Question

Label each of the following as being a strong base, a weak base, or a species with negligible basicity. In each case write the formula of its conjugate acid, and indicate whether the conjugate acid is a strong acid, a weak acid, or a species with negligible acidity: \((\mathbf{a})\mathrm{CH}_{3} \mathrm{COO}^{-},(\mathbf{b}) \mathrm{HCO}_{3}^{-},(\mathbf{c}) \mathrm{O}^{2-},(\mathbf{d}) \mathrm{Cl}^{-},(\mathbf{e}) \mathrm{NH}_{3}\)

Short answer

a. CH3COO- is a weak base. Its conjugate acid is CH3COOH, which is a weak acid. b. HCO3- is a weak base. Its conjugate acid is H2CO3, which is a weak acid. c. O2- is a strong base. Its conjugate acid is OH-, which is a weak acid. d. Cl- has negligible basicity. Its conjugate acid is HCl, which is a strong acid. e. NH3 is a weak base. Its conjugate acid is NH4+, which is a weak acid.

Step-by-step solution

a. CH3COO-

CH3COO- is acetate ion, which is a weak base because when it reacts with water, it forms a weak acid (acetic acid, CH3COOH) and hydroxide ion (OH-). Its conjugate acid is CH3COOH, which is a weak acid.

b. HCO3-

HCO3- is bicarbonate ion, which is a weak base as well because it forms H2CO3 (carbonic acid) and OH- when it reacts with water. Its conjugate acid is H2CO3, which is also a weak acid.

c. O2-

O2- is oxide ion, and it is a strong base because when it reacts with water, it forms hydroxide ions (OH-) and water. Its conjugate acid is OH-, which is a weak acid.

d. Cl-

Cl- is chloride ion, and it has negligible basicity because it forms a very weak base (HCl, which is a strong acid) when it reacts with water. Hence, there is no significant reaction of Cl- with water. Its conjugate acid is HCl, which is a strong acid.

e. NH3

NH3 is ammonia, which is a weak base because it forms NH4+ (ammonium ion) and OH- when it reacts with water. Its conjugate acid is NH4+, which is a weak acid.

Key concepts

Conjugate Acids and Bases

In the world of acid-base chemistry, the concept of conjugate acids and bases helps us understand chemical behavior.
When an acid donates a proton (\( ext{H}^+ \)), it forms its conjugate base. Conversely, when a base accepts a proton, it forms its conjugate acid.
This pair forms a conjugate acid-base pair, showcasing their intertwined nature in chemical reactions.
For example, the acetate ion (\( ext{CH}_3 ext{COO}^- \)) acts as a base. When it gains a proton, it becomes acetic acid (\( ext{CH}_3 ext{COOH} \)), its conjugate acid.
Understanding the relationship between these pairs helps predict the outcome of chemical reactions.

Strong Acids and Bases

Strong acids and bases dissociate completely in water, meaning they release all of their hydrogen or hydroxide ions into the solution.
For example, hydrochloric acid (\( ext{HCl} \)) is a strong acid because it fully dissociates to produce \( ext{H}^+ \) and \( ext{Cl}^- \) ions.
This complete dissociation is what makes an acid or a base strong.

• Strong acids donate all protons and have negligible conjugate bases.
• Strong bases accept protons completely, like the oxide ion (\( ext{O}^{2-} \)) forming hydroxide ions (\( ext{OH}^- \)).

Knowing the strength of an acid or base helps predict their reactions and properties.

Weak Acids and Bases

Weak acids and bases only partially dissociate in water, which means they do not release all of their ions.
This partial dissociation results in an equilibrium between the undissociated molecules and the liberated ions.
Consider acetic acid (\( ext{CH}_3 ext{COOH} \)), a weak acid that only partially ionizes to form ions in solution.
Similarly, ammonia (\( ext{NH}_3 \)) is a weak base forming ammonium ions (\( ext{NH}_4^+ \)) in water.

• Weak acids have stronger conjugate bases compared to strong acids.
• Weak bases have corresponding conjugate acids that are not too strong.

Understanding the dynamic balance of weak acids and bases helps in recognizing their behavior in chemical environments.

Chemical Reactions with Water

The interaction between acids and bases with water is fundamental in acid-base chemistry.
When a substance reacts with water, it can donate or accept protons, affecting the properties of the solution.

• For example, oxide ion (\( ext{O}^{2-} \)) reacts with water to form hydroxide ions, significantly increasing the solution's basicity.
• In contrast, bicarbonate ion (\( ext{HCO}_3^- \)) balances a weakly acidic and basic nature in water.

These reactions determine a solution’s pH and its capacity to conduct electricity.
By knowing how different species react with water, we can better anticipate and control chemical processes.