Questions: Factors Affecting Reaction Rates and Speed

Question 1

Temperature has a much larger effect on reaction rate than simply increasing collision frequency would predict. What is the primary reason?

• A. Higher temperature increases the activation energy, making bonds more reactive
• B. Higher temperature shifts the Maxwell-Boltzmann distribution so that a much larger fraction of molecules have energy exceeding the activation barrier — a small temperature rise can double the rate
• C. Higher temperature increases molecular size, improving the probability that collisions occur with the correct orientation
• D. Temperature primarily works by reducing solvent molecule concentration, clearing the path for productive collisions

Answer: (B)

The key insight is that most molecules at room temperature have energies below the activation barrier — they collide but don't react. The fraction that exceeds the threshold depends exponentially on temperature (via the Boltzmann factor e^{−Ea/RT}). Even a modest temperature increase disproportionately enlarges the high-energy tail of the Maxwell-Boltzmann distribution, so the fraction of productive collisions rises much faster than collision frequency alone. Temperature does not increase activation energy (option A) — it increases the fraction of molecules already above the fixed activation barrier. This exponential dependence explains why a 10°C rise can double the reaction rate.

Question 2

A chemist adds a platinum catalyst to a reaction and observes the rate increase dramatically. A student concludes that the catalyst must have increased the collision frequency between reactants. What does the catalyst actually do?

• A. The student is correct — the solid catalyst surface concentrates reactants, dramatically increasing collision frequency
• B. The catalyst provides an alternative reaction pathway with lower activation energy, so a much larger fraction of existing collisions have enough energy to produce products
• C. The catalyst raises the temperature of the reaction mixture by releasing stored chemical energy
• D. The catalyst changes the equilibrium constant, shifting the reaction toward products and speeding up the forward rate

Answer: (B)

A catalyst works by providing an alternative mechanism — a different sequence of steps with a lower activation energy barrier. Because the activation energy is lower, a much larger fraction of collisions (which occur at the same frequency) now have sufficient energy to cross the barrier. Critically, a catalyst does not change the thermodynamics: ΔH and the equilibrium constant are unchanged, because the energy difference between reactants and products is unchanged (option D is wrong). The catalyst lowers the hill that molecules must climb, not the difference in elevation between the starting valley and the destination valley.

Question 3

Adding a catalyst to a reaction changes both the reaction rate and the overall energy change (ΔH) of the reaction.

• T. True
• F. False

Answer: False

A catalyst increases rate but does NOT change ΔH, the equilibrium constant, or the identity of the products. It provides an alternative pathway with lower activation energy — lowering the barrier between reactants and products — but the energy levels of the reactants and products themselves are unchanged. ΔH depends only on the difference in bond energies between reactants and products, which is independent of the path taken. A catalyst equally accelerates both the forward and reverse reactions (consistent with an unchanged equilibrium constant), which is why it cannot shift the final equilibrium position.

Question 4

For a heterogeneous reaction involving a solid reactant, grinding the solid into a fine powder increases reaction rate by increasing the surface area available for collisions with the other reactant.

• T. True
• F. False

Answer: True

In heterogeneous reactions (reactants in different phases), only surface atoms of the solid participate in collisions with the other reactant — the interior is inaccessible. Grinding into powder enormously increases the surface-area-to-volume ratio, proportionally increasing the number of reactive collision sites. This is why finely powdered metals can be highly reactive or even explosive while the same metal in bulk form is relatively inert — the same chemistry, but vastly different effective collision opportunities. This factor connects directly to the underlying principle: rate depends on how often effective collisions occur, and surface area directly controls that frequency for heterogeneous systems.

Question 5

Explain why all five factors affecting reaction rate (concentration, temperature, nature of reactants, surface area, and catalysts) can be understood through a single unifying principle. What is that principle?

Model answer: The unifying principle is: for a reaction to occur, reactant particles must collide with (1) sufficient energy to overcome the activation barrier and (2) the correct orientation to allow bond-breaking and bond-forming. Every factor works by changing one or both of these requirements. Concentration increases collision frequency (more molecules per volume = more collisions). Temperature increases the fraction of collisions with energy above the activation barrier (and slightly increases frequency). Nature of reactants determines the activation energy itself (how strong the bonds to be broken are). Surface area increases the number of accessible collision sites in heterogeneous reactions. Catalysts lower the activation energy, increasing the fraction of collisions that succeed without changing collision frequency.

Framing all five factors through this single collision principle makes the concepts predictive rather than just a memorized list. If a reaction is too slow, you can systematically ask: should I increase collision frequency (concentration, surface area), increase the energy of collisions (temperature), or lower the threshold energy needed (catalyst)? The 'nature of reactants' factor reminds you that some reactions are intrinsically fast (ion recombination in solution) or slow (breaking triple bonds) regardless of conditions — the activation energy is set by the chemistry itself, not by external manipulation.