Video Transcript
In the first picture, 20 mice are
eating two pieces of cheese. In the next picture, 10 smaller
pieces of cheese are being eaten by two mice each, and the cheese is being consumed
more quickly. The comparison between the mice
eating the cheese in these two pictures could be used when explaining how the rate
of reaction is affected by a certain variable. What is this variable? (A) The volume, (B) the catalyst,
(C) the concentration, (D) the surface area, or (E) the temperature.
In this question, we want to
compare the example of mice eating cheese to a chemical reaction. For a chemical reaction to take
place, reactant particles must successfully collide. How quickly a reaction takes place
is known as the rate of reaction. The rate of reaction describes how
the concentration of a reactant or product changes over time.
Many different factors can affect
the rate of reaction. Common examples of these factors
include the presence of a catalyst, the concentration of reactants, the reactant’s
surface area, or the temperature. Volume is not generally considered
to affect the rate of a reaction, although it may have an influence in terms of
concentration.
Catalysts increase the chance of
successful collisions by providing an alternative reaction pathway. A catalyst lowers the energy needed
for a reaction to take place and can speed up the rate. When we increase the concentration
of a reactant, more particles are present and available to collide with each
other. This increases the chances of a
successful collision and thus increases the rate of reaction. Increased surface area increases
the number of particles available to participate in the reaction. More reactant particles are able to
come into contact with each other to collide successfully, which increases the rate
of reaction. When we increase the temperature,
the particles move more quickly and have greater energy. This increases the frequency of
collisions and the chance that when particles do collide, they will actually
react. So, these four variables can
increase the rate of reaction.
Let’s now apply this to the mice
and their cheese. The cheese has been cut up into
smaller pieces from the first picture to the second. This allows the 20 mice to more
easily get to the cheese and consume it without getting in each other’s way or the
cheese in the center of the block being protected by the outer layers of cheese. The surface area of the cheese has
been increased by decreasing the size of the pieces.
We could think of this in chemistry
terms by comparing a piece of magnesium ribbon to magnesium powder. If hydrochloric acid were reacted
with both, magnesium powder would react much faster because of its increased surface
area. Therefore, the variable that can be
explained by this comparison between the mice eating cheese in these two pictures is
answer choice (D), the surface area.
