Day 23 MCAT Practice Question

Passage 4: Hydrogen Peroxide

Hydrogen peroxide (H2O2) is a common component in fuels, antiseptics, and

bleaching agents due to its strong oxidizing properties. It decomposes into water

and O2 gas via the following relationship:

2H2O2 (aq) = 2H2O(l) + O2 (g) ΔH: -98kJ/mol (Reaction 1).

Researchers performed decomposition of H2O2 with potassium iodide (KI) at

various temperatures. Specifically, they measured the rate of oxygen gas evolution

at varying concentrations of H2O2 and KI as well as varying temperatures. Results

are summarized in Table 1:

The specific heat capacity of the hydrogen peroxide solution was measured to be

2.5 J/g°C and was slightly more dense than water at 1.45 g/cm3. Hydrogen peroxide

is quite environmentally friendly as an oxidizing agent because of this harmless

decomposition into water and oxygen gas.

At the end of the experiment, the researchers noticed that no potassium iodide

had been consumed over the course of their project, although its presence always

increased reaction rate relative to the same reaction run at the same temperature

and concentrations in its absence.

Which component of the arrhenius equation does the presence of KI affect,

and how?

A) KI presence increases the pre-exponential term; it increases the collision

frequency of the reactants

B) C. KI presence increases the exponential term; it lowers the activation

energy and makes the transition state easier to reach

C) KI presence decreases the exponential term; it lowers the temperature at

which the forward reaction is favorable

D) KI presence decreases the pre-exponential term; it decreases the fraction

of possible transition states that are chemically unproductive