A catalyst is added to an exothermic reaction. Which of the following correctly describes what changes and what stays the same?
ABoth the activation energy and ΔH decrease — the catalyst makes the reaction more thermodynamically favorable
BThe activation energy decreases but ΔH is unchanged — the catalyst changes the route without changing the energy difference between reactants and products
CThe activation energy increases but ΔH decreases — the catalyst sacrifices some kinetic efficiency for thermodynamic stability
DBoth Ea and ΔH stay the same — the catalyst only increases collision frequency, not energy barriers
A catalyst provides an alternative reaction pathway with lower activation energy. Crucially, it does not change the reactants, the products, or the energy difference between them (ΔH). The thermodynamics of the reaction — which direction is energetically favorable — is unchanged; the catalyst only affects kinetics (how fast the reaction reaches equilibrium). The reaction coordinate diagram shows the same starting and ending energy levels but a lower hill in the catalyzed pathway.
Question 2 Multiple Choice
An enzyme reduces activation energy by 35 kJ/mol at 37°C (310 K). Using the relationship that rate ∝ e^(-Ea/RT), approximately how does this affect the reaction rate?
AThe rate increases by roughly 35-fold — activation energy reduction produces linear rate increases
BThe rate approximately doubles — a common rule of thumb for every 10 kJ/mol reduction
CThe rate increases by millions- to billions-fold — the exponential dependence makes large Ea reductions produce enormous rate accelerations
DThe rate increases by about 350-fold — it scales as Ea/RT
The Arrhenius equation, k = Ae^(-Ea/RT), shows an exponential dependence on Ea. At 310 K, RT ≈ 2.6 kJ/mol. A 35 kJ/mol reduction in Ea changes the exponent by 35/2.6 ≈ 13.5, giving a rate increase of e^13.5 ≈ 730,000-fold. This is why enzymes are so extraordinarily effective — small reductions in Ea produce enormous rate accelerations. A reduction of even 10 kJ/mol gives roughly 50-fold acceleration at room temperature. The exponential is the key: linear thinking dramatically underestimates catalyst effectiveness.
Question 3 True / False
A catalyst increases reaction rate by raising the temperature of the reaction mixture, giving more molecules sufficient energy to react.
TTrue
FFalse
Answer: False
A catalyst does not raise temperature. It provides an alternative reaction pathway with a lower activation energy, allowing a larger fraction of molecules at the existing temperature to undergo successful collisions. The Maxwell-Boltzmann distribution of molecular speeds doesn't shift — instead, the threshold energy that counts as 'sufficient' is lowered. Catalysts increase the proportion of successful collisions at unchanged temperature; they do not add energy to the system.
Question 4 True / False
A catalyst that lowers activation energy does not change the energy difference between reactants and products.
TTrue
FFalse
Answer: True
True. The reaction coordinate diagram makes this clear: the starting energy level (reactants) and ending energy level (products) are the same in the catalyzed and uncatalyzed reaction — only the height of the energy hill between them changes. ΔH is a thermodynamic quantity that depends only on the initial and final states, not on the pathway. Because catalysts only change the pathway (the route over the hill), they cannot change ΔH. This is why catalysts cannot make thermodynamically unfavorable reactions favorable — they can only accelerate reactions that are already thermodynamically feasible.
Question 5 Short Answer
Explain why even a small reduction in activation energy produces a disproportionately large increase in reaction rate.
Think about your answer, then reveal below.
Model answer: Reaction rate depends exponentially on activation energy through the Arrhenius equation: k = Ae^(-Ea/RT). The fraction of molecules with enough energy to react is e^(-Ea/RT), which changes exponentially as Ea changes. A small decrease in Ea increases this fraction by a multiplicative factor of e^(ΔEa/RT), not by an additive amount. At room temperature (RT ≈ 2.5 kJ/mol), lowering Ea by 10 kJ/mol multiplies the fraction of successful collisions by e^4 ≈ 55-fold. Because of this exponential sensitivity, catalysts that shave even modest amounts off the activation energy produce dramatic rate accelerations.
The misconception to overcome is thinking about activation energy linearly. Students often reason 'lower barrier = somewhat faster' when the reality is 'lower barrier = exponentially faster.' This is why enzymes can accelerate reactions by 10^6 to 10^17 fold — they achieve this through the same exponential mechanism applied to large Ea reductions at enzyme active sites.