Question

What is meant by the term amphoteric? Give two chemical equations that illustrate the amphoteric nature of water.

Short answer

An amphoteric substance can act as either an acid or a base. Water demonstrates this amphoteric behavior by reacting with NH₃ to form NH₄⁺ and OH⁻ (water as an acid), and with HCl to form H₃O⁺ and Cl⁻ (water as a base).

Step-by-step solution

Define Amphoteric Substance

Amphoteric refers to a substance that has the ability to react both as an acid and as a base. In the context of the Brønsted-Lowry theory, an amphoteric substance can either donate a proton (H⁺) when acting as an acid or accept a proton when acting as a base.

Identify Water as an Amphoteric Substance

Water (H₂O) is a classic example of an amphoteric substance because it has the ability to act as both an acid and a base depending on the other substances in the reaction. To demonstrate this, we will need to provide two chemical equations in which water acts in these two capacities.

Write Chemical Equation Showing Water as an Acid

To show water acting as an acid, it needs to donate a proton to another substance, a base. A typical reaction is with ammonia (NH₃). The equation for this reaction is: H₂O(l) + NH₃(aq) ⟶ NH₄⁺(aq) + OH⁻(aq)

Write Chemical Equation Showing Water as a Base

To display water acting as a base, it has to accept a proton from an acid. For instance, in the reaction with hydrogen chloride (HCl), water accepts a proton, forming hydronium ions. The chemical equation is: H₂O(l) + HCl(g) ⟶ H₃O⁺(aq) + Cl⁻(aq)

Key concepts

Brønsted-Lowry Theory Explained

The Brønsted-Lowry theory is instrumental in understanding acid-base reactions. According to this theory, an acid is any substance that can donate a proton (which is a hydrogen ion, represented as H⁺), while a base is defined as a substance that can accept a proton.

The theory extends the notion of acids and bases beyond just the substances that produce H⁺ or OH⁻ in water. It acknowledges the role of solvent systems and the concept of conjugate acid-base pairs: when an acid donates a proton, it becomes a base, termed its conjugate base, and vice versa for a base that accepts a proton, becoming its conjugate acid. This reciprocal relationship helps to explain why substances like water, which can both donate and accept protons, are amphoteric.

For example, in water's interaction with ammonia, NH₃, water acts as a Brønsted-Lowry acid because it donates a proton to the ammonia, forming NH₄⁺ and OH⁻. The formation of these ions is both crucial to the concept and reflective of water's amphoteric nature, a core topic explored in the original exercise.

Understanding Acid-Base Reactions

Acid-base reactions are fundamental to chemistry and involve the transfer of protons from acids to bases. Using the Brønsted-Lowry definition, we can see that these reactions are all about the movement of protons.

When an acid-base reaction occurs, there is a direct exchange of protons between molecules. This simplicity belies the complexity of reactions that can occur in solution. For instance, the interaction of water with hydrogen chloride (HCl) shows water acting as a base, accepting a proton from HCl to form H₃O⁺ and Cl⁻. This concept epitomizes the dynamic nature of water and its pivotal role in acid-base chemistry, which is highlighted in the textbook exercise.

These reactions are not limited to liquids; they can take place in all phases of matter. However, when they happen in aqueous solutions, we see unique dynamic equilibria and pH level changes that are essential for various biological and industrial processes.

Deciphering Chemical Equations

Chemical equations are the language of chemists, providing a concise way to represent chemical reactions. Every chemical equation tells a story, involving reactants transforming into products through chemical processes.

To understand these equations, we need to recognize that they obey the law of conservation of mass; atoms are neither created nor destroyed. Therefore, equations must be balanced, with the same number of each type of atom on both sides of the equation. As seen in the examples demonstrating water's amphoteric behavior, balanced equations provide insight into the stoichiometry—the quantitative relationship between reactants and products in a chemical reaction.

In the context of the exercise, the two equations involving water interacting with NH₃ and HCl are balanced, illustrating how water can play the dual role of an acid and a base. The ability to interpret such equations is vital for students in predicting the outcomes of reactions and understanding chemical phenomenology.