Question

What is the difference between a monoprotic acid and a diprotic acid? Give an example of each.

Short answer

Monoprotic acids donate one proton (e.g., HCl), while diprotic acids donate two protons (e.g., H₂SO₄).

Step-by-step solution

Understanding Acids

Acids are substances that can donate protons (H⁺ ions) when dissolved in water. The ability to donate one or more protons is what classifies an acid as either monoprotic, diprotic, or polyprotic.

Defining Monoprotic Acid

A monoprotic acid is an acid that donates only one proton (H⁺ ion) per molecule when it is dissolved in water. It has one hydrogen atom capable of dissociation. An example is hydrochloric acid (HCl), which dissociates to form H⁺ and Cl⁻ in water.

Defining Diprotic Acid

A diprotic acid is an acid that can donate two protons (H⁺ ions) per molecule in a stepwise fashion when dissolved in water. It contains two hydrogen atoms that can dissociate. Sulfuric acid (H₂SO₄) is an example; it dissociates in two steps: first releasing one H⁺ ion to form HSO₄⁻, and then another H⁺ ion from HSO₄⁻ to form SO₄²⁻.

Example Identification

To provide examples, a monoprotic acid is hydrochloric acid (HCl), and a diprotic acid is sulfuric acid (H₂SO₄). This identification aligns with each acid's ability to donate protons.

Key concepts

Monoprotic Acid

Monoprotic acids are acids capable of donating one proton (\( H^+ \) ion) per molecule. These acids have a single hydrogen atom that dissociates when the acid is dissolved in water. The process is straightforward, as there is only one hydrogen ion to consider.
A common example of a monoprotic acid is hydrochloric acid (\( \text{HCl} \)). When \( \text{HCl} \) is dissolved in water, it separates into \( \text{H}^+ \) and \( \text{Cl}^- \) ions.
This makes the categorization of monoprotic acids easy to understand:

• They always release one hydrogen ion per molecule.
• They play crucial roles in chemical reactions as reactants and catalysts.
• They include compounds like \( \text{HCl} \), \( \text{HNO}_3 \), and \( \text{CH}_3\text{COOH} \), which is acetic acid.

Diprotic Acid

Diprotic acids are a bit more complex as they give up two protons (\( H^+ \) ions) per molecule in a sequential manner. These types of acids contain two hydrogen atoms that can be ionized in separate stages.
The dissociation process of a diprotic acid involves two distinct steps, offering a characteristic example through sulfuric acid (\( \text{H}_2\text{SO}_4 \)):

• In the first dissociation step, sulfuric acid releases one \( H^+ \) ion, forming \( \text{HSO}_4^- \).
• In the second dissociation, \( \text{HSO}_4^- \) now releases another \( H^+ \) ion, resulting in \( \text{SO}_4^{2-} \).

This staged dissociation process:

• Allows diprotic acids to act as buffers in different pH ranges.
• Includes common acids like \( \text{H}_2\text{CO}_3 \) (carbonic acid) and \( \text{H}_2\text{C}_2\text{O}_4 \) (oxalic acid).

Proton Donation

Proton donation is a fundamental concept in understanding how acids behave in chemical reactions. When an acid donates a proton (\( H^+ \) ion), it changes the chemical environment, often leading to an increase in acidity or a change in pH.
The process of donation is pivotal because:

• It defines the strength of an acid; strong acids tend to donate protons more readily in aqueous solutions.
• It results in the formation of a conjugate base, which further participates in balancing chemical reactions.
• It influences the acid's role in catalyzing or controlling the speed of reactions.

Understanding proton donation helps to explain why compounds like hydrochloric acid are potent in reactions, particularly in producing salts and liberating hydrogen gas.

Hydrochloric Acid

Hydrochloric acid (\( \text{HCl} \)) is one of the most widely known and used monoprotic acids. It is often encountered in laboratory and industrial settings due to its strong acidic nature and reactive properties.
Some of its key characteristics include:

• Full dissociation in water, producing \( H^+ \) and \( Cl^- \) ions.
• Usage in applications like pickling of steel, cleaning agents, and the production of organic compounds.
• A powerful reactivity that demands careful handling in diluted forms to prevent harm.

Additionally, \( \text{HCl} \) is naturally found in the stomach, where it aids digestion by breaking down food and killing pathogens. Its simplicity and effectiveness are why it's frequently considered a model acid for studying chemical reactions.

Sulfuric Acid

Sulfuric acid (\( \text{H}_2\text{SO}_4 \)) serves as a classic example of a diprotic acid and is renowned for its wide array of industrial applications. It is an essential component in various chemical processes and products around the world.
Key aspects of sulfuric acid include:

• Its capability to dissociate in water through two stages, releasing two protons.
• Extensive use in producing fertilizers, refining oil, and manufacturing chemicals like detergents and explosives.
• Its role as a dehydrating agent, making it useful in drying processes.

Despite its utility, sulfuric acid is corrosive and requires careful, controlled handling. Its propensity to release protons readily makes it a preferred acid in many commercial applications, serving as both a reactant and a catalyst.