Enzymes do not change $\Delta G$: the catalyst lowers the hill, not the destination

01The mistake

Students say an enzyme makes a reaction possible, or that it makes an unfavourable reaction favourable. Asked whether an enzyme-catalysed reaction would occur without the enzyme, they say no. In their model the enzyme supplies whatever the reaction was missing, which merges the kinetic barrier with the thermodynamic outcome into one idea of “can this happen.”

The sharper form is the equilibrium claim: students assert that adding enzyme shifts the reaction toward products, or increases the yield. It does neither. It reaches the same equilibrium faster, and it accelerates the forward and reverse reactions by the same factor. This is where the misconception becomes measurable, because the student's model makes a prediction that is straightforwardly false.

Watch for “enzymes provide energy” as a related surface form (U3-BIO6). It follows naturally: if the enzyme makes an unfavourable reaction go, it must be supplying something, and energy is the available noun. Students who say this are being consistent with their model rather than adding a second error.

The diagnostic question is the reverse reaction. Ask whether an enzyme speeds up the reverse reaction too. A student who has fused rate and favourability says no — the enzyme is pushing in one direction — and that answer reveals the whole model in a word.

02Why it makes sense to the student

The energy diagram is drawn once and read wrong. The picture has two features: a hill and a net drop. Students who have not been made to distinguish them read the hill as the whole story, and lowering the hill then looks like it should change where the reaction ends up. The diagram contains the correction, but only for a student who has been shown which feature is which.

Biology courses reach enzymes before most students have a stable thermodynamics vocabulary. Activation energy and free energy change arrive in the same week, both are energies, both are drawn on the same axes, and one of them is new. Merging them is the path of least resistance.

Cells genuinely cannot run these reactions without enzymes, so “the enzyme makes it possible” is true in the only context students care about. At body temperature and on biological timescales, an uncatalysed reaction may as well not occur. The statement is right about the biology and wrong about the mechanism, which makes it very hard to dislodge.

And “catalyst” is defined in most students' memory as something that makes a reaction happen, rather than as something that is not consumed and does not alter the equilibrium. The definition they carry is about enabling, not about rate.

03The correction

Split the diagram explicitly and label both features every time you draw it. The hill is the activation energy $E_a$: how much energy is needed to reach the transition state. The net drop from reactants to products is $\Delta G$: whether the reaction is favourable and by how much. An enzyme lowers the hill. It does not move either endpoint, so it cannot touch $\Delta G$.

State the consequence that students can check: because the endpoints are unchanged, the equilibrium constant is unchanged. An enzyme accelerates the forward and reverse reactions equally. Add enzyme to a reaction at equilibrium and nothing happens at all — no shift, no change in yield, no net conversion. That prediction is the cleanest possible test of the correct model against the incorrect one.

Then close the door on “the enzyme makes it possible.” An enzyme-catalysed reaction is one that would proceed without the enzyme, just far too slowly to matter. Carbonic anhydrase is the example worth quoting: the uncatalysed hydration of CO₂ does happen, and the enzyme speeds it up by a factor of roughly a million. The reaction was always favourable; it was only slow.

Handle the genuinely unfavourable cases separately and directly, because this is where students' intuition is actually onto something. Cells do drive unfavourable reactions — by coupling them to ATP hydrolysis, which changes the overall $\Delta G$ of the combined process. That is a different mechanism from catalysis, and teaching it alongside enzymes is what prevents students from assigning the job to the enzyme.

A useful classroom test: “A reaction has $\Delta G > 0$. You add the appropriate enzyme. What happens?” The answer is essentially nothing — the reaction is still unfavourable and will not proceed to product. Students holding the misconception predict it now runs, and the question separates the two models without any calculation.

04A sample question

05What each wrong answer reveals

• A Rate and yield fused. The student expects a catalyst to produce more product, because in their model the enzyme pushes the reaction along and pushing should get you further. Notice they may well know the definition of a catalyst — this is not a vocabulary gap. What is missing is that equilibrium is a destination set by thermodynamics and the enzyme only affects the journey.
• B Correct. An enzyme lowers $E_a$ for both directions equally, so it changes how quickly equilibrium is reached but not where it lies. At equilibrium, adding enzyme produces no net change.
• C Activation energy and free energy merged by name. This student has attached “lowers the energy” to the wrong energy. It is the most direct statement of the misconception and points straight at the energy diagram as the repair — they need the hill and the net drop labelled as different quantities before anything else will land.
• D One-directional catalysis. The most revealing distractor in the set. This student has correctly identified that enzymes affect rates rather than energetics — a genuine conceptual advance over A and C — and then made the effect directional. The repair is a single fact: the transition state is shared, so lowering the barrier necessarily speeds both directions. This student is one sentence from correct.

C and D sit at opposite ends of understanding despite both being wrong. C has not yet separated the two energies; D has separated them and needs only the symmetry of the barrier. A is closest to everyday intuition and is the most common. Reading which of the three dominates tells you whether to spend the period on the energy diagram or on the transition state, and those are not the same lesson.

06Try it in Mistake Master