that has a [OH-] = 0.00060 M. Calculate the [H+] = ______________

The pH = ______ The pOH = ______

b) now use your values of [H+] and [OH1-] to make a particle diagram simulating how many H+ and OH1- ions are found in the beaker of water below:

1. [H+] = 1.6 x 10^-11

2. pH = 10.8

3. pOH = 3.2

Circle the appropriate answer to fill in the blanks:

1. In a concentrated acid, the amount of H+ ions is very high/very low.

2. In that same concentrated acid, the amount of OH- ions is very high/very low.

3. In a concentrated base, the amount of H+ ions is very high/very low.

4. In that same concentrated base, the amount of OH- ions is very high/very low.

1. very high

2. very low

3. very low

4. very high

Classify each of the following as strong or weak. Finish any ionization reactions that are incomplete.

Simple ionization: H₂SO₄ —→

2H⁺ + SO₄^2-

more complex ionizing to make hydronium ex:

H₂SO₄ + H₂O ——>

acid/base neutralization ex:

1. Ba(OH)₂ + HC₂H₃O₂ ——>

2. H₃PO₄ + NH₄OH ——>

3. (fun one!) H₇VoO₈ + Vo(OH)₄ —→

• don’t forget to balance!

You titrate some 1.25 M H₂SO₄ into an unknown concentration of Ca(OH)₂. It takes 15.7 mL of the acid to neutralize 19.5 mL of the base. What is the concentration of the base?

1.01 M

Weak formic acid (HCHO2) has a Ka = 1.8 x 10-4 and the original acid's concentration is [HCHO2] = 0.72 M.

What is the pH?

1.9

a) Using the first (simplified) ionization for carbonic acid, write out the Ka expression: (the 2nd H+ can also come off but we’ll ignore it to keep it simple for this example)

• first ionization: H₂CO₃ (aq) ——> H⁺ (aq) + HCO₃^1- (aq)

b) At equilibrium, [H2CO3] = 0.250 M and Ka = 4.3 x 10^-7. What is the [H⁺] at equilibrium? (hint: assume [H⁺] = [HCO^3-] and assume only a tiny bit of the original acid ionizes)

c) what is the pH of this solution?

A 0.105 M solution of HCN has a pH = 1.9. Calculate the K_a of this solution:

K_a = 1.6 × 10^-3

You have a strong acid, HNO3 with an initial concentration of 5.5 M. What is its pH?

pH = -.74

Hydrofluoric acid (HF) is a weak acid with a Ka = 7.1 x 10^-4. What would the pH of this acid be if it was a 0.088 M solution?

pH = 2.1

Label the following: Bronsted acid (BA), BB, conjugate acid (CA), or CB. Then draw lines connecting acid-base pairs.

1) HCl + H₂O <—→ Cl^- + H₃O⁺

2) HCO^3- + OH^{- <}—→ CO₃^2- + H₂O

3) PO₄^3- + H₃O⁺ <—→ HPO₄^2- + H₂O

4) H₂O + CH₃COO^- ←——> CH₃COOH + OH^-

5) NH₄¹⁺ + F^1- ←——> NH₃ + HF

H₃PO₄ (Hydronium ion and Polyprotics stuff ex)

Titration procedure

Normality

• the moles of H⁺ or OH^- per liter of solution

• like Molarity but it measures how many H⁺ and OH^- ions are present in the solution, which is important for neutralization

Normality = molarity x (# of H⁺ or OH^-)

• So Normality tells you how many moles of H⁺ and moles of OH^- are accessible for neutralizing

• When you neutralize an acid and a base the moles have to be equal

Procedure for a titration:

• End point – the point when the indicator changes color

• Phenolphthalein – changes from clear to pink at about pH = 8.0

  • (close enough to neutral for us!)

Titrations

acid: HCl = 2.0 M, 27.0 mL added base: Al(OH)3 = 0.10 M, ? mL added

write out the neutralization reaction:

acid: H2SO4 = ? M, 75.0 mL added base: Ca(OH)2 = 0.145 M, 16.5 mL added

write out the neutralization reaction:

You start with 15.0 mL of 0.050 M H₃PO₄. You add 4.50 mL of an unknown concentration of Ca(OH)₂ and it neutralizes. What is the Normality and the Molarity of the Ca(OH)₂?

(0.050 x 3) x 15.0 = Nb x 4.50

N = 0.50 N

M = 0.25 M

a) You start with a stock solution of 0.0080 M KOH. What is its pH (assume 100% ionization)?

b) You place 20.0 mL of this base in a flask and add 3 drops of phenolphthalein. It turns pink. You start adding a solution of H₃PO₄. After you add 12.5 mL of acid you reach the endpoint and the solution turns clear (neutralized). What is the Normality of your H₃PO₄?

c) What is the Molarity of the acid?

a) [OH^- ] = 0.0080 M —> pOH = -log (0.0080) = 2.1 —> pH = 11.9

b) N_a x 12.5 = 0.0080 x 20.0 —> Na = 0.013 N

c) N = 3 x M —→ M = 0.013 ÷ 3 = 0.0043 M

An acid has a molarity of 0.450 M and a normality of 1.35 N. Is it a monoprotic, diprotic, or triprotic acid?

triprotic acid

Label the following as Arrhenius acid, Arrhenius base, or Salt:

1. KBr

2. H₃PO₄

3. HC₂H₃O₂

4. Mg(NO₃)₂

5. Ba(OH)₂

1. salt

2. acid

3. acid

4. salt

5. base

How can HCl be a stronger acid than H₃PO₄, even though H₃PO₄ gives off 3H+?

only a small % of H₃PO₄ ionizes, making fewer H+ than the same amount of HCl (100% ionizes)

HCl is a stronger acid than (H₃PO₄) Phosphoric acid because acid strength depends on how easily an acid releases its protons, not on the total number of protons available. While H₃PO₄ has three protons, it is a weak acid that only partially dissociates, whereas HCl is a strong acid that completely dissociates in water.

• HCl (Strong Acid): When dissolved in water, almost 100% of HCl molecules split into H⁺ and Cl^-. The reaction goes completely to the right

  • HCl ——> H⁺ + Cl^-

• H₃PO₄ (Weak Acid): Only a small fraction of H₃PO₄ molecules ionize in water. Most of the molecules remain as H₃PO₄. The first dissociation is only partial:

  • H₃PO₄ —→ H⁺ + H₂PO₄^-

Why can HNO3 considered strong, even when it is very diluted?

strength is related to % ionization, and HNO₃ nearly ionizes 100%, no matter what concentration

What is the Ka of a 1.5 M solution of HCN that has a pH of 2.65?

K_a = 3.3 × 10^-6