Question

Is it possible for a chemical to behave as an acid in one instance and as a base in another instance?

Short answer

Yes, some chemicals can behave as both an acid and a base (amphiprotic substances).

Step-by-step solution

Understanding the Concept of Acids and Bases

In order to determine if a chemical can behave both as an acid and a base, we must first understand what an acid and a base are. According to the Brønsted-Lowry theory, an acid is a substance that can donate a proton (H⁺ ion), while a base is a substance that can accept a proton.

Introducing Amphiprotic Substances

An amphiprotic substance is one that can both donate and accept protons. This means it can act as an acid in some instances and a base in others. A common example of an amphiprotic substance is water (H₂O), which can act as an acid and donate a proton to become hydroxide (OH⁻), or act as a base and accept a proton to become hydronium (H₃O⁺).

Final Analysis

Based on our understanding of amphiprotic substances, it is indeed possible for a chemical to behave as both an acid and a base depending on the reaction it is involved in. Substances like bicarbonate (HCO₃⁻) also demonstrate this dual behavior, acting as acids when reacting with stronger bases and as bases when reacting with stronger acids.

Key concepts

Brønsted-Lowry theory

The Brønsted-Lowry theory is a fundamental concept in the study of acids and bases. It defines acids as substances that can donate a proton, which is a hydrogen ion (\(\text{H}^+\)), and bases as substances that can accept a proton. This theory broadens the traditional definition of acids and bases, moving beyond just their taste or pH level.According to this theory:

• Acids are proton donors – they give away a hydrogen ion in a reaction.
• Bases are proton acceptors – they receive a hydrogen ion during a reaction.

This perspective allows for a wider range of compounds to be classified as acids or bases, depending on their behavior in chemical reactions. It helps us understand that some substances can act like both, depending on their environment or what they are reacting with.

Acid-base reactions

Acid-base reactions are chemical processes where acids donate protons to bases. These reactions are vital to many biological and chemical processes.In everyday life, these reactions can be seen in:

• Neutralizing acidic substances like vinegar with a base such as baking soda.
• The digestion of food in the stomach, where gastric acids play a role.

The understanding of acid-base reactions is crucial as these reactions are reversible. It means the product of one reaction can split back into proton donors and acceptors, highlighting their dynamic nature. For example, when water (\(\text{H}_2\text{O}\)) acts as an acid by donating a proton, it forms hydroxide (\(\text{OH}^-\)). Conversely, it can act as a base by accepting a proton to form a hydronium ion (\(\text{H}_3\text{O}^+\)). This reversible reaction showcases the balance in acid-base chemistry.

Proton donation and acceptance

Proton donation and acceptance are key components of acid-base chemistry. These actions determine whether a substance behaves as an acid or a base in a particular reaction.Proton Donation
When a substance donates a proton, it behaves as an acid according to the Brønsted-Lowry theory. For example, in water reacting with ammonia (\(\text{NH}_3\)), water donates a proton and therefore acts as an acid, transforming into hydroxide.Proton Acceptance
Conversely, when a substance accepts a proton, it acts as a base. For instance, ammonia accepts a proton from water, becoming ammonium (\(\text{NH}_4^+\)), and thus acts as a base.These interactions highlight the versatility of substances like water, which are amphiprotic, meaning they can either donate or accept protons depending on the reaction they are involved in. This dual capability is what allows amphiprotic substances to behave both as acids and bases, adapting to the specific chemical interactions they encounter.