Catalysis (Homogeneous, Heterogeneous, Enzymatic)

Catalysis is one of those concepts that sounds simple — catalysts speed up reactions — but the MCAT probes the details in ways that trip up a lot of students. A catalyst works by providing an alternate reaction pathway with a lower activation energy (Ea). That's it. It doesn't change the thermodynamics of the reaction: ΔG, ΔH, and the equilibrium constant K are all untouched. The three types you need to know are homogeneous (catalyst and reactants in the same phase), heterogeneous (different phases — think solid metal catalyst with gas-phase reactants), and enzymatic (biological catalysts that are proteins). Each type shows up in passage contexts where you need to identify what kind of catalysis is occurring and predict its effects.

The MCAT tests this from several angles. Reaction coordinate diagrams are the most common — you'll be shown an energy diagram and asked to identify where Ea appears, how a catalyst changes the diagram, or what happens to the reverse reaction's Ea. The cross-disciplinary angle connects catalysis to biochemistry: enzymes follow Michaelis-Menten kinetics, but the underlying principle is still just Ea lowering. Students who compartmentalize bio and chem struggle here because they forget enzymes obey the same rules as any other catalyst.

The biggest source of errors on this topic isn't forgetting definitions — it's misapplying them under pressure. Students confuse 'speeds up the reaction' with 'shifts equilibrium toward products,' or they think because a catalyst appears in a mechanism it must be consumed. Both are wrong, and both are actively tested. If you can't articulate exactly why these are wrong, you'll lose points on questions that look like they're about something else entirely.