Video Transcript
Manganese dioxide is used as a
catalyst in the decomposition of hydrogen peroxide to form water and oxygen. Which statement is untrue when
using MnO2 as a catalyst? 2H2O2 aqueous produces 2H2O liquid
plus O2 gas. (A) The oxygen will be formed more
quickly. (B) More oxygen will be
produced. (C) The mass of MnO2 before and
after the reaction will be the same. (D) An alternative reaction pathway
is provided by the catalyst. (E) The catalyst remains unchanged
at the end of the experiment.
In this question, we’re being asked
about how a catalyst, which is manganese dioxide, behaves in a decomposition
reaction. In this decomposition reaction, a
single reactant, the hydrogen peroxide, decomposes to form new products. The products are water and oxygen
gas. This decomposition reaction is very
slow under normal conditions at room temperature. We would not observe many bubbles
of oxygen gas coming from the hydrogen peroxide solution at all. This situation would change rapidly
if a small amount of the solid catalyst, manganese dioxide, were added to the
hydrogen peroxide solution.
Rapid fizzing or effervescence
would be observed as soon as the black powder is added to the hydrogen peroxide. The manganese dioxide catalyst will
increase the rate of decomposition of the hydrogen peroxide. More oxygen gas, seen as bubbles,
will be produced per unit of time as the reaction rate has been increased. Oxygen gas will certainly be
produced more rapidly. This statement is true, so it’s not
the correct answer. Remember, in this question, we’re
looking for an untrue statement.
Notice that the oxygen gas, which
is one of the products, originates from the hydrogen peroxide molecules. According to the balanced equation,
two molecules of hydrogen peroxide are required to produce one molecule of oxygen
gas. If we have a fixed amount of
hydrogen peroxide molecules at the start of the reaction, we can only produce a
fixed amount of oxygen molecules during the decomposition reaction. Adding the manganese dioxide
catalyst does not change the amount of oxygen gas produced. It simply increases the rate of
reaction.
The same amount of oxygen gas is
produced in much less time. The manganese dioxide does not
appear in the overall reaction equation. The amount of oxygen gas obtained
will be the same with or without the catalyst present. This means that the statement that
more oxygen gas is produced is untrue. Therefore, it’s likely to be the
correct answer.
