Video Transcript
Experiment A relies upon a catalyst. If a greater quantity of the catalyst is used, what changes to the graph would be
most likely? (A) The graph would initially be less steep, and the final volume of gas collected
would be the same. (B) The graph would be the same shape, and the final volume of gas collected would be
greater. (C) The graph would initially be steeper, and the final volume of gas collected would
be the same. (D) The graph would initially be less steep, and the final volume of gas collected
would be lesser. (E) The graph would be the same shape, and the final volume of gas collected would be
lesser.
We are told in the question that an experiment relies upon a catalyst. A catalyst is a substance that increases the rate of reaction without undergoing a
permanent chemical change. The reaction between nitrogen and hydrogen to produce ammonia gas requires a lot of
energy to occur. The addition of an iron catalyst lowers the activation energy barrier by providing an
alternate pathway for the reaction to occur. Let’s examine the alternate reaction pathway involving a catalyst.
First, the reactant molecules adsorb onto the iron catalyst surface and dissociate
into atoms of nitrogen and atoms of hydrogen. Then, three hydrogen atoms react with and bond to a nitrogen atom one at a time. Then, the ammonia gas is released or desorbed from the catalyst surface. Although the iron catalyst is involved in the reaction, at the end of the reaction it
is chemically unchanged. This means that the iron catalyst can be used again to catalyze more nitrogen and
hydrogen molecules into ammonia.
The question asks, what would happen if a greater quantity of catalyst is used? We can see from our example that a catalyst is involved in a chemical reaction, and
it can only adsorb so many reactants onto its surface. If a greater quantity of catalyst is used, then more reactant molecules can adsorb
onto the surface. This ultimately means that the reactant molecules will be converted into product
molecules faster. So a greater quantity of catalyst will increase the rate of reaction even
further.
We should also recognize that catalysts are not reactants. So while increasing the amount of a catalyst will increase the rate of reaction and
the product will be produced more quickly, it will not increase the total amount of
product produced.
With this in mind, let’s consider how a greater quantity of catalyst would affect the
provided graph. The graph shows the volume of gas collected over time. We know that increasing the amount of catalyst will increase the rate of
reaction. So if a greater quantity of catalyst is used, more gas will be collected in a shorter
amount of time and the graph would initially be steeper. But as the same amount of product can be produced, the graph would level out and the
total volume of gas collected would be the same as experiment A.
In conclusion, if a greater quantity of catalyst is used, the graph would initially
be steeper and the final volume of gas collected would be the same, answer choice
(C).
