Question

What is a polyprotic acid?

Short answer

A polyprotic acid is an acid that can donate more than one proton or hydrogen atom per molecule to an aqueous solution.

Step-by-step solution

Define an Acid

An acid is a substance that donates hydrogen ions. In other words, an acid is a proton donor. This is according to Brønsted-Lowry acid definition. The acidity of a substance is directly related to the concentration of its hydrogen ions.

Explain Protons

Protons are a type of subatomic particle that each has a positive electric charge. Protons are found in the nucleus of every atom. When we say that an acid donates a proton, this means that it donates a hydrogen ion, because a hydrogen ion is just a proton.

Explain Polyprotic Acid

A polyprotic acid is an acid that can donate more than one proton per molecule. In the case of a diprotic acid, like sulfuric acid (H2SO4), it can donate two protons per molecule. Similarly, a triprotic acid, like phosphoric acid (H3PO4), can donate three protons per molecule. Hence, a polyprotic acid is known as a multi-proton donor.

Key concepts

Brønsted-Lowry Acid

A Brønsted-Lowry acid is a fundamental concept in chemistry, primarily defining what an acid is in terms of its behavior during chemical reactions. Specifically, a Brønsted-Lowry acid is a substance that acts as a proton donor. This definition helps chemists understand and categorize substances based on their ability to release hydrogen ions (H⁺), which are essentially protons in the atomic world.
The concept is crucial for analyzing reactions, especially in aqueous solutions where the transfer of protons plays a key role. Understanding this helps in predicting how substances will react with one another, affecting everything from acidity to stability of chemical compounds.

Proton Donor

When we talk about acids in chemistry, referring to them as proton donors can simplify complex reactions. A proton donor is simply an acid that releases a hydrogen ion (H⁺). This is because a hydrogen ion is essentially a proton, a positively charged particle devoid of electrons.
This release of a proton is an essential part of many chemical reactions, particularly those in water. By donating protons, these substances directly affect the pH and chemical balance of a solution. Understanding the role of proton donors helps in the analysis of acid-base reactions, including neutralization and buffering.

Diprotic Acid

Diprotic acids are a category of polyprotic acids that are able to donate two protons per molecule. They include substances like sulfuric acid (H₂SO₄).
This ability to donate two protons makes diprotic acids unique as they often undergo two distinct dissociation steps in a solution:

• The first dissociation typically results in the donation of one proton, often completely in strong acids.
• The second dissociation might be less complete, especially in weaker acids, and depends heavily on the solution's pH.

This two-step process affects the acid's strength and behavior, offering insights into its reactivity and the overall nature of the solution.

Triprotic Acid

Triprotic acids can donate three protons per molecule, such as phosphoric acid (H₃PO₄). These acids have three potential stages of proton donation, creating a more complex sequence than diprotic acids.
The triprotic nature leads to:

• Multiple dissociation steps, each with a unique equilibrium constant, affecting the concentrations of ions in the solution.
• The ability to act as a buffer in multiple pH ranges, making them useful in various industrial and biological contexts.

This characteristic defines their versatility in chemical reactions, influencing everything from soil chemistry to metabolic pathways.