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Acid-Base Definitions

Let’s begin our exploration by delving into acid-base definitions. While it might be convenient to have a single, memorizable definition, the world of chemistry is anything but simple. The understanding of these essential chemical species has given rise to not one, but three primary definitions. Each of these definitions provides a distinct perspective, offering us a unique lens through which to comprehend the intriguing world of acids and bases.

According to the Arrhenius Theory, acids are substances that, when dissolved in water, release hydrogen ions (H+), while bases release hydroxide ions (OH-).

The Brønsted-Lowry Theory, on the other hand, characterizes acids as substances capable of donating a proton (H+ ion) to another substance and bases as substances capable of accepting a proton from another substance.

Lastly, the Lewis Theory defines acids as substances that can accept an electron pair and bases as substances that can donate an electron pair.

While it is great to know what the definitions are the AAMC is often going to ask you to spot or categorize different molecules into one of the three categories. Knowing what to look for ahead of time makes this much easier.

Regardless of the type of acids or base involved we are going to follow a couple of general ideas when determining what type of acid or base something is. Then round out our understanding of the topic with some practice questions.

Examine Chemical Formulas

Our first technique revolves around looking for specific features in the chemical formula of an acid or base to determine it’s identity. 

Arrhenius Acid and Base:

  • Acids: Look for a chemical formula that contains H in its chemical formula, typically at the beginning of the formula, such as HCl (hydrochloric acid) or H2SO4 (sulfuric acid).
  • Bases: Identify substances that contain a hydroxide group (-OH) at the end of it’s formula such as NaOH (sodium hydroxide) or KOH (potassium hydroxide).

Brønsted-Lowry Acid and Base:

  • Acids: Focus on substances that can donate a proton (H+ ion) to another substance this means looking for a chemical formula with an H in it.
  • Bases: Identify substances that can accept a proton (H+ ion) from another substance. Look for compounds that have available electron pairs to bond with incoming H+ ions. For example, ammonia (NH3) can accept an H+ ion to form NH4+.

Lewis Acid and Base:

  • Acids: Search for substances that can accept an electron pair. These are often compounds with partially positive charges that can accommodate an incoming electron pair. Transition metals, such as AlCl3 (aluminum chloride), can act as Lewis acids by accepting electron pairs.
  • Bases: Identify substances that can donate an electron pair. These are typically compounds with lone pairs of electrons that can form a coordinate bond with a Lewis acid. For instance, ammonia (NH3) can donate a lone pair to a Lewis acid.

Know Overlaps:

Every substance that qualifies as an Arrhenius acid or base also falls under the categories of Brønsted-Lowry acids or bases. Furthermore, any substance recognized as a Brønsted-Lowry acid or base can also be classified as a Lewis acid or base, given that these definitions encompass a broader range of reactions.

However, it’s important to note that the reverse isn’t necessarily true. Just because a substance is identified as a Brønsted-Lowry base, it doesn’t automatically mean it can be classified as an Arrhenius base. Similarly, a substance meeting the criteria for being a Lewis acid or base doesn’t guarantee that it will fit the definitions of Brønsted-Lowry or Arrhenius acids or bases. So, while there’s a hierarchy that ensures overlap when moving “up” the definitions, moving “down” doesn’t necessarily imply the same degree of overlap.

Key Terminology: 

The final aspect to pay attention to, key terms, pertains exclusively to the Lewis definition of acids and bases. Below are the essential terms you should familiarize yourself with:

  • Nucleophiles are another name for Lewis bases as they describe electron donors
  • Electrophiles are another name for Lewis acids as they describe electron acceptors
  • Complexes involve central metal ions (Lewis acids) and ligands (Lewis bases) in coordination chemistry.