Chemistry Exam 3

Consider the following reaction and use it to answer the following questions.

PCl5(g)⇌PCl3+Cl2(g)      ΔHo<0

A) Which direction does the reaction shift if the temperature of the reaction is increased?

B) Which direction does the reaction shift if the volume of the reaction vessel is decreased?

A) Towards the reactants

B) Towards the reactants

During the solution of an equilibrium problem you derive this expression:

Kc=3.7×10^-10 = x2/ (0.97-x)^2

In this case, what is the ‘x is small’ assumption? (≈ means ‘approximately equal to”)

0.97-x = about 0.97

What can be said about a reaction where Grxno<0 ?

It is spontaneous under standard conditions

The following reaction is exothermic.

2CO(g)+O2(g)⇌2CO2(g)

What happens to Kp with increasing temperature?

Kp decreases

For reaction X, ΔGrxno = 23.7 kJ

For reaction Y, ΔGrxno = 56.4 kJ.

What can be said about the relationship between the Keq values for X and Y?

The Keq value for reaction X is larger

For the following reaction,

A+B⇌C+D    Keq<1

What is the sign of ΔGrxno?

Positive

Consider the following reaction.

H2O(ℓ)+NH3(aq)⇌OH-(aq)+NH4+(aq)

A) Which of the following is a Brønsted-Lowry acid in the reaction above?

B) Which of the following is a conjugate acid in the reaction above?

A) H2O

B) NH4+

Which of the following is a representation of the ion product of water?

H2O(ℓ)+H2O(ℓ)⇌H3O+(aq)+OH-(aq)

Kw= [H+][OH-]

If the pH of a solution is 4.83, what is the pOH?

14 - 4.83= 9.17

During the solution of an equilibrium problem you derive this expression:

Kc=4.2×10^-4= x^2/ (0.02-x)

In this case, is the “x is small” approximation valid?

No, because x > 5% of 0.02

Consider the following reaction.

HNO2(aq)+CH3NH2(aq)⇌NO2-(aq)+CH3NH3+(aq)

A) Which of the following is a Brønsted-Lowry base in the reaction above?

B) Which of the following is a conjugate base in the reaction above?

A) CH3NH2

B) NO2-

Which answer correctly describes the pH of a 1.0×10^-7 M solution of HBr?

Less than pH 7.0

Without doing a calculation, choose the answer that best describes the pH of a 0.01 M solution of H2SO4. Note that -log(0.01) = 2

The pH is slightly less than 2.0

Phosphoric acid has three protons that can ionize. The Ka values are as follows:

Ka1=6.9×10^-3      Ka2=6.4×10^-8       Ka3=4.8×10^-11

Which of these constants would you use to calculate the pH of a 0.1 M solution of H3PO4?

Ka1 only

Choose the answer that best describes the relative acidity of hypobromous (HBrO) and hypoiodous (HIO) acids.

Note: although by convention the H is written first, it is actually attached to the oxygen in each case.

HBrO is more acidic than HIO because Br is more electronegative than I

Rank the following three acids in terms of decreasing acid strength.

HBrO4          HBrO2           HBrO3

HBrO4 > HBrO3 > HBrO2

In a diprotic acid, why does the second ionization have a smaller Ka?

Because after the first ionization the acid has a negative charge which attracts the remaining proton

Consider the acid base reaction below and choose the acid and conjugate base from the list.

CH3NH2(aq)+H2O(ℓ)⇌CH3NH3+(aq)+OH-(aq)

H20 and OH-

Consider the acid base reaction below and choose the base and conjugate acid from the list.

CH3NH2(aq)+H2O(ℓ)⇌CH3NH3+(aq)+OH-(aq)

CH3NH2 and CH3NH3+

Rank the following three acids in terms of increasing acid strength.

HClO3          HClO           HClO2

HClO < HClO2 < HClO3

Citric acid has three protons that can ionize. The Ka values are as follows:

Ka1=7.4×10-4       Ka2=1.7×10-5         Ka3=4.0×10-7

Which of these constants would you use to calculate the pH of a 0.1 M solution of citric acid?

Ka1 only

Which of the following is a neutral salt?

• LiBr(s)

• H2SO4(aq)

• Na2CO3(s)

• NH4Cl(s)

• NH3(aq)

LiBr(s)

Which of the following is a basic salt?

• KHSO4(s)

• Na2CO3(s)

• KI(s)

• H2SO4(aq)

• NH4Br(s)

Na2CO3(s)

Which of the following is an acidic salt?

• Na2CO3(s)

• NH4Cl(s)

• H2SO4(aq)

• KCN(s)

• CaCl2(s)

NH4Cl(s)

Are either of the products in the following reaction acidic or basic?

NaNO2(s)→Na+(aq)+NO2-(aq)

 NO2-(aq) is a basic anion

Which of the following is the correct ionization reaction for K3PO4 (s)?

K3PO4(s) → 3K+ (aq) + PO4-(aq)

If Tiffany added 0.1 M NaNO2 to 0.1 M HNO2, would the pH change? (see the reaction below)

HNO2(aq)+H2O(ℓ)⇌H3O+(aq)+NO2-(aq)

Yes, because of Le Châtlier's Principle

If Rylee added 0.30 M of NaHSO3 added to a solution of 0.1 M H2SO3 (see reaction), how will the pH change?

H2SO3(aq)+H2O(aq)⇌H3O+(aq)+HSO3-(aq)

It gets more basic, the pH increases

Which of the following constitute a pH buffer system?

• 0.1M HNO2 and 0.1M NaCl

• 0.1M HCl and 0.1M NaOH

• 0.1M NH3 and 0.1M NH4Cl

• 0.1M HF and 0.1M NaOH

• 0.1M HNO3 and 0.1M NaNO3

0.1M NH3 and 0.1M NH4Cl

or a buffer system, what is the pH when the [base] = [acid]?

pH = pKa

Buffer capacity is the ability of a buffer to withstand the addition of acid or base without a significant change in pH. The buffer with the highest buffer capacity will have which of the following?

• The highest pKa

• The highest concentration of buffer components

• A 10:1 ratio of the acid and base buffer components

• The lowest pKa

• The lowest concentration of buffer components

The highest concentration of buffer components

Which of the followig weak acid/conjugate base combinations would make the best bufffer at pH 3.3?

• H2CO3(aq) and HCO3-(aq) pKa=6.37

• CH3COOH(aq) andCH3COO-(aq) pKa=4.75

• HCN(aq) and CN-(aq) pKa=9.21

• HClO(aq) and ClO-(aq) pKa=7.45

• HF(aq) and F-(aq) pKa= 3.14

HF(aq) and F-(aq) pKa= 3.14

Strong Acids:

HCl

HBr

HI

H2SO4

HNO3

HClO4

Strong Bases:

NaOH

LiOH

KOH

Ca(OH)2

Ba(OH)2

Sr(OH)2

The Kb of aminoethanol, HOCH2CH2NH2, is 3.1 x 10-5.

-Is aminoethanol a stronger or weaker base than ethylamine, pKb = 3.36? Explain your answer.

Weaker. The stronger base will have the lowest pKb and the largest Kb.

For ethylamine, pKb = 3.36, so Kb = 10^-3.36 = 4.365 × 10^-4

For aminoethanol Kb = 3.1 × 10^-5, so pKb = -log(3.1 × 10^-5)  = 4.50

Compare pKa Ka

Ethylamine 3.36            4.36 × 10^-4

Aminoethanol 4.50            3.1 × 10^-5

-ethylamine is stronger, aminoethanol is weaker

Sulfuric acid (H2SO4) and selenic acid (H2SeO4) are both diprotic acids.

1. Which is the stronger acid? Explain your answer

2. Which will have the largest pKa1?   HSeO4–

3. Which will have the largest Ka2?    H2SO4

1. H2SO4. sulfuric acid is the strongest acid because sulfur has a higher electronegativity than selenium and so the HSO4– anion is more stable than the HSeO4– anion.

2. HSeO4–

3. H2SO4

Which has the highest Ka1 phosphorous acid (H3PO3) or phosphoric acid (H3PO4)? Explain your answer.

Phosphoric acid has more oxygens to delocalize the negative charge of the anions so it will be the stronger acid.

Shown at right are piperidine  and morpholine. Which will have the higher Ka. Explain your answer.

Morpholine. The electronegative oxygen pulls electrons away from the nitrogen-hydrogen bond. This weakens the nitrogen-hydrogen bond in morpholine and increases the acidity.

Note: morpholine is greater than 500x more acidic than piperidine

1. A buffer is prepared by dissolving NH3(aq) and NH4Cl(s) in water. Write equations to show how this buffer neutralizes both added H+ and added OH–.

1. Neutralizes H+:

2. Neutralizes OH-:

3. Explain:

1. Neutralizes H+:  NH3 + H3O+  ⇋  NH4+ + H2O

2. Neutralizes OH-:  NH4+ + OH−  ⇋  NH3 + H2O

3. Buffers work because small amounts of added acid (H+) or base (OH−) shift the equilibrium only slightly so that there are only small changes in the pH.