Question

Label each of the following as being a strong acid, a weak acid, or a species with negligible acidity. In each case write the formula of its conjugate base, and indicate whether the conjugate base is a strong base, a weak base, or a species with negligible basicity: \((\mathbf{a}) \mathrm{HCOOH},(\mathbf{b}) \mathrm{H}_{2},(\mathbf{c}) \mathrm{CH}_{4},\) (d) \(\mathrm{HF}\) (e) \(\mathrm{NH}_{4}^{+}\).

Short answer

(a) HCOOH: Weak acid, Conjugate base: \(HCOO^-\), Weak base (b) \(H_2\): Negligible acidity, No conjugate base (c) \(CH_4\): Negligible acidity, No conjugate base (d) HF: Weak acid, Conjugate base: \(F^-\), Weak base (e) \(NH_4^+\): Weak acid, Conjugate base: \(NH_3\), Weak base

Step-by-step solution

Analyzing HCOOH

First, we need to determine if the formic acid (HCOOH) is a strong acid, weak acid, or has negligible acidity. Formic acid is a weak acid because it can donate a proton (H+), but it does not completely ionize in water.

Conjugate Base of HCOOH

The conjugate base is formed by removing one proton (H+) from the acid. So, the conjugate base of HCOOH is HCOO⁻. HCOO⁻ is a weak base because it can accept a proton (H+) but does not ionize completely in water. (a) HCOOH: Weak acid, Conjugate base: HCOO⁻, Weak base

Analyzing H2

Hydrogen gas (H₂) does not donate any protons, as it is a neutral molecule and does not act as an acid. (b) H₂: Negligible acidity, No conjugate base

Analyzing CH4

Methane (CH₄) does not donate any protons, as it is a neutral molecule and does not act as an acid. (c) CH₄: Negligible acidity, No conjugate base

Analyzing HF

Hydrofluoric acid (HF) is a weak acid because it can release a proton (H+) but does not completely ionize in water.

Conjugate Base of HF

The conjugate base of HF is F⁻, which is formed by removing one proton (H+) from the acid. F⁻ is a weak base since it can accept a proton (H+), but does not ionize completely in water. (d) HF: Weak acid, Conjugate base: F⁻, Weak base

Analyzing NH4+

Ammonium ion (NH₄⁺) is a weak acid because it can donate a proton (H+) but does not ionize completely in water.

Conjugate Base of NH4+

The conjugate base of NH₄⁺ is NH₃, which is formed by removing one proton (H+) from the acid. NH₃ is a weak base because it can accept a proton (H+) but does not ionize completely in water. (e) NH₄⁺: Weak acid, Conjugate base: NH₃, Weak base

Key concepts

Conjugate Base

In acid-base chemistry, a conjugate base is what remains after an acid donates a proton (H⁺). When an acid loses its proton, it becomes a conjugate base, because it now has the potential to gain back the proton.
For example, the conjugate base of formic acid (HCOOH) is the formate ion (HCOO^-). This transformation occurs when the acid donates its H⁺, leaving HCOO^-, which can potentially accept a proton again.

• The concept of conjugate bases helps in understanding the acid-base balance.
  
• It is important to note that conjugate bases can be weak bases, strong bases, or possess negligible basicity depending on their ability to re-gain a proton.
  

This is crucial in predicting the behavior of substances in chemical reactions, and in identifying their strength as a base.

Weak Acids

Weak acids are those that do not fully dissociate in solution. This means that only a small fraction of the acid molecules ionize, releasing protons (H⁺) into the solution.
One classic example of a weak acid is acetic acid (CH₃COOH), which only partially dissociates in water.

• Because they don't fully ionize, weak acids tend to have higher pH values compared to strong acids.
  
• As a result, their conjugate bases (formed when the acid loses a proton) tend to be partially stable.
  

The concept of weak acids is important for understanding many biological and environmental processes, where reactions need to be controlled and wouldn't work as well with strong acids.

Negligible Acidity

Negligible acidity refers to substances that essentially do not release any protons into solution; they do not act as acids.
For instance, hydrogen gas (H₂) and methane (CH₄) hold their hydrogen atoms tightly, not donating them as protons.

• Such substances have negligible acidity because they do not contribute significantly to the proton concentration in a solution.
  
• Therefore, they are not considered in acid-base reactions for their proton exchanging abilities.
  

Understanding negligible acidity is particularly useful for identifying which reactions are unlikely to occur spontaneously and thus require additional conditions or catalysts.

Strong Acids

Strong acids are characterized by their complete ionization in water. This means that in solution, they donate all of their protons (H⁺) and form a large amount of hydronium ions (H₃O⁺).
Common examples include hydrochloric acid (HCl) and sulfuric acid (H₂SO₄).

• Because they completely dissociate, strong acids lead to low pH values, indicating high acidity.
  
• Their conjugate bases, usually remain in a relatively stable form and do not engage in reversing the ionization under normal conditions.
  

Strong acids play a critical role in chemistry because of their ability to drive reactions to completion and their widespread use in analytical chemistry, industry, and research.
Recognizing strong acids is key when predicting the outcomes of chemical reactions.