Acid-base chemistry has long been, and still is, a high-yield topic on the MCAT. In chemistry, there are three approaches to classifying acids and bases. Each approach has its own weight on the exam and tends to appear more often in one section. In this blog, we will introduce all three, but focus on the two highest-yield areas.

Approach 1 – Arrhenius Theory

The most specific definition of acids and bases is the Arrhenius approach. Arrhenius theory posits that in order for a substance to be an acid or a base, it must release either H+ or OH– ions (i.e. the compound must contain that ion). This definition covers many of the commonly known acids and bases (H₂SO₄, HNO₃, Ca(OH)₂, NaOH, KOH, etc.). However, this approach limits acidic and basic species to the aqueous phase. An Arrhenius acid is a compound that releases H+ ions when added to water, and an Arrhenius base is a compound that releases OH– ions when added to water. Arrhenius theory is much more limited than the other two theories so it is the least tested on the MCAT. For example, Arrhenius theory fails to explain the basic behavior of amines (e.g. ammonia, NH₃) which, in the presence of water, increase hydroxide ions concentration in solution, but do not contain any OH– ions. This leads to the next, broader definition of acids and bases.

Approach 2 – Bronsted-Lowry Theory

The Bronsted-Lowry theory posits that an acid is any species which is a proton (H+ ion) donor and a base is any species which is a proton acceptor. This includes species like ammonia and water. Water itself can act as both a BL-acid and a BL-base, making it an amphoteric species. A basic salt, such as Ca²+(F–)₂, will generate OH– ions in water by taking protons from water itself (to make HF). This approach forms the basis for pH, pOH, K_a, K_b, and K_w. These values are tested quite often in the areas of general chemistry and biochemistry. An MCAT favorite in this area is to ask students to recognize the charges that can appear or disappear on amino acids and their side chains. The relative strengths of acid/base species is also based on the Bronsted-Lowry theory. This content area is math-heavy, so be sure to practice your calculations, work on your test day scratch work, and utilize your Next Step materials to learn valuable shortcuts to avoid math whenever possible.

Approach 3 – Lewis Theory

The most general school of thought on amino acids is the Lewis theory. This theory does not define acids or bases based on ions or proton exchange, but on the transfer of electrons between species. A Lewis acid is an electron pair acceptor while a Lewis base is an electron pair donor. Lewis bases (e.g. NH₃) are generally the same molecules seen in the Bronsted-Lowry definition but Lewis acids are different. Since Lewis theory posits that an acid species must be able to accepting an electron pair, many are species like H+ ions (protons) that are cations. Many Lewis acids will have a positive charge and an empty orbital that can hold the electron pair. Other Lewis acids need not be ionic at all. Classic MCAT Lewis acids include BF₃ and AlCl₃, the latter of which is a common catalyst for electrophilic aromatic substitution reactions (aromatics are fair game for the exam). This definition of acid base chemistry tends to show up more in the organic chemistry concepts, but may be combined with biochemistry. Keeping track of electrons is a big part of understanding the biochemistry tested on the MCAT.

Bonus – Schiff Base

A creative approach by the new MCAT is the concept of Schiff bases. With carbonyl derivatives, the AAMC has long tested the idea that the electrophilic carbonyl carbon is a ripe target for nucleophilic attack. If the attacking nucleophile is an amine, and the electrophile is an aldehyde or ketone, the product of this reaction is a compound in which the C=O double bond (carbonyl) is replaced by a C=N double bond (imine). This imine is also known as a Schiff base. Schiff base formation has important biological applications to metabolism, aldol reactions, antibiotic mechanisms, and the reactions of B vitamins. This concept is most likely to appear in Biological/Biochemical Foundations passages, but can also be tested under the guise of organic chemistry basics (e.g. nucleophilic attack).

No matter the school of thought, the test makers will expect you to recognize the actions of acids and bases in many contexts, so be sure to practice plenty of MCAT-style questions once you think you have a handle on the content. Looking to get more practice? Our online Qbank contains over 1,000 discrete MCAT science questions plus CARS practice passages! Add the Qbank to your NextStepMCAT.com account here.