Q: A reaction has an equilibrium constant $$K_{eq} = 1.0 \times 10^3$ at 298 K. Calculate $$\Delta G^\circ$$ for the reaction. (R = 8.31$ J/mol·K)

$$\Delta G^\circ = -8.31 \times 298 \times \ln(1.0 \times 10^3$)$$

Question 1

Which of the following statements best describes the difference between an Arrhenius acid and a Brønsted-Lowry acid?

Accepted Answer

An Arrhenius acid produces H$$^+$$ ions in water, while a Brønsted-Lowry acid donates a proton to another species.

Question 2

In the reaction $$\mathrm{HCl(aq) + H_2O(l) \rightarrow Cl^-(aq) + H_3O^+(aq)}$$, which species is the conjugate base of the acid?

Accepted Answer

$$\mathrm{Cl^-}$$

Question 3

Which of the following best explains why the negative charge on the methanethiolate ion ($$\mathrm{CH_3S^-}) is $$more delocalized than on the methoxide ion ($$\mathrm{CH_3O^-}$$)?

Accepted Answer

Sulfur is larger than oxygen, so the negative charge is spread over a larger volume.

Question 4

Given the following p$$K_a$$ values: Ethanol (15.9) and Carbonic Acid (6.36), which compound is the stronger acid and why?

Accepted Answer

Carbonic acid, because it has a lower p$$K_a$$ value.

Question 5

Which of the following best describes the effect of increasing s-character in the hybrid orbital of the atom bearing the negative charge on acidity?

Accepted Answer

Increasing s-character increases acidity because electrons are held closer to the nucleus.

Question 6

Consider the acid-base reaction: $$\mathrm{CH_3COOH + NH_3$ \rightleftharpoons CH_3COO^- + NH_4$^+}. $$Using the p$$K_a$$ values (acetic acid: 4.76, ammonium ion: 9.24), which direction does the equilibrium favor?

Accepted Answer

Toward the products (right)

Question 7

Which of the following is a Lewis acid in the reaction $$\mathrm{BF_3$ + NH_3$ \rightarrow F_3B-NH_3$}$$?

Accepted Answer

$$\mathrm{BF_3$}$$

Question 8

Calculate the equilibrium constant ($$K_{eq}) $$for the reaction $$\mathrm{C_6H_5OH + $$HCO_3$$^- \rightleftharpoons C_6H_5O^- + H_2CO_3}$$, given $$pK_a($$phenol) = 10.0 and $$pK_a($$carbonic acid) = 6.36.

Accepted Answer

$$K_{eq} = 10$$^{(6.36-10.0)}$$

Question 9

Which of the following best describes the inductive effect in organic acids?

Accepted Answer

It is the polarization of electron density in a bond due to the electronegativity of a nearby atom.

Question 10

Which of the following is the correct expression for the acid dissociation constant ($$K_a) $$for the reaction $$\mathrm{HA + H_2O \rightleftharpoons A^- + H_3O^+}$$?

Accepted Answer

$$K_a = \frac{[A^-][H_3O^+]}{[HA]}$$

Question 11

A reaction has an equilibrium constant $$K_{eq} = 1.0 	imes 10^3$ at 298 K. Calculate $$\Delta G^\circ$$ for the reaction. (R = 8.31$ J/mol·K)

Accepted Answer

$$\Delta G^\circ = -8.31 	imes 298 	imes \ln(1.0 	imes 10^3$)$$

Question 12

Which of the following best explains why 2,2,2-trifluoroethanol ($$pK_a = 12.4) is a $$stronger acid than ethanol ($$pK_a = 15.9$$)?

Accepted Answer

The fluorine atoms increase the inductive effect, stabilizing the conjugate base.

Question 13

Which of the following is true regarding the relationship between acid strength and conjugate base strength?

Accepted Answer

The stronger the acid, the weaker its conjugate base.

Question 14

Which of the following best describes the transition state in a reaction coordinate diagram?

Accepted Answer

It is the highest energy point, representing the activated complex.

Question 15

Which of the following species can act as both a Brønsted-Lowry base and a Lewis base?

Accepted Answer

$$\mathrm{NH_3$}$$

Question 16

Which of the following is the best explanation for why carboxylic acids (p$$K_a 4-5) $$are more acidic than alcohols (p$$K_a 15-18$$)?

Accepted Answer

The negative charge on the carboxylate ion is delocalized by resonance.

Question 17

Which of the following is the correct curved arrow notation for the proton transfer in the reaction $$\mathrm{CH_3CH_2OH + NH_2$^- \rightarrow CH_3CH_2O^- + NH_3$}$$?

Accepted Answer

Arrow from the lone pair on $$\mathrm{NH_2$^-} to $$the hydrogen of $$\mathrm{CH_3CH_2OH}$$, and arrow from the O-H bond to the oxygen atom.

Question 18

Which of the following best describes a nucleophile in the context of Lewis acid-base theory?

Accepted Answer

A species that donates a lone pair of electrons.

Acids and Bases in Organic Chemistry: Concepts, Mechanisms, and Applications

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