Video Transcript
In which of the following diagrams
will the rate of reaction be slowest?
The reaction rate quantifies how
quickly reactants turn into products. We can use a model called collision
theory to explain how reactions occur and at what rate. Collision theory states that a
reaction occurs when particles collide with appropriate orientation and enough
energy. Many factors affect how often
reactant particles collide and, therefore, how quickly they turn into products. The temperature affects the
reaction rate because it determines the kinetic energy of the reactant
particles.
Particles move faster at a higher
temperature and consequently collide and turn into products more often. Lower-temperature systems generally
have a lower reaction rate because their particles have less kinetic energy and thus
collide less frequently. The temperature of the water is a
factor in the reactions of our answer choices. The reactions are occurring at
either 60 degrees Celsius or 30 degrees Celsius. Since the question is asking us to
determine which will have the slowest rate of reaction, we know we must select a
beaker with water at a temperature of 30 degrees Celsius, not 60 degrees
Celsius. So, we can eliminate answer choices
(A), (C), and (D).
Let’s look now at another factor
that determines the reaction rate: concentration. The concentration of a substance
can be summarized as the amount of particles per volume of space. There is relatively little space
between highly concentrated reactants, which causes them to collide more
frequently. At a low concentration, reactant
particles collide less frequently and turn into products at a slower pace. The reactant hydrochloric acid is
either at a lower concentration with a molarity of 0.5 or at a higher concentration
with a molarity of 2.0.
It is likely that answer choice (B)
would occur at a slower reaction rate because of its lower concentration. But let’s have a look at the final
factor featured in this problem. The surface area of reactant
substances is a determinant of the reaction rate as well. Surface area can be equated to the
size of solid reactants in a reaction. If a solid reactant is in large
chunks, then only the outermost particles can initially be involved in the
reaction. If the large chunks are ground into
a powder, then the surface area of the reactant increases and more particles are
available to react. So, for a slower rate of reaction,
solid reactants generally have a lower surface area.
The solid reactant magnesium
carbonate in the answer choices is either powdered or in lumps. A larger size of lumps would likely
result in a slower reaction rate, as we have in answer choice (B). Therefore, the answer choice that
would have the slowest rate of reaction due to lower temperature, lower
concentration, and lower surface area would likely be answer choice (B).
