17.1.2 Strong Acid-Weak Base and Weak Acid-Weak Base Reactions Flashcards Preview

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Flashcards in 17.1.2 Strong Acid-Weak Base and Weak Acid-Weak Base Reactions Deck (12)
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1
Q

Strong Acid-Weak Base and Weak Acid-Weak Base Reactions

A

• The direction of a strong acid-weak base reaction can be predicted using equilibrium constant (K eq ) information.
• The direction of a weak acid-weak base reaction can be predicted using K eq information.
• The pH of a system can be predicted by comparing the acid-dissociation constant (K a ) of the acid and the
base-dissociation constant (K b ) of the base.

2
Q

note

A
  • The direction of a strong acid-weak base reaction, and thus, whether a particular acid-base pair will react, can be predicted using equilibrium constant (K eq ) information.
  • For the reaction of a weak base (such as ammonia) with a strong acid, K eq is calculated from the base-dissociation constant (K b ) and the dissociation constant of water (K w ).
  • A large K eq indicates that a reaction will favors products. A small K eq indicates that a reaction will favor reactants. In this example, K eq = 1.8 x 10 9 (at 25 ̊C), so the reaction favors products.
  • The direction of a weak acid-weak base reaction, and thus, whether a particular acid-base pair will react, can be predicted using K eq information.
  • For the reaction of a weak acid (such as acetic acid) with a weak base (such as ammonia), K eq is calculated from the acid-dissociation constant (K a ) of the weak acid, the base-dissociation constant (K b ) of the weak base, and the dissociation constant of water (K w ). In this example, K eq = 3.2 x 10 4 (at 25 ̊C), so the reaction favors products.
  • To approximate the pH of a solution, simply compare the Ka of the acid and the K b of the base. If the K a of the acid is greater than the K b of the base, the solution is acidic (pH < 7). If the Ka of the acid is less than the K b of the base, the solution is basic (pH > 7). If the K a and K b values are about equal, the solution is neutral.
3
Q

In the reaction

H3O+(aq) + Cl-(aq) + NH3(aq) <==> NH4+(aq) + Cl-(aq) + H2O(l)

H3O+ and Cl − are intermediate reactants. Which statement best explains why this occurs?

A

This occurs because hydrochloric acid, a strong acid, completely dissociates in the aqueous solution, resulting in the two ions of H3O+ and Cl −.

4
Q

A strong acid, HCl, is combined with a weak base, NH3, yielding ammonium chloride, NH4Cl(aq).

HCl(aq) + NH3(aq) <==> NH4Cl(aq)

The first intermediate reaction is the result of the strong acid, HCl(aq), and the weak base, NH3, interacting with the aqueous solution (i.e., water). The resultant reaction is:

H3O+(aq) + Cl-(aq) + NH3(aq) <==> NH4+(aq) + Cl-(aq) + H2O(l)

Which of the following correctly states what has occurred so far in the reaction?

A

HCl and NH4Cl have completely dissociated, while NH3 has remained as an ammonia molecule.

5
Q

In the following reaction,

HCl(aq) + NH3(aq) <==> NH4Cl(aq)

the first intermediate reaction is the result of the strong acid, HCl(aq), and the weak base, NH3, interacting with the aqueous solution (i.e., water). This is also the final reaction.

First intermediate reaction:

H3O+(aq) + Cl-(aq) + NH3(aq) <==> NH4+(aq) + Cl-(aq) + H2O(l)

Second intermediate reaction:

NH3(aq) + H2O(l) <==> NH4+(aq) + OH-(aq)

Third intermediate reaction:

H3O+(aq) + OH-(aq) <==> 2H2O(l)

Final reaction:

H3O+(aq) + NH3(aq) <==> NH4+(aq) + H2O(l)

If all the reactants and products in the two intermediate reactions are added, the result is the final reaction. If Kb is the equilibrium constant for the second intermediate reaction and (1 / Kw ) is the equilibrium constant for the third intermediate reaction, what is the equilibrium constant for the final reaction, KFinal?

A

KFinal = Kb (1 / Kw ) = Kb / Kw

6
Q

Which of the following is the key question for accurately assessing a strong acid-weak base reaction at equilibrium?

A

What is the Kb for this reaction?

7
Q

Look at the same intermediate reaction.

H3O+(aq) + Cl-(aq) + NH3(aq) <==> NH4+(aq) + Cl-(aq) + H2O(l)

Identify the spectator ions and explain why H2O is a product.

A

The spectator ions are the Cl − ions on either side. H2O is a product because it is the conjugate base of H3O+.

8
Q

Look again at the reaction.

HCl(aq) + NH3(aq) <==> NH4Cl(aq)

The first intermediate reaction is the result of the strong acid, HCl(aq), and the weak base, NH3(aq), interacting in the aqueous solution. The resulting reaction is

H3O+(aq) + Cl-(aq) + NH3(aq) <==> NH4+(aq) + Cl-(aq) + H2O(l)

Notice that the NH3 has not changed. Why does it not dissociate in water?

A

The NH3 is such a weak base that it does not naturally dissociate in water.

9
Q

The next intermediate reaction that we can examine shows the first reaction,

H3O+(aq) + Cl-(aq) + NH3(aq) <==> NH4+(aq) + Cl-(aq) + H2O(l)

with the spectator ions removed.

H3O+(aq) + NH3(aq) <==> NH4+(aq) + H2O(l)

Which statement about the reaction is not correct?

A

We now have all the information that we need to determine the K value for the overall reaction.

10
Q

Look at the reaction in which a weak acid, CH3COOH, and a weak base, NH3, are combined in an aqueous solution. The products are the ammonium ion (NH4+) and the acetate ion (CH3OO-)
CH3COOH(aq) + NH3(aq) <==> NH4+(aq) + CH3OO-(aq)
There are three intermediate reactions for this overall reaction:
1. CH3COOH(aq) + H2O(l) <==> CH3OO-(aq) + H3O+(aq)
2. NH3(aq) + H2O(l) <==> NH4+(aq) + H3O+(aq)
3. H3O+(aq) + OH-(aq) <==> 2H2O(l)
The overall reaction is CH3COOH(aq) + NH3(aq) <==> NH4+(aq) + CH3OO-(aq)
Which of the following statements about this overall reaction and the intermediate reactions is not correct?

A

The final K value (and the equilibrium) for the reaction is most affected by the base or the acid that dissociates the most with water.

11
Q

A weak acid and a strong base are combined in solution. Which of the following describes the key step in the analysis of this reaction?

A

Finding Ka value (for the dissociation intermediate reaction of the strong base in aqueous solution).

12
Q

Look at the reaction of hydrochloric acid, a strong acid, and ammonia, a weak base, to (potentially) yield ammonium chloride.

HCl(aq) + NH3(aq) <==> NH4Cl(aq)

One of the ions created by the dissociation of HCl in water is Cl −. What is the charge on the ammonium ion before it bonds with the Cl − ion?

A

+1

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